Author Topic: Vacuum Fluorescent Display Driver  (Read 10221 times)

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Offline bostonmanTopic starter

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Vacuum Fluorescent Display Driver
« on: April 09, 2024, 03:31:30 am »
I have an Agilent 53132A counter with a slightly messed up 12-digit display.

The odd part:

The display is fine during different stages. If I inject 10MHz, the display is fine except the 12th digit, segment C (right bottom vertical segment), remains on (something I could live with), however, if I access a menu, the segment is fine.

Injecting around 15MHz sometimes (I emphasize that word) causes digits four and five to have extra segments illuminated; and the same if I access menu options (AUTO TRIG is displayed as AUTOOTRIG). Other times extra commas and/or periods are displayed in multiple locations (hard to tell if it's trying to show a comma or it's just a bright period) Usually the wrong commas and/or periods are also in the fourth and fifth digit.

I'm not asking for help diagnosing the issue (unless someone has repaired a similar issue), but mainly I suspect the display driver IC (SN75518FN) because the frequencies seem to be correct if I watch the fourth and fifth digit change from correct to incorrect.

The driver chip datasheet (see attached) isn't exactly helpful because I'm unfamiliar with fluorescent displays. The circuit (see attached) uses 32 of the output pins on the chip which correlates to the datasheet. Unfortunately I can't locate the datasheet for the fluorescent display.

My initial assumption was this is similar to a seven-segment display and would be 84 driver lines (7 * 12), but this isn't the case since it's only using 32. My next thought is this is scanning using the internal shift register, but this still doesn't mathematically add.

The reason(s) I'd like to understand this: obviously to learn, but also maybe to use a logic analyzer (I have a 16-channel unit) to confirm the output isn't matching the input data.

Update: looking more, maybe I understood this a bit more than I thought. I'm assuming G1 through G12 activate each digit as the chip scans left to right (or right to left?). Since the chip also outputs SEG-A thru SEG-Q (17 outputs), looking at the display closer reveals each digit is comprised of 14 segments (rather than 7 like I thought), a comma, period, and a dot, this would add to 17.

If this is accurate, can I still measure this on a logic analyzer to confirm the chip is bad, and, how are other things on the display shown such as MHz, freq, per, Ch1, Ch2, etc...?
« Last Edit: April 09, 2024, 03:46:11 am by bostonman »
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #1 on: April 09, 2024, 05:56:53 am »
The display looks (see attached) to multiplex 12 digits with each digit consisting of 18 segments:
  8 line segments (standard a..g) with a split g segment,
  4 diagonals,
  2 center verticals,
  3 colon dots and comma (rightmost digit replaces the dots and comma with annunciators)
  1 annunciator.
I can't explain where two additional driver (SN75518FN) lines are used on the display.

How to drive VFDs can be found here

In summary, one digit (grid) line is energized (to +38VDC) along with selected segments of that digit (also to +38VDC); all other lines grounded. Then the next digit (grid) + segments are lit, etc.  Since the driver is just a SPI consumer, expect a stream of 32 bits * 12 * Hz refresh rate.

There is a low AC voltage across the FIL1+FIL2 lines providing a stream of electrons, with the mid-point (split) AC voltage biased (via zener) a few volts above ground so that any grounded driver lines keep that segment or grid off completely (no bleeding from adjacent segments).

I'm not sure which (display or driver) is causing the misfiring segments that you're seeing. This is a known problem and has been documented with other HP VFD products such as the 34401A on this forum. 

You could use your LA to compare the SPI stream with the driver output but be wary of the +38VDC so use a divider.  ie. an unchanging SPI stream should produce a unchanging output line state. If not, the driver is bad.  If true, and the display shows changing segments, then the display is bad.
« Last Edit: April 09, 2024, 06:02:25 am by pqass »
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #2 on: April 09, 2024, 06:13:02 am »
The display is multiplexed, 14 segments plus colon (DP2), decimal pt (DP1), comma (SEG-COM), annunciators (SEG-COM) and a few others Freq, Gate, Ch1. as simply extra segments per digit.
Segments go positive voltage wrt cathode to light them but digit (screen) grids must also go positive to enable a digit, so the segments can light.
Note the directly-heated cathode (filaments) are biased at 6.2V up. Not to be confused with end-end voltage which is the filament heater voltage. VFD power supply DC-DC converter U30-A outputs +38V and the filament bias I think done by VR1 or VR2 zeners on the main board.
So the VFD sees +31.8V (when '75518 is outputting +38V) at the grid and segments, or -6.2V (relative) (when '75518 is outputting 0V) to bias the VFD into cutoff.
If you get segment or digit overlap or smearing, I would suspect the 75518 but also confirm the cathode bias is OK at +6.2V and the anode supply is around +38V. I'm not sure how that Endcott DC-DC converter module is, what's inside.
Careful the logic analyzer can handle high voltage +38V signals.
You can use a scope and look for the signal swing say 36 to 5V low (@1mA) it has push-pull outputs and can source+sink current.
You can force the issue with the '75518, if it has weak outputs by stressing an output pin by adding load resistor and seeing what the output voltage does. Say a 20k to a VFD pin should not cause much change, either to GND or +38V - if the IC is OK.
Or the IC is switching slow and overlap between digits is what you are seeing.

HV518PJ is in production.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #3 on: April 09, 2024, 04:51:24 pm »
Quote
I'm not sure which (display or driver) is causing the misfiring segments that you're seeing. This is a known problem and has been documented with other HP VFD products such as the 34401A on this forum.

Prior I did some research and found exactly what you referred to. I was trying not to guess and just swap without measuring and tried to understand the datasheet.

The explanations are helpful, but wow, still a bit confusing. Does each digit turn on and off rapidly? If I'm thinking about this correctly, let's say it's a four-digit display and 1, 2, 3, and 4, come into the serial input. Is '1' sent on all 17 output lines and the associated Gx output selects which digit to light (the far left digit in this example)? After '2' is received, the '1' digit is turned off, and '2' is shown in the second far left digit? It's done rapidly, so to the human eye it's not noticed?

My logic analyzer (Kingst 16-channel model LA2016 I believe) shows it can handle +/-50v. I'm uncertain how easy this will be to look at the serial data, figure out which bit is the fourth and fifth digit, and then see that it's outputting something other than the bit in the serial data.


 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #4 on: April 09, 2024, 06:08:23 pm »
The SN75518FN is a 32-bit serial-in shift register with latched outputs. This is similar to a cascade of 4, 74HC595s.

This means that it can receive, via SPI, 32 bits to fill its shift register WHILE the output lines still holds the previous 32 bits steady. The MCU then toggles the LATCH EN pin which moves the serial shift register [internal] outputs into the output latches which will now show a new state on the output lines (all at once). The STROBE input just briefly blanks all outputs (sets to GND) during the transition when the output latches are loaded with new serial-in data (since STROBE is tied to LATCH EN in the schematic).

So, as for your example display, the MCU sends 32 CLKs and DATA, then LATCH EN is toggled (LOW-HIGH-LOW) which causes those 32 bits to now appear on the output lines.  The MCU waits 4.2ms then outputs the next 32 bit digit data stream. After 4, 32bit streams+4.2ms waits, it cycles back to digit 1.  Only 1 grid output is ever enabled at a time otherwise the same segment data will be on in multiple digits.

Why a 4.2ms wait? A: You'll want to refresh each digit at least 60 times per second, so for your 4 digit example that's 4 * 60 = 240Hz or 1/240s = 4.2ms.  In the 53132A with 12 digits, you'll need at least 12 * 60 = 720Hz or 1/720s = 1.4ms wait.  The 32 bit stream needs to be fast enough to complete within one wait, obviously.

See attached Noritake diagram. The serial stream of grids and segments appears as a column (say T1). During T1 time, a new 32 bit stream is sent then transitioned to the output latches during the dotted vertical lines time where all output lines are blanked (GNDed) momentarily before showing T2 state on the output lines.  I think the reason for the blanking is to make sure all digits and segments are completely off so your eye won't perceive a dim segment that was on in the previous digit.
« Last Edit: April 09, 2024, 06:16:33 pm by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #5 on: April 09, 2024, 06:54:19 pm »
Quote
Only 1 grid output is ever enabled at a time otherwise the same segment data will be on in multiple digits.

So as I assumed, each Gx selects the associated digit? Since 1, 2, 3, and 4, arrive as serial data, 1 is placed on the output, a blank pulse, Gx selects digit one, and a 1 is shown.

The diagram you provided definitely simplifies things.

Unfortunately my 16-channel logic analyzer doesn't have enough channels to look at all 32-bits and the input serial line simultaneously; or 17-bits if I ignore the Gx lines. Maybe I can look at 15-bits and see something obvious that will point to the output data being wrong whereas the input serial data is correct confirming the chip is bad.

Also, what is the purpose of the serial output pin? I'm guessing it's to feed the selected menu options back to the main board so it knows which settings were selected such as 'Auto Trig'.

On a side note, during my initial investigating, I discovered the Microsemi/Microchip replacement IC part number. I need to confirm mine is surface mount, but I was happy to see the chips (or replacement chip in this case) exist.

 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #6 on: April 09, 2024, 08:38:38 pm »
Yes, each Gx corresponds to one digit.  Both the segment data and grid (digit) to enable appear on the driver output lines at the same time.  The total display blanking (STROBE=HIGH) only lasts for the duration of the LATCH EN=HIGH toggle (LOW-HIGH-LOW); about 10-50 usecs as per section 4.2 in the Noritake docs.

You don't have to monitor all driver output lines.  If you know a particular anomaly happens on one digit, then just monitor the segments when that grid pin is enabled.  We know the first SPI bit out of the MCU corresponds to Q32 (pin 3) output line which is G1, the next bit is G2, ... bit 12 is G12.  The segments bits 13..32 are not readily obvious so you could decode them if you ramp up a frequency input while you monitor a particuar digit.  Once all bit positions are decoded to which segment they enable, you can monitor your problem digit while watching the display. If the bit stream doesn't change but the output lines do, that means: bad chip.

I'm not sure why they route the serial out of the driver back to the MCU.  The MCU knows what it sent.  Serial out is used to cascade to the next 75518.  Maybe there is another serial-in shift register elsewhere? ie. more than 32 bits are sent before LATCH EN is toggled.   The keyboard input is another set of 6 RnCm+READRLC+LKBD lines.
« Last Edit: April 09, 2024, 09:22:38 pm by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #7 on: April 10, 2024, 03:01:32 am »
Hopefully I'll have time this weekend to take some measurements. To be safe, maybe I'll use a voltage divider to avoid stressing the inputs to the logic analyzer (although it's a bit of work since I'll have to make one for all the channels).

Briefly looking at the board tonight to confirm it's a surface mount chip (although the BOM did confirm and it also shows the part number of HV518PJ which I didn't realize - I was looking at the part number next to the IC on the schematic), trying to connect to pins will not be easy without soldering fly leads; something I'm uncertain about since I'd like to avoid damaging anything.

The other unfortunate part: the display needs to be removed to access the chip.

I didn't see another 75518 for the serial out pin, but did trace it to pin 3 of a 74AC251; seems to be the only location it connects.

The schematics seem to have a few errors and/or some weird designs. The first error is in the attached, 5v into a LM317 and 5v out? The other weird thing is the 50ohm input option. Channel 1 uses 51ohms 1W, Ch2 uses two 100ohms 0.25W in parallel.

Although some weird things and/or errors, I'll take having schematics so I can repair my electronics than not having them.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #8 on: April 10, 2024, 03:37:22 am »
I think it's most likely an analog problem so the logic analyzer may not be useful. Just scope the display pins - that will reveal plenty.
If the '75518 outputs are weak and instead of say +36V/+3V signal swings, you get +35V/+8V the VFD could nuisance glow weak, not be blanked (for that seg/digit).
Is that a "1" or a "0" to the logic analyzer?

What complicates working on this problem is the VFD has different transconductance depending on its overall age and the seg/digit age. So the same voltages on a commonly used segment/digit verses a rarely lit segment/digit can make a difference between it working or staying stuck on dimly.

It's not a fun repair - the IC is stashed underneath the VFD. I had to remove the metal shields, all the while not smashing the glass or nipple around too much. Need extra care and patience. To desolder VFD Kovar pins, I will heat the pin on the board top-side using a soldering iron, and then vac desolder on the back-side. It works fast. The VFD pin and hole size is large.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #9 on: April 10, 2024, 07:42:53 am »
I don't think you need to take apart anything just yet.  You have access to the VFD leads so it's just a question of understanding the VFD pinout. Once the pinout is identified and you've done some preliminary 'scope probing (below), you can then use your LA (via a divider).

The filament pins are almost always the outer pair which, according to the schematic, are pins 1 and 18. Start with an assumption (see attached). It could be numbered according to the white set or the red set. Or bottom white and top red or vice versa.  Or 1 to 18 on top and 19 to 36 on bottom.   The pinout is right in the service docs board layout (page 3). DUH!

You should be able to measure with a multimeter (in VAC mode) about 5.5Vrms between the FIL stubs and either the upper outer leads or lower outer leads. Identifying the filament leads will identify which row the grids are located in, therefore, the opposite row will have most of the segments.

Using an oscilloscope, you should be able to probe any lead other than the filaments. You should see square waves from GND to +38V; fast rise/fall-time edges, no runts. I think it's safe to attach your probe GND clip to the chassis which is likely connected to board ground (labelled as DCOM) though I couldn't find any connection between DCOM and PE/chassis in the schematics.  If you want to probe between filament leads only do so in differential mode; ie. using two 'scope channels, NO GND CLIPS, math set to A-B channel. That way you won't accidentally short one filament to ground (via GND clip) and possibly kill the DC-DC.

FYI: I found an earlier post of mine which identified a datasheet for the DC-DC converter. The schematic pinout for the part is off by one. The converter actually produces up to 44V which is lowered to 38V via 6.2V zener VR1.

As for the 75518 data out going back to the MCU...  on further inspection, the SPI topology used isn't the usual MISO/MOSI/SCLK common bus with individual SS per slave. Nooooo.... lets use a star topology where every MOSI/SCLK/SS has it's own 74AC138 and MISO is funnelled through a MUX.  The MCU then only needs MISO, MOSI, SCLK, PCS0..2 to select 1 of 8, and PCS3 to the enable pins on the '138s.  I still can't explain why it needs to re-read its own output. Display failure detection? Completeness?
« Last Edit: April 10, 2024, 03:29:22 pm by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #10 on: April 10, 2024, 05:21:04 pm »
Quote
FYI: I found an earlier post of mine which identified a datasheet for the DC-DC converter. The schematic pinout for the part is off by one. The converter actually produces up to 44V which is lowered to 38V via 6.2V zener VR1.

The link doesn't appear to work, however, I probably already downloaded the same datasheet earlier this week; or maybe the weekend. Is this the one that supplies the FL1 and FL2 (?) power? If so, yes, I got that datasheet.

Also, I read through the thread from the link you provided. The second last message, or one of the last messages, provides an eBay link to "new" VFD's sold out of China. I'm wondering whether to buy a backup (assuming I repair this unit) should mine eventually burn out, dim, or I break it while replacing the driver chip.

Last night I had to place a DigiKey order and ordered a few driver chips. It seemed worth the gamble for $7.xx cost of the part. At least this way I'll have the chip on hand should I confirm the driver is bad, suspect, or be unable to get an adequate amount of measurements.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #11 on: April 10, 2024, 09:23:58 pm »
Yes. This one (attached).
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #12 on: April 11, 2024, 02:08:17 am »
Yes, that's the datasheet I found the other night. Thanks for sending the corrected link regardless.

 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #13 on: April 13, 2024, 12:22:31 am »
I took some quick measurements. While the displayed was showing “AUTOOTRIG”, the G5 pin was snowing a high.

This tells me the chip is feeding the wrong data to the display.

Just wanted to ask this question before I take more measurements this weekend.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #14 on: April 13, 2024, 01:10:18 am »
I took some quick measurements. While the displayed was showing “AUTOOTRIG”, the G5 pin was snowing a high.

This tells me the chip is feeding the wrong data to the display.

Just wanted to ask this question before I take more measurements this weekend.

Sure seems like G4 and G5 are both on as witnessed by the double "O" character. That shouldn't happen.  Although you didn't describe double/superimposed characters in your original post; just extra segments.  Since it's doubtful that the serial stream has both G4 bit and G5 bit on, I'd say it's a bad VFD driver.
« Last Edit: April 13, 2024, 03:25:32 am by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #15 on: April 13, 2024, 01:39:36 am »
Sorry…. Unless I’m wrong, I discovered the double O after my initial post; but mentioned it  in another follow up.

Ideally, just for my own interest, I’d like to see the incoming data and individual digit data for educational purposes, so I’ll take more measurements. I believe the replacement chips are arriving Monday or so, however, I won’t have time mid week to replace it, so updates may be delayed.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #16 on: April 17, 2024, 02:48:32 am »
I'm slightly confused.

Attached are two scope measurements. The first one named 'All Hyphens' is the display showing all hyphens and not changing (I set auto trig to 'external' without having an external trigger, so it froze the display). The other is measuring the pins on the VFD (not the IC) - file named accordingly showing which channel is connected to which pin and ('AUTOOTRIG' displayed on the screen).

My confusion is not knowing which segment data line lights which display segment (this is something I should and can figure out on my own since my goal is to measure the data lines). Excluding not knowing which data line lights which segment, when a particular G line is activated and shows on the scope, the other channel on the scope should have that segment lit showing 40v. I'm getting confused over seeing a G line go high and then the data just jumps around (obviously different data for different digits), but how do I capture the particular bit for that G line?

The other confusion is having all hyphens on the screen. Even though every digit had a hyphen, channel 1 on the scope doesn't show the G line being activated. How can every digit display a hyphen if the G line isn't activated? This would imply that particular digit shouldn't have anything displayed, however, it has a hyphen.

The more I thought about things tonight, the more I realized maybe trying to take measurements with a logic analyzer is too involved. If I make voltage dividers for all 16-channels (even though my logic analyzer can handle up to 50v - I'd rather not chance damaging it due to any voltage spikes), I need to connect each divider to each data line - seems I'm increasing my chances of shorting something.

My initial concern was to confirm the incoming data to the VFD driver chip was correct. I'm sure the VFD driver chip is the culprit, but I was trying to confirm with seeing the data in versus out. Maybe I may rethink things and just replace the chip.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #17 on: April 17, 2024, 03:07:36 pm »
"I'm getting confused over seeing a G line go high and then the data just jumps around..."
"... but how do I capture the particular bit for that G line?"

As for the "All Hyphens - Ch1 Pin 28 Ch2 Pin 6-1.pdf" file, you're capturing SEG-A (pin 28) which as expected, is 0V.  You should be probing SEG-G (pin 35) or SEG-H  (pin 36) assuming SEG-A thru SEG-G mean the same segments as the typical 7-segment display; with SEG-H being left or right-half of the center horiz. segment.   As to why you're seeing other segment info, try using normal trigger mode, not auto, or lower your trigger voltage to say, 10V. See auto/normal explained.

"The other confusion is having all hyphens on the screen. Even though every digit had a hyphen, ... doesn't show the G line being activated."

Maybe SEG-H isn't the other half of the center horiz. line segment.  Maybe there is some multiplexing activity that outputs segment data temporarily without any Gx enabled (which won't be visible to the user). MCU funny business.

To determine which pin corresponds to which SEG-x, I'd put channel 1 on, say, G2 (pin 6), set trigger to normal on channel 1 to <10V, then use channel 2 to probe around pins 19-36, 3, 4.  Then change the value on the G2 digit (2nd from left) and re-probe to confirm the lit segments again.

« Last Edit: April 17, 2024, 03:21:14 pm by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #18 on: April 17, 2024, 03:20:07 pm »
Quote
As for the "All Hyphens - Ch1 Pin 28 Ch2 Pin 6-1.pdf" file, you're capturing SEG-A (pin 28) which as expected, is 0V.  You should be probing SEG-G (pin 35) or SEG-H  (pin 36) assuming SEG-A thru SEG-G mean the same segments as the typical 7-segment display; with SEG-H being left or right-half of the center horiz. segment.

Wow..... one thing I can't understand about myself is how I dive into these projects more than some, yet, I miss the obvious. You are absolutely correct, I should have considered the fact I was on a data line that was blank.

Per your suggestion on probing the other pins to determine the segment order, I somewhat did that. My scope captures unfortunately have both channel 1 and 2 on the same 0v point, so seeing individual pulses is difficult. Ideally I'd like to determine which data lines control which segments so I can provide the information to this forum (assuming it hasn't already been done) to help others who experience the same issues.

The annoying part is I'm dealing with too much fluctuations with the unit. Sometimes the display is correct, other times it doesn't. By the time I try capturing the screen with incorrect displays, it changes again. Due to having the cover off, I use the lead of a resistor to short the pads so I can change menu options in attempt to "freeze" the counter (much like I did by setting it to 'external' trigger).

Then the data on the scope is jumping around, so I can't tell if I'm not triggering (although I'll try your suggestion on triggering) or if the data line is jumping.

Finally, I don't want to accidentally short anything, so I turn off the counter when I move the probe to another pin.

I'm either being too cautious, or over thinking. :)
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #19 on: April 25, 2024, 01:15:53 am »
After building a voltage divider and setting up for measuring with the logic analyzer, I finally had time earlier, however, I think more questions than answers came out of it.

My logic analyzer is 16-channels, but the chip has seventeen outputs (not including two additional outputs for decimal places). I made the error of accidentally connecting one of my logic analyzer channels to DP2 meaning two of my logic analyzer channels weren't connected to two of the SEG outputs (one accidentally connected to DP2 and unable to connect to the seventeenth data line due to my logic analyzer being sixteen channels).

The logic analyzer results was not what I expected. Attached are four CSV files (they may not be worth assembling to show the logic because it contains many data points). The CSV files are named accordingly and I'll also upload some pictures of the display which will show the display at the time of the logic analyzer capture (give me a few minutes).

The reason the results weren't as expected: if I kept the logic analyzer set to keep polling, the pulses changed. I expected different because when the display showed (as in one file) all 1's and I removed the input so the display remained frozen, the logic analyzer (as I expected) shouldn't show different pulses each time it polled.

If the entire display is twelve digits, and each digit has fourteen (?) segments, and two decimal places, that uses sixteen out of the seventeen data lines (not sure where the seventeenth is used). The G lines are twelve total (for twelve digits?).

Due to the display having 'period', 'frequency', 'ch1', etc... and assuming each G line is for each digit, that leaves the question of how does the other stuff get lit? My guess is that more than one G line is on so the data lights MHz, CH1, etc...

Unfortunately I didn't have time to investigate this. If my assumption is wrong, then I should see the logic analyzer output to have each channel in sequential order (or no two G lines on simultaneously).

Due to the complexity of trying to not damage anything, I'm close to pulling the trigger and just swapping the IC.
« Last Edit: April 25, 2024, 01:18:26 am by bostonman »
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #20 on: April 25, 2024, 03:33:46 am »
It's very difficult to fully troubleshoot the problem because it's likely inside an IC - the '518 or MCU that has crosstalk.
In the pics I can see right-most digit has "C" segment stuck on, is that the only malfunction in the pics?
I would scope that pin on the VFD to see if the '518's output driver has a problem like a weird weak voltage. But it is odd that segment prefers that digit. I was thinking of DRAM/SRAM stuck bits.
If you are pulling the VFD, I suggest putting a soft piece of tubing over the nipple, it is extremely fragile and easy to bang. Or scotch tape something maybe.
Getting the shields off does take a bit of extra heat, as I mentioned my technique is to use a soldering iron heating on the pin topside, and vac desolder on the other side. A two-handed affair. The VFD holes on the PCB are large though.

Some notes I found on the '518 protocol, to help you out if you are still digging:

Code: [Select]
For 53131A

The protocol is similar to SPI. It is clocked at ~1 MHz, with 4 byte word bursts at about 1 kHz rate. It is 5 V logic.

The lines and their SPI equivalents are:
VFDSCLK - Clock, data sampling on rising edge
VFDSOUT - Data, from MCU to display
VFDSIN  - Data, from display to MCU, 32 bit shift register
VFDSEN  - Enable, active high, inverted compared to a SPI /SS

VFDSCLK:
____   _   _   _       _   _   _____
    \_/ \_/ \_/  ... _/ \_/ \_/
VFDSOUT:
________________     _______________
    \___X___X___ ... ___X___X___/
VFDSEN:
   _____________     ___________
__/              ...            \___
Bit: (MSB first)
      0   1   2  ...  29  30  31

Every ~976 us (~1024 Hz), there is a 32 bit word, with bits clocked at ~934 kHz. After the first 16 bits, the clock makes a small pause of about 0.5 bit cycle.

Each 32 bit word enables one gate (character position) and any combination of segments for that position. The first 12 bits are gate enablers, the first/highest bit enabling the leftmost/most significant character. The segments are 14 bits for the segments of the digit/character display, 3 bits for the separator segments for ".",
",", ":" and ";" and 1 bit for the text label under each character. The rightmost character is special; instead of separators the bits enable the "u" (mu) and "s" unit indicators and the "Gate" text label, and it has two extra bits for the "M" and "Hz" unit indicators.

A frame has 16 words - 1 for each of the 12 characters, and 4 extra that can be highlighted, x0-x3 below. The frame rate is ~1024 / 16 = ~64 Hz.

Below data is shown as big endian, first byte leftmost, and with first bit on SPI as most significant.

Bits:
0xfff00000 - gate enablers (character position)
0x0000fcff - 14 segment character
0x00070000 - separators, on rightmost position "u", "s" and "Gate"
0x00080000 - text label
0x00000300 - on rightmost position units "M", "Hz"

Examples:

Text: "FREQUENCY 1 "; Labels: "Freq", "Ch1"
1000CC8C # char 8 "Q"
  100000 # char 0 empty
2000C087 # char 9 "E"
  202040 # char 1 "1"
4008888F # char 10 "R", label: "Freq"
  400000 # char 2 empty (space)
80008087 # char 11 "F"
  802030 # char 3 "Y"
80000000 # x3 - no highlighting
 108C084 # char 4 "C", label: "Ch1"
80000000 # x2 - no highlighting
 2008C2C # char 5 "N"
80000000 # x1 - no highlighting
 400C087 # char 6 "E"
80000000 # x0 - no highlighting
 800C40C # char 7 "U"

When a character is blinking, it changes between being normally displayed and being highlighted. The character is powered an extra time per frame using word x0-x3 which makes it brighter. The flashing, changing between the character and empty, is done in the normal word for that character. Only the 14 segments of the character seems to be
flashing and highlighted, not the indicators or the text labels.
As there are only 4 words x0-x3, only 4 characters can be highlighted using this mechanism, and it seems also never more than 4 characters are highlighted and blinking.

"LIM TEST: OFF", where "OFF" is highlighted and blinking
10000000 # char 8 space
  108087 # char 0 "F"
2000843C # char 9 "M"
  208087 # char 1 "F"
400860C0 # char 10 "I", label: "Freq"
  40C48C # char 2 "O"
8000C004 # char 11 "L"
  800000 # char 3 space
80000000 # x3 - empty, no highlight
 10B20C0 # char 4 "T", separator ":", label "Ch1"
  108087 # x2 - highlight char 0 "F"
 20044A1 # char 5 "S"
  208087 # x1 - highlight char 1 "F"
 400C087 # char 6 "E"
  40C48C # x0 - highlight char 2 "O"
 80020C0 # char 7 "T

10000000 # char 8 space
  100000 # char 0 empty ***
2000843C # char 9 "M"
  200000 # char 1 empty ***
400860C0 # char 10 "I", label: "Freq"
  400000 # char 2 empty ***
8000C004 # char 11 "L"
  800000 # char 3 space
80000000 # x3 - empty, no highlight
 10B20C0 # char 4 "T", separator ":", label "Ch1"
  108087 # x2 - highlight char 0 "F"
 20044A1 # char 5 "S"
  208087 # x1 - highlight char 1 "F"
 400C087 # char 6 "E"
  40C48C # x0 - highlight char 2 "O"
 80020C0 # char 7 "T
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #21 on: April 25, 2024, 03:48:33 am »
Quote
In the pics I can see right-most digit has "C" segment stuck on, is that the only malfunction in the pics?

In the pics I posted, if I'm not mistaking, the commas and/or periods are wrong, and it's not showing "MHz".

Sporadically, digits four and five are wrong, commas and/or periods are wrong, and, usually, in all cases (I believe) "AUTOOTRIG" is displayed.

Unfortunately I don't have a solder VAC just solder braid and a solder plunger (the manual pump and push button release type).

I think to save time and reduce the risk of damage, I may just move forward with replacing the HV518 chip.

I was hoping to figure out which data line and G line toggled which segment(s) in each digits, and not only gain some knowledge, but provide the information back to this thread for others to learn from and/or help diagnose issues on their end.

Since I have the logic analyzer out and the display sitting outside the unit, I'll probably measure the G lines on the logic analyzer; and then move forward with replacing the chip.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #22 on: April 25, 2024, 04:07:07 am »
If desoldering a few VFD pins doesn't go well, I'd suggest getting a decent handheld vac desoldering tool. I find braid is hard on boards, for heat and abrasion on larger TH pins. Unless you have a good technique.
We used Soldapullt back in the day they were good if greased well. The chinese ones I hear many are useless. The test was to see how much suction it had on your finger, the piston should move slow if the tip is blocked.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #23 on: April 26, 2024, 12:45:57 am »
The G lines are equally confusing. This is with a frozen 5MHz (5.0000000000) on the screen.

The data on the analyzer lines that are not labeled weren’t connected. For some reason they are picking up stray signals and showing on the captures.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #24 on: April 26, 2024, 03:12:07 am »
I don't see G12, G5, G2 being enabled at all and G1 signal looks scrambled with four extra pulses, offset in time as well. The shift-register looks defective  :-//
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #25 on: April 26, 2024, 03:19:25 am »
The shift register inside the HV518 chip?

I agree that the signals don't look correct, however, oddly enough, the display is almost correct (12th digit has the segment C lit and nothing else - segment C being the right vertical bottom).

Hopefully this weekend I'll have time to replace the chip.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #26 on: April 26, 2024, 03:34:41 am »
I would look at the G1 with a scope to see actual voltages. G1 might be not be driven (floating) or the 518's shift register is wonky.
This thread, it sorta seems like the OP found VFD had internal leakage but not sure about the English, his bench test of the VFD:
https://www.eevblog.com/forum/repair/hp-agilent-53181a-display-error/msg1399630/#msg1399630
Inside the VFD is a set of close PCB traces that very rarely have some ion migration happen.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #27 on: April 26, 2024, 01:17:15 pm »
I'll check with a scope.

I read that thread. personally I'd like the path of bench testing the VFD solo as a second step rather than risk damaging it (assuming it's not the culprit).

Since the fourth and fifth digit work sporadically, my assumption is: this isn't a short in the VFD. Before it seemed the fourth and fifth digit was failing around >15MHz, but then it began working correctly. Sometimes the display would show the frequency in ten or maybe eleven digits, other times the full twelve; but it was always the correct frequency (ignoring the in correct decimal and comma locations - at least I remember/believe they were wrong).

The only consistent error is the 12th digit segment C (?) remaining illuminated. Maybe periods/commas, but I believe those shift too.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #28 on: April 26, 2024, 10:48:53 pm »
I scoped G1 and it had many more pulses than G2 and G3, so I moved forward with replacing the chip.

The good news: the new chips seems to have fixed it. Can’t say for sure, but I didn’t see digit twelve have the lit segment. I wanted to see if it displayed AUTO TRIG or AUTOOTRIG (somewhat of a true test), tried pushing the menu options, accidentally got it in count mode where it was counting pulses, so I turned off power and repowered it to get the default stage.

The bad news: when it repowered, the display may have come on for a split second (I wasn’t staring at it) and vanished. LEDs light and got a brief moment I saw a faint sign of life on the display where it showed something.

I didn’t move anything, nothing was near it to short, etc… Just a simple repower.

FL1 and FL2 measure 5.8v DC, so I’m assuming the supply blew, but don’t know why.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #29 on: April 27, 2024, 09:31:56 pm »
Look if the VFD filament wires are glowing dim orange. Recheck your soldering, especially to the (filament) pins which are on the far outside ends of the tube.
I'm not clear what it is doing now.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #30 on: April 27, 2024, 11:49:09 pm »
I assume you mean the wires behind the glass?

Nothing seems to glow even faintly. Tomorrow I’ll reflow the pins, but I tend to go heavy with the solder regardless. A quick look earlier didn’t reveal any indication of bad solder or lack of, however, anything is possible.

Odd it was working and nothing more than a power recycle resulted in the display not lighting.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #31 on: April 28, 2024, 01:58:50 am »
With power off, I would measure ohms and look for a short, on the HV rail and around the new 518 IC. If it bagged and is shorted, it would overload the DC-DC converter and possible its supplying rail. It might be why you see no filaments, although you said you measure 5.8VDC. I thought the DC-DC output AC to the filaments?

Once I had an Aliexpress special IC, lasted about 1 minute and cratered after a power down/up sequence. Thanks liaoxiyuan/SICSTOCK store for that waste of time.

The HV518 is susceptible to latch-up, they actually have a power up sequence in Sect. 3.2; The 5V rail must be the first to come up and last to go down, wrt VPP. I put a Schottky diode between the two rails to do this, but not something to do wrong and maybe your rail capacitance is low value. It's just a strange happening.

Worst case the VFD has an internal low resistance or short, since the 518 is push-pull output, it could roast if two outputs arm wrestle each other. Does that make sense? Any solder bridge between two outputs could do that as well.
You can check ohms at the display pins, as if adjacent on the 518.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #32 on: April 28, 2024, 02:11:46 pm »
Quote
I thought the DC-DC output AC to the filaments?

At the time I only had schematics handy and saw it's a "DC-DC" convertor, so I assumed it was DC out, however, I should have included that I measured the AC as well, it was approximately 600mV; but I measured this with respect to ground and maybe needed to measure across.

The datasheet specifies pins 5 and 7 are AC, but doesn't give a voltage. The schematic shows FL1 and FL2 are on pins 4 and 6, so that's a bit confusing.

Later today or tomorrow (hopefully) I'll take the steps you suggested. Usually I start with the obvious such as do I have voltage(s). My personal rule is first figure out what happened between when something worked and when it didn't. Since the only anomaly in that process was me replacing the chip, seems the obvious route is to start with soldering issues.

My other thought is the replacement IC takes more current and damaged the convertor. My somewhat focus is on the fact the display was working until I recycled power. As you suggested, a power up sequence failure, shorted pins, etc... so I'll look and report what I find.

Update: just for reference, this is the chip I ordered and installed    HV518PJ-G (DigiKey part number: HV518PJ-G-ND)

« Last Edit: April 28, 2024, 02:18:47 pm by bostonman »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #33 on: April 29, 2024, 01:04:10 am »
All the solder connections on the VFD looked good, however, I re-flowed them anyway including the ribbon cable.

FL1 and FL2 to ground is high impedance, 9.7ohms across the two pins FL pins, and approximately 1.3ohms across the two FL pins when the ribbon cable is connected to the unit. All other pins on the VFD to ground was (I believe) 4xx k. Not sure if these are good, but seems correct based on the FL lines being AC (most likely this is the resistance of the output transformer).

I found a previous message that states the DC/DC pins are off by one which makes sense when compared to the schematic.

Tomorrow I'll measure the SEG lines to see if they are going to approx. 38V, but what should the AC voltage be?

As for measuring the new HV518, obviously the VFD needs to be removed again, and this eliminates measuring voltages under a load since the VFD isn't connected. The other thing I'm keeping in mind is the start up sequence.

 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #34 on: April 29, 2024, 02:43:37 am »
I would check the VFD's getter has not gone white due to air ingress in the tube.
A '518 output short to another or to ground would heat up the IC and possibly damage it. Look for any solder bridges or just beep it out checking continuity. You can do this with the display in, also ohmmeter the pins to ground, look for shorts.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #35 on: April 30, 2024, 02:12:36 am »
Doesn’t look cracked, so this is a good thing.

The DC-DC seems different than the schematic, but I’m not seeing 38v.

The G lines and segment lines are all DC sitting around 5 and 3v. I did a quick test to see if anything is shorted, but I didn’t realize the IC pins are in a different order than the VFD pins, so I need to do more measuring.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #36 on: May 01, 2024, 12:58:52 am »
The DC-DC seems different than the schematic, but I’m not seeing 38v.

The G lines and segment lines are all DC sitting around 5 and 3v. I did a quick test to see if anything is shorted, but I didn’t realize the IC pins are in a different order than the VFD pins, so I need to do more measuring.

You should see +38VDC at the J1 (display-main PCB) connector pin 21 and pin 23 (GND).  The DC-DC is actually outputting more; approx +44VDC.  It's lowered at the J1 connector due to the 6.2V zener VR1.
ACV between J1 pin 22 and pin 24 should be approx. 5.5Vrms.
Of course measuring any G or segment VFD pin will just show you the average voltage due to the multiplexing.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #37 on: May 01, 2024, 02:32:50 am »
What would I see if the getter or glass was cracked? You mentioned white, but wanted to ask if it could show something else too.

I already tried measuring the 38v at J1 and the voltage wasn't present. For all I know the IC is pulling down the 38v and the DC/DC is fine, but who knows.

Hopefully tomorrow evening I'll have time to measure the IC pins via the VFD pins for shorts. I can't imagine a solder short between two pins since the display worked for a short time, but I was the only anomaly between the time it worked and the time it didn't, so I'm not ruling out a solder short.

On another note, I thought maybe I was onto something because yesterday I realized the fan wasn't spinning. Seems the fan has trouble starting without a little manual push, however, initially I thought maybe something was wrong in the AC/DC so some excitement began building. If the display ever works again, looks like the unit will need a new fan.

 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #38 on: May 01, 2024, 03:22:38 pm »
If the DC-DC converter is overloaded, it will get hot. See if the heatsink is a finger burner or not. It would also load up the supplying +5V rail.
HP p/n 0950-2325, ERG E2577VF is just a TDK CD-1848P rated 5.2Vrms 120mA, 43VDC 38.6mA (similar also used in HP-3458a I believe).

If air gets in a tube, the Getter turns white and can flake off inside. The filaments will draw a ton of current because they don't make it to their hot resistance, they stay cold.
I mentioned that possibility as a reason for the DC-DC converter getting overloaded. Otherwise it's a soldering fault or the 518 is bagged, possibly due to a defective (shorted) VFD.

It's not easy fixing this, takes a lot of patience. Hang in there.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #39 on: May 03, 2024, 02:34:31 am »
Earlier I took measurements to see if I could find any shorted pins.

I made a few errors due to not realizing pins 21-24 weren't in sequential order with the SEG lines, so I measured DP2 to SEG-E (by way of measuring the VFD pins directly) thinking the pins on the IC were next to each other.. By the time I realized this error, it was late and had to call it a day.

From the measurements I took, none of the pins were shorted, however, one odd thing I noticed was SEG-COM wasn't connected to ground (unless I'm wrong, I've been using chassis ground as it appears ground is connected to chassis ground).

Maybe SEG-COM is floating, but thought to mention it.

After measuring for shorts, I powered the unit and took scope measurements (see attached - file names are according to which pin was measured). Upon powering it, I checked the heatsink (again) and it was fairly cool. After keeping the unit powered for about fifteen-minutes (the longest I've kept it powered since the display stopped working) while taking scope measurements, I turned off the unit and touched the heatsink. This time it burned the back of my index finger; believe this indicates something is wrong.

The 5v measures about 4.8v I believe, however, whichever the voltage is falls within the range in the service manual.

Update: I didn't think to include my next step(s). Looking at the schematic tells me possibly one of the two capacitors are bad, however, looking at the board, I don't see an electrolytic; nor do I see the same components on the board as the schematic.

I'll check the resistance to ground before the diode, but it's looking like I may need to remove the VFD again.
« Last Edit: May 03, 2024, 03:27:25 am by bostonman »
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #40 on: May 03, 2024, 06:01:42 am »
There's a number of issues I see....

First of all, I believe SEG-COM is the comma segment; not a ground.  Your 'scope ground clip should be on J1 pin 23 (or any point marked DCOM).   The chassis is probably connected to DCOM but I can't find that explicitly in the schematics.

Tantalum caps are known to short.  Given that the 5V supply is 4.8V, check C62 (at pin 1 of the DC-DC) but that could be any other tantalum elsewhere too.  The 38V waveform seems to be loaded down (to ~13V). Check C129 and C243** (at pin 3* of the DC-DC). This is the likely problem (of all segments off) and yet there isn't any activity on G5 or SEG-A either (may be multiple problems here).

Clearly there is something wrong with VFDSEN as it seems to show activity but I would expect the waveform low levels be close to ground (at mid screen). Seems a bit too noisy... May not have a good ground connection on the probe.
 
VFDSDOUT and VFDSDIN similarly never make it to ground (at mid screen).  It's hard to tell if VFDSDIN is showing data but has a bad ground or is just induced noise (from adjacent traces) on line that's stuck high.  If it's the latter, then I'd expect all segments be on.  Probably a bad probe ground.

FL1, FL2 appear to be above ground by 6V as expected due to VR2 zener (at pin 5* of the DC-DC) and are 5V peak-to-peak (if you ignore the weird spike to ground).  I guess that's a pass-able waveform for a center-tapped transformer secondary.
 
 
*: pin marked incorrectly in schematic vs. DC-DC datasheet.
**: if your board is the 60024 PCB version on page 53 vs without on the 60014 PCB version on page 22.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #41 on: May 03, 2024, 01:16:22 pm »
I agree about things seeming odd and the possibility of multiple issues.

Upon soldering the new IC, I should have measured all the pins to check for shorts. The soldering was done under a microscope and I was confident bridges didn't occur, but anything is possible. With J leads, my limited experience has always been more about trying to get solder between the pin and the pad (which occurred several times while soldering this chip).

Not having activity on the G line (I measured a random G line just to show the signal) I thought could be attributed to the low 38v. Due to the DC/DC getting hot leads me to believe this is the issue to chase, but at some point this weekend (if I have time) I'll remove the board and look around the DC/DC board.

Am I wrong, or is the schematic different than the DC/DC board? If you look at the picture of my DC/DC board I included above, it shows four resistors (or a combination of resistors and inductors) but the schematic shows three (if you include whatever E4 is), also my board has three diodes, and the schematic shows two.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #42 on: May 03, 2024, 03:09:13 pm »
Am I wrong, or is the schematic different than the DC/DC board? If you look at the picture of my DC/DC board I included above, it shows four resistors (or a combination of resistors and inductors) but the schematic shows three (if you include whatever E4 is), also my board has three diodes, and the schematic shows two.

The schematic doesn't speak to the components on the DC/DC PCB itself; it's a black box figuratively and literally with the 0950-2325 label below it.  The parts on the schematic are on the green PCB surrounding the DC/DC; they're silkscreened: VR2, VR1, C62, E4, L3, ...      E4 is a SMT ferrite bead.

What's also interesting is the E2577VF part that I thought was the OEM has a different pinout to your CD 1848P TDK part (see "B. Positive voltage output type" on pg 2) which matches the schematic pin numbering.  Nevermind what I said earlier about off-by-one pin numbering.


Not having activity on the G line (I measured a random G line just to show the signal) I thought could be attributed to the low 38v. Due to the DC/DC getting hot leads me to believe this is the issue to chase, but at some point this weekend (if I have time) I'll remove the board and look around the DC/DC board.

Regardless of the low 38V supply, if there is no G-line activity that tells me that serial info isn't being received or gated to the output via enable pin.  If your probing was correct, then the TTL levels are off; they never get to ground (just flapping between 3-5V)
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #43 on: May 03, 2024, 05:04:27 pm »
In case I didn’t specify, or specify correctly, all my measurements were to chassis ground.

Next time I’ll try a different ground; and shorter.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #44 on: May 04, 2024, 12:47:27 am »
Just measure ohms/continuity to confirm your measurement ground is correct, from the BNC's, the chassis, the PC board ground etc.

If the scope traces are right then I think the new '518 has shorted. Any output shorted to another, the shields etc. the IC would be overloaded. Or I wonder if the new '518 got zapped with ESD somewhere?
I'd measure diode-test/ohms from each output to adjacent one, to GND to +HV etc. to fish for a problem. You can do that at the VFD pins.

Usually I cut the pins to take out a PLCC instead of hot air, it's easier on a board. If you replace the '518 you could leave the display out to make sure things work to it.
Tiny chance the VFD has an internal short - but you would see it with an ohmmeter right now.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #45 on: May 06, 2024, 01:24:59 pm »
I may have been confused about the CD-1848P. I thought this was a DC/DC converter, however, yesterday I removed the board and it looks like a transformer. Underneath the silscreen reads 'trans', I saw magnetic wire, and it looked like multiple windings.

The transistor (NEC C2334) that gets hot (next to the CD-1848P) had two pins with low resistance, but, after removing the board from the main PCB, the transistor measures normal; I'm assuming the low resistance is due to connections through the transformer windings that are made when the board is soldered onto the main PCB.

Yesterday I measured all the pins on the new 518 and didn't find any shorts. At this point I plan to remove this chip, power the unit, and see if the 34v 38V returns to normal.

Seems hard to believe I got a bad batch of ICs, but I guess it's possible. I'm considering buying a socket rather than soldering another IC onto the board, but one capacitor may be in the way, and it's also a week of waiting for them to arrive from DigiKey.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #46 on: May 07, 2024, 02:23:06 am »
I successfully removed the new IC and now the +38V measured approximately 44V on the meter (it was approximately 13V with the 'new' IC. The higher voltage is probably due to it being unloaded.

Also, the transistor with the heat sink remained cool, however, it's not supplying any current at the moment, so I wasn't surprised.

After giving consideration to using a socket, I decided to order three (two for a backup) and shipped them three-day; along with the transistor that's on the heat sink. I planned to replace other parts too such as the smaller transistor since maybe it took some high current too, but decided to wait to see if I repair this unit.

At least now I'll (hopefully) have a socket in place and can remove the IC much easier if need be.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #47 on: May 07, 2024, 03:27:31 am »
Do confirm there is enough room (height) available as I believe it has to fit underneath the display.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #48 on: May 07, 2024, 02:30:53 pm »
Believe it or not, I realized the potential height issue a few hours after ordering the sockets.

I devoted time thinking whether using one was worth it, saw the surface mount capacitor was far enough away, and forgot the display was going on top.

Edit: let me rephrase that and say the desire to get the sockets delivered quickly overshadowed my thought process on whether they'd physically fit.
« Last Edit: May 07, 2024, 03:06:49 pm by bostonman »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #49 on: May 07, 2024, 04:36:47 pm »
The socket I ordered is: 8444-21A1-RK-TP

Looking at the IC, it has a maximum height of 0.18 with a normal of 0.172. The socket has a height of 0.181, however, without realizing, I ordered the one with location post which adds height; but I can cut them.

Providing the IC sits flush (or below) in the socket, looks like the socket is the same height based on maximum height of the IC.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #50 on: May 10, 2024, 12:59:49 am »
Well, the sockets arrived and I wasn't comfortable with the height. They appeared borderline (after cutting the positioning posts), and, inserting an IC to test whether the IC would extend even further would result in trying to yank it out without damaging it.

Rather than risking it, I soldered in the new chip even though I really wanted a socket.

After soldering, I measured pin-to-pin to check for solder shorts (didn't find any), and checked all solder connections with the board at an angle under the microscope.

An initial power up (without the display) showed the 38V was up around 44V (probably due to being unloaded), the G lines were clocking with nice pulses, SEG lines were clocking with nice square waves pulses, and nothing was getting warm at all.

Now the question is whether installing the display will result in the chip getting damaged again.

Keep in mind, none of the display pins were shorted to (chassis) ground, however, I'm also uncertain exactly how the display works to have an idea which pins could be susceptible to shorting or cause the IC to blow.

Any suggestions on what to look for before I solder the display? The time to solder it isn't necessarily an issue, it's more than I don't want to keep applying heat to all the pads. The IC pads have already taken a beating with hot air twice and soldering twice; not including having to reflow pins that didn't flow correctly to the pads.
« Last Edit: May 10, 2024, 03:32:45 am by bostonman »
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #51 on: May 10, 2024, 04:44:36 am »
Now the question is whether installing the display will result in the chip getting damaged again.

Keep in mind, none of the display pins were shorted to (chassis) ground, however, I'm also uncertain exactly how the display works to have an idea which pins could be susceptible to shorting or cause the IC to blow.

Any suggestions on what to look for before I solder the display? The time to solder it isn't necessarily an issue, it's more than I don't want to keep applying heat to all the pads. The IC pads have already taken a beating with hot air twice and soldering twice; not including having to reflow pins that didn't flow correctly to the pads.

With the display free, I'd confirm:
a. that there is some non-zero but non-open resistance between the filament pins, and
b. no continuity exists between a filament pin and any grid pin or segment pin or between grid and segment pins. 
   ie. MM in continuity mode, black lead on pin 1, scan with red lead from pin 2 onward.  Then, black on pin 2, scan with red lead from pin 3 onward. Etc.
       There should only be a small-ish resistance between 1 and 18 (filament pins).  All other pairs should be open.

If the above passes, you can quickly confirm that everything works by...
just spreading the display pins out a bit and place them into their PCB holes; holding the display down with an elastic or just your fingers.  There should be enough springiness to maintain a connection.

Then turning on the power for a few seconds to confirm that the segments light.

 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #52 on: May 10, 2024, 01:26:34 pm »
Thanks for the feedback, I'll make sure to take all those measurements.

Quote
Then turning on the power for a few seconds to confirm that the segments light.

Maybe I'm wrong because it was hard to see if every segment in every digit lit correctly, but after installing the first IC (the one that blew), the display seemed to work fine. It was re-powering that seemed to cause things to go south. Either way, all I can do is take initial measurements based on your feedback and keep my fingers crossed.

Most likely I'll have updates by late weekend.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #53 on: May 10, 2024, 05:17:10 pm »
We don't know why the new IC failed, so I'd grasp at a few straws.

While the VFD is out, I would rule out any shorts within the VFD. I don't have direct experience with this, nothing low enough resistance to blow the driver IC, so this could all be a goose chase.
I'm not sure an ohmmeter would find them. I'd use a higher voltage test, say 25V with 10k series resistor, probing between between each of the segments, and then the digits etc. Leave the filament out/off. Just looking for a mystery low resistance occurring above a few volts.

Inside the tube, spacings are very small. I have seen some contamination/ion migration between segments cause dim or fuzzy appearance due to aging. Another person thought their short was the fine PCB-like traces running along inside the back of the tube, where the bridge occurs. You can look at the VFD's backside to see what I am talking about.
So at least look for leakage (resistance) between VFD elements.

I power a VFD up on the bench, it's not too hard just need two power supplies and a few resistors. I rejuvenate them in two ways and have good success. The contamination/ion migration between grid and segments I have no idea what it is but am able to see and reverse it. If I can find my notes maybe spell it out for people to try. If the VFD's phosphor has aged or is burned, nothing can be done there but the VFD's emission can be helped I find.

Last straw to grasp, otherwise- as I mentioned a power cycle can make a '518 latch-up if the 5V rail comes up late or falls first during a power up/down per the datasheet. I think it's a separate 5V reg on the board. Maybe look at the 5V reg output cap and others, see if its value is OK.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #54 on: May 11, 2024, 05:21:10 pm »
I found this pic in my collection and somebody seems to have added socket pins  :-//
although the VFD pins are flat Kovar with some oddball plating to make solder stick.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #55 on: May 11, 2024, 09:21:18 pm »
Wow, those pins look useful. Maybe I’ll try those around my fifth IC replacement :)

I’m about to go take some measurements and maybe solder the VFD back in afterwards.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #56 on: May 11, 2024, 11:46:38 pm »
Update.

I measured all the pins on the VFD. Filament pins were about 9.3ohms, 2- to 36 were shorted (but the traces have them tied together), all other pins were high impedance.

Soldered the VFD, soldered a fly lead to the 38V, connected the scope to it, powered, and the display worked (all segments seemed to be correct). The 38V was a solid 38V.

Left it powered for about 90s, turned off power, left it unpowered for about 3-5s, turned on power, no display, and the 38V was about 12V. Several power recycles and still no display.

I’m so confused

Edit: wanted to add that the counter was free running meaning I didn't have anything connected to the BNCs, so it was displaying random numbers, however, if this matters, it didn't seem it wanted to display twelve-digits and was maybe ten instead. I noticed this with the original (bad) chip and thought maybe it auto selects the number of digits based on frequency or whatever.
« Last Edit: May 12, 2024, 02:27:46 pm by bostonman »
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #57 on: May 13, 2024, 12:52:57 am »
It seems to be eating HV518PJ's  :( I can hear your screams
Sounds a bit like what happened here: https://www.eevblog.com/forum/repair/hp-agilent-53181a-display-error/ shorted segments inside the VFD. I would PM the guy to confirm.
Originally I saw that you had SEG-C lit when it should have been off (when showing 2) and thought it was the '518. 

Is the VFD shorting after being warmed up a bit, or is the IC latching-up, or getting very hot (due to VFD short) and then it dies? That's why I advocated testing the VFD out of circuit. It could be a very weird Itron/Noritake failure mode.

edit: notice the tight spacings on the VFD internal bus from a similar (not the same part number) display
« Last Edit: May 13, 2024, 01:00:14 am by floobydust »
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #58 on: May 13, 2024, 01:22:56 am »
I measured all the pins on the VFD. Filament pins were about 9.3ohms,

Good, as expected.    I have a large format clock display with a 6R filament.

Quote
2- to 36 were shorted (but the traces have them tied together), all other pins were high impedance.

Shorted?  I don't understand.    Oh, you mean only the pin2-to-pin36 pair was shorted.  That may be acceptable since pin 2 and pin 17 aren't normally connected; ie. may serve an internal structural need.

Other than between pins 1 and 18, and ignoring pins 2 and 17, all other pin pairs should be open (have high resistance when the display is out-of-circuit).   If not, this is what's killing the driver chip.

You can test the display out of circuit with two DC power supplies (with common ground); +5V and +30V.   Set current limit of both supplies to 100mA max.
+5V ---47R filament resistor--- pin1,  then pin 18 to ground.
+30V ---10K grid resistor--- Gn pin  (where n=1 thru 12)
+30V ---10K segment resistor--- SEG_m pin  (where m=A thru Q, DP1, DP2, COMM)
All three must be connected for one segment of one digit to light. 
Iterate for all segments of all digits to find the short (the one that needs more current to light).
You shouldn't need less than 10K (to force higher current) for the segments to light but if you do find a short, may be intentionally driving 100mA through it (via 47R 330R vs 10K) may fix it. <== speculation on my part.
The display is likely FUBAR.

See this on how to test VFDs.
« Last Edit: May 13, 2024, 05:50:36 am by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #59 on: May 13, 2024, 02:21:48 am »
Quote
Shorted?  I don't understand.    Oh, you mean only the pin2-to-pin36 pair was shorted.  That may be acceptable since pin 2 and pin 17 aren't normally connected; ie. may serve an internal structural need.

I used the word short (which they were), but it could have been a bit misleading. The follow up somewhat clarified they are suppose to be "shorted" since I followed the trace showing they are connected electrically.

I'm only thinking out loud and NOT rejecting testing the VFD and also elaborating a bit. Upon first powering after installing the new chip and the display worked, the DC/DC was not getting warm at all. It was actually cold. If the VFD had a short, rhetorically asking, why would the 518 continue working and the DC/DC staying cold? Also, this display worked fine (ignoring the bad segments) until I began installing the replacement 518's somewhat implying the VFD isn't shorted and something funky is going on with these new chips.

Upon repowering, it's not even the display shows something for a brief moment, it just doesn't come on at all, the DC/DC begins getting warm, and the +38V never goes beyond approx. 12V. I did notice the +38V takes time to discharge when I turned off the unit after the first power up and wonder if this is an issue.

The failure mode in my opinion indicates the IC is latching, but, if it's latching, why does it power fine the first time (unless the +38V taking long to discharge after the first power up/down is a hint).

Quote
+5V ---47R filament resistor--- pin1,  then pin 18 to ground.
+30V ---10K grid resistor--- Gn pin  (where n=1 thru 12)
+30V ---10K segment resistor--- SEG_m pin  (where m=A thru Q, DP1, DP2, COMM)

I'm scared of blowing up the VFD, so I want to confirm.

Connect +5V to to pin 1 with a 47ohm resistor in series. Pin 18 gets connected directly to ground?

+30V to Gn pin with a 10k resistor in series - obviously to pin 1 - 12 one at a time. The ground on the +30V channel also gets connected to pin 18 (or the ground banana jack on channel 1 that's being used for +5V)?

Now I take the same 10k that's connected to (let's just say G1) and also touch it to segment A (then B, C, etc...) at the same time? I know this is the answer because segment A can't light until a G line activates the proper digit, but I want to make sure. One wrong move with the VFD and I'm really in trouble.

On a side note, I may have only one IC left. So one more failure, and I'll be ordering more and/or repairing pads that eventually lift.

 

Online PCB.Wiz

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Re: Vacuum Fluorescent Display Driver
« Reply #60 on: May 13, 2024, 02:48:26 am »
Left it powered for about 90s, turned off power, left it unpowered for about 3-5s, turned on power, no display, and the 38V was about 12V. Several power recycles and still no display.
Bummer !
Could be turn off, or turn on, or turn on into a partially charged load, that kills things.
It survives initial ON, so a cold start appears to be OK ?
 

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Re: Vacuum Fluorescent Display Driver
« Reply #61 on: May 13, 2024, 02:54:08 am »
I did notice the +38V takes time to discharge when I turned off the unit after the first power up and wonder if this is an issue.
Maybe it does not like re-powering into a charged load ?
You could add some overvoltage clamps and ab active discharge, that is off with LV rail present, put pulls HV down when LV is gone ?

On a side note, I may have only one IC left. So one more failure, and I'll be ordering more and/or repairing pads that eventually lift.
Maybe time to fit the socket ?
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #62 on: May 13, 2024, 02:56:18 am »
I was about to update my previous post, but saw another reply.

Quote
Bummer !
Could be turn off, or turn on, or turn on into a partially charged load, that kills things.
It survives initial ON, so a cold start appears to be OK ?

Just to clarify: install new chip, power on, everything works, power off, power on, and display no longer works.

I'm re-reading the datasheet about the power down sequence. It states Vpp should be powered down, all inputs, and then Vdd. Now if the +38V (Vpp) is slowly discharging, maybe it remains high too long and the inputs along with Vdd have already turned off.

Maybe this is the issue and I need to make the +38V turn off quicker?
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #63 on: May 13, 2024, 05:50:04 am »
Quote
Shorted?  I don't understand.    Oh, you mean only the pin2-to-pin36 pair was shorted.  That may be acceptable since pin 2 and pin 17 aren't normally connected; ie. may serve an internal structural need.

I used the word short (which they were), but it could have been a bit misleading. The follow up somewhat clarified they are suppose to be "shorted" since I followed the trace showing they are connected electrically.

I see. I think you should ignore pins 2 and 17 for any testing purposes since the schematic has them unconnected.

Quote
I'm only thinking out loud and NOT rejecting testing the VFD and also elaborating a bit. Upon first powering after installing the new chip and the display worked, the DC/DC was not getting warm at all. It was actually cold. If the VFD had a short, rhetorically asking, why would the 518 continue working and the DC/DC staying cold? Also, this display worked fine (ignoring the bad segments) until I began installing the replacement 518's somewhat implying the VFD isn't shorted and something funky is going on with these new chips.


I agree that on first power up, given that the display is working, does seem to confirm that the filament nor the grid and segments aren't too much of a load on the driver chip.   However, testing the display out-of-circuit will confirm that there's no excessive draw. ie. that a 47R filament resistor, 10K segment and 10K grid resistors is enough to light a segment. And to iterate over all grids and segments to confirm there isn't a bad one in the bunch.

Quote
Upon repowering, it's not even the display shows something for a brief moment, it just doesn't come on at all, the DC/DC begins getting warm, and the +38V never goes beyond approx. 12V. I did notice the +38V takes time to discharge when I turned off the unit after the first power up and wonder if this is an issue.

The failure mode in my opinion indicates the IC is latching, but, if it's latching, why does it power fine the first time (unless the +38V taking long to discharge after the first power up/down is a hint).


All I could find in the HV518 datasheet (page 11, section 3.2) is that Vdd (+5V) should be powered before Vpp (+38V) and Vpp should be powered-down before Vdd.  Maybe by adding a 10K resistor between Vpp and ground will provide a minimal load (3.8mA) to drop that rail before the bulk caps drop the +5V rail.

Quote
Quote
+5V ---47R filament resistor--- pin1,  then pin 18 to ground.
+30V ---10K grid resistor--- Gn pin  (where n=1 thru 12)
+30V ---10K segment resistor--- SEG_m pin  (where m=A thru Q, DP1, DP2, COMM)

I'm scared of blowing up the VFD, so I want to confirm.

Connect +5V to to pin 1 with a 47ohm resistor in series. Pin 18 gets connected directly to ground?

Yes.  See attached diagram.
Quote

+30V to Gn pin with a 10k resistor in series - obviously to pin 1 - 12 one at a time. The ground on the +30V channel also gets connected to pin 18 (or the ground banana jack on channel 1 that's being used for +5V)?

Yes.  See attached diagram.
Quote

Now I take the same 10k that's connected to (let's just say G1) and also touch it to segment A (then B, C, etc...) at the same time? I know this is the answer because segment A can't light until a G line activates the proper digit, but I want to make sure. One wrong move with the VFD and I'm really in trouble.

A different 10K (from +30V) to the segment of choice.  See attached diagram.

Note, the grid and segment voltages don't have to be exact; just not more than +38V.  It should light with anything north of 20V.
Just make sure that the filament doesn't see more than 100mA (hence the 47R) and NEVER shows a red glow.  And if you do find a segment or grid that won't light with a 10K resistor in series, then lower it no less than 330R (ie. no more than allowing 100mA on any grid or segment).

The 47R and 10K resistors should limit the current fine but if you have constant current capable power supplies, set them no more than 100mA just in case of oopses.
Quote

On a side note, I may have only one IC left. So one more failure, and I'll be ordering more and/or repairing pads that eventually lift.

It sucks to find out the hard way.
« Last Edit: May 13, 2024, 06:10:34 am by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #64 on: May 13, 2024, 01:40:04 pm »
Quote
A different 10K (from +30V) to the segment of choice.  See attached diagram.

Rough day of yardwork and lack of sleep, I should have realized a different 10k resistor.

Thankfully I didn't use too much solder to reinstall the VFD (planned to reflow after confirming the unit was fixed), so unsoldering should be easier.

I fully agree that testing the VFD is important to ruling out at least one anomaly. If I were giving advice to someone, asking them to rule out other pieces in the puzzle would be my first approach. I'll place an amp meter in series to note the current for each segment.

As for the slow discharge of the +38V, it's not just slow, I mean (at 10V/div) the DC line crawls. I didn't measure the time, but I'd estimate a good ten-seconds and it was still >20V.

Before I remove the VFD, I'll put probe on the 5V and +38V (now approx. 12V) and check the timing during power down. I'm guessing the 5V will reach 0V long before, however, due to the chip being blown, the measurement will be skewed. It's unfortunate the datasheet doesn't specify what happens if the 'shut down' sequence isn't followed; maybe at some point tech support will help, but let me confirm the VFD first.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #65 on: May 14, 2024, 02:24:40 am »
Attached are two scope measurements - file names are named accordingly. Edit: note that Vcc was taken from pin 10 of the ribbon cable (main board end).

Remember the +38V is now being loaded by the damaged IC and measuring approx. +12V (for those who are new to this thread)

As I expected, the discharge rate on the +38V is quite fast since (I'm assuming) the chip is low impedance and sinking the voltage(s) quicker.

It's unfortunate I didn't measure the turn off rate of the +38V when the chip was working, but I remember it being quite slow. If the 5V turns off at the same rate regardless if the chip is good or bad, then a good possibility is the +38V turns off before thus going against the datasheet suggested turn-off sequence.
« Last Edit: May 14, 2024, 02:29:49 am by bostonman »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #66 on: May 14, 2024, 03:40:47 pm »
Just wanted to clarify one thing: I still plan to test the VFD out of circuit. As I previously mentioned, before removing it, I wanted to measure the +38V discharge rate.

Hope nobody thought I was ignoring the suggestions of testing it out of circuit.

 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #67 on: May 14, 2024, 11:32:50 pm »
What V/div? I see Ch. 2 at 1V/div for a noisy 5V, and Ch. 3 at 2V/div for 11V.

For testing a VFD, I'll use a 10Ω 5W resistor in series with a bench PSU to power the filament. This is just to limit inrush current and give coverage for an ill-behaved power supply doing some overshoot, some are terrible at power on/off or going in/out of CC mode if you use it, they surge. The tube is around 10Ω cold and 40Ω hot. To rejuvenate an old tube, I will slowly ramp up to 2x filament voltage to burn off oxides/contaminants, hold for 30 seconds and then let the VFD cool down for a few minutes. Do not bang or move the VFD at all when doing this.

The HV518PJ appears different (BIDFET) than the SN75518 (Bi-CMOS) output pin uses a BJT vs MOSFET to source current.
My money is on the tube having a short inside. Or a 5V rail cap is low value, it looks noisy.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #68 on: May 15, 2024, 01:11:46 am »
Quote
What V/div? I see Ch. 2 at 1V/div for a noisy 5V, and Ch. 3 at 2V/div for 11V.

Yes, 1V/div and 2V/div.

Quote
The HV518PJ appears different (BIDFET) than the SN75518 (Bi-CMOS) output pin uses a BJT vs MOSFET to source current.
My money is on the tube having a short inside. Or a 5V rail cap is low value, it looks noisy.

The difference in ICs wasn't something I analyzed, but my thought was the ICs are different and need a different turn-off sequence than what the unit was designed for.

Fast forwarding and assuming the VFD isn't shorted (I'll update once I get data), I've been thinking about the next step and how to somehow test the IC with the display rather than solder it in. The time and money for new ICs isn't an issue, it's more I'm worried about pads lifting due to excess heat. Also, since I don't have a clue why the ICs are failing, then it's a waste to continue soldering new ones in.

I probably know the answer to this, but is the 10ohm value essential? I have 15ohms, 50W and 20ohms, 20W I can put in parallel.

Most likely the 15ohms is fine (50W is overkill, but all I have in high wattage), but wanted to double check.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #69 on: May 15, 2024, 01:59:23 am »
Given that the DC-AC (TDK CD-1848P) filament output spec is 5.2V@120mA, a 1W filament resistor would be overkill.
The higher the resistor value, the dimmer the display. If high enough, you won't see anything at all.
However, given 40R when operating (therefore, 5V/40R=125mA), adding 15R won't likely lower the current such that you won't see anything (5V/(40+15)R = 90mA).
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #70 on: May 15, 2024, 02:57:07 am »
I mentioned the resistor I used 10Ω 5W because the PSU voltage has to be set higher and it's what I had it my junkbox. Just being lazy about the power rating including at rejuvenating overvoltaging it.
The 47Ω 2W you'd need almost 13V to get ~6V on the filament and a person might fret over that. 15Ω 1/2W I estimate 8V to get ~6V on the filament.
Also, I think 38V for HV is high because on the bench the VFD is not being multiplexed, so 25-30V is plenty bright for static drive. But the 10kΩ will limit current too.

I've had a couple weird effects happen with these VFD's- they don't behave like even a triode.
Even the filament glow along only half its length, ending at a digit grid with +HV on it. Segments can have an odd blue glow that goes away after a few seconds, with the anode left open. That glow never returns so I assume some contaminant between segments and something else, migrated. The segments were no longer fuzzy.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #71 on: May 15, 2024, 03:27:40 am »
On the power sequencing, where is the trace of the 38V rail?
The DC-DC converter is powered from SW+5V which is switched by Q12/C159, so it should fall fast. But C129 is huge 470uF 63V makes me wonder why.
The 5V rail for the '518, as you mentioned schematic shows U7 LM317 but it's probably a 7805? I'm not sure where the feed is from.
I'm just looking for what could scuttle the power supply sequencing.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #72 on: May 15, 2024, 02:58:36 pm »
Quote
On the power sequencing, where is the trace of the 38V rail?

I assume you're referring to my two scope captures? Channel 2 is Vcc (the green) . It's set to 1V/div and channel 3 (purple) is the +38V (but it's only measuring +13V since the IC is in "failure mode"). The time per division is 20ms.

Not sure if you already knew all this, but wasn't sure which portions of the captures you were inquirin about.

Vcc was taken from pin 10 of the ribbon cable on the main board end (I'm assuming it's the same Vcc that feeds the HV518, but the VFD is connected at the moment so I can't be 100% certain). +38V was taken directly at the ribbon cable pin on the front panel end.

As previously mentioned, upon first installing the new IC and turning off power with the VFD installed, if I remember correctly, the +38V took a very long time to discharge. Unfortunately I wasn't measuring it with intent of checking the timing and/or comparing with the Vcc discharge rate.

Also, per my statement(s) of this slow discharge being a problem, the more I thought about it, upon first installing the new IC, I powered the unit without the VFD. The chip didn't blow thus possibly eliminating the power down sequence being an issue (unless it powers down differently without the VFD connected due to a different load). Maybe the VFD truly has a short, but still remain baffled why it would work fine until power is turned off and back on.

 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #73 on: May 15, 2024, 07:29:59 pm »
Rail sequencing is complicated. I was interested in the rail sequencing with a good IC, not with an overloaded DC-DC converter. So we don't know what the 38V rail normally does. The 38V rail cap C129 is huge.

Where does the 5V for the HV518 come from? There is a DPAK linear regulator but what provides the input?
If Q12 is shorted then SW+5V could stay up too long keeping the 38V rail up after the 5V rail is down, on power down.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #74 on: May 16, 2024, 01:06:34 am »
Quote
I was interested in the rail sequencing with a good IC, not with an overloaded DC-DC converter. So we don't know what the 38V rail normally does.

You're absolutely correct and wish I had taken such a capture. The scope captures with the "blown" IC was for general reference.

Quote
If Q12 is shorted then SW+5V could stay up too long keeping the 38V rail up after the 5V rail is down, on power down.

Another good piece of advice. Keep in mind though, this unit worked fine with the original IC (except segments were in correct) and seems to work fine with a new IC providing I don't turn off power. Not to say something didn't get damaged during my repairing.


Quote
Where does the 5V for the HV518 come from? There is a DPAK linear regulator but what provides the input?
If Q12 is shorted then SW+5V could stay up too long keeping the 38V rail up after the 5V rail is down, on power down.

I'm not quite sure the schematic makes sense to me. The front panel schematic (page 4) shows 5V into a LM317 and 5V out; that's obviously wrong. Page 53, pin 10, of the ribbon cable, shows Vcc and I believe this is the Vcc to the HV518. If I remember correctly, 5V net name gets converted to Vcc somewhere. Maybe I'm wrong and it's SW+5V goes through Q12 and is named Vcc. Edit: also I see there is a "power up" reset on page 21.

Maybe I'm wrong, and due to being uncertain about some things, I try to add as much detail in my replies to reduce any confusion.

Tomorrow, or possibly Friday, I'll remove the VFD and perform tests. I'm thinking of soldering twelve momentary push button switches with 10k resistors on a perf board to reduce constantly fiddling with clips on the leads of the VFD. Also, it will reduce time because I can power one segment, push each of the switches, and see the same segment on each digit before moving onto the next segment.

Earlier I was thinking ahead and wondering how I could perform testing on the IC without soldering it back onto the board. I thought about soldering the IC socket to a perf board, running jumper wires from the ribbon cable for the data line and other essential signals, but then I'm stuck wiring all the segments and G lines manually.

The advantage is I can blow IC chips without the need to solder, still a $8 loss each time, but better than soldering.

The only other thought was to layout a PCB with sockets for the VFD pins (or hope the pins make a connection on unsoldered holes). I'd still need to layout a PCB, cost, and wait for it to arrive.



« Last Edit: May 16, 2024, 01:16:24 am by bostonman »
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #75 on: May 16, 2024, 02:52:05 am »
"Agilent 53131A Universal Counter Component Level Information" 5989-6307EN Feb. 2007 pdf does list the Supertex HV518PJ for U2, same p/n 1820-5330 as the TI SN75518FN. I say the IC is a drop-in replacement but we'd need to compare an old (75518) schematic to see if changes were made. Only the 53131/132 use that DC-DC converter.
I can only find that C129 is way too big, the hold-time of that 38V rail is large and the display likely gets blanked fast on power-down, which decreases the current drain and lengthens the ramp down.

You have be careful C129 470uF 63V (on the main board) is totally discharged before plugging in the ribbon cable or it will zap the IC. That would kill it.

The schematic shows U7 LM317 jumpered IN-OUT, front panel board +5V coming in from J1-10, J1-20. Back to J6-10, J6-20 and a ferrite bead to SW+5V. So it's the same rail as the DC-DC converter power. The 38V will lag on power-on but any possible problem is power-off, it stays up too long.
The one VFDSDIN data line from main board U19 is powered from VCC (before the SW+5V switch) but I can't see that causing 518 latchup.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #76 on: May 16, 2024, 02:59:36 am »
Quote
he schematic shows U7 LM317 jumpered IN-OUT

I completely missed the jumper. All I saw was the LM317 and +5V. Think I may have just lost some credibility on here :(

Quote
The schematic shows U7 LM317 jumpered IN-OUT, front panel board +5V coming in from J1-10, J1-20. Back to J6-10, J6-20 and a ferrite bead to SW+5V. So it's the same rail as the DC-DC converter power. The 38V will lag on power-on but any possible problem is power-off, it stays up too long.

Wow, you really managed to follow this well. I hate following drawings in PDF format because after jumping to another page, I forget what the other page(s) looked like. I tried printing, but for whatever reason, it printed very light; else I would have considered having them printed on C size paper at Staples.

« Last Edit: May 16, 2024, 03:01:50 am by bostonman »
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #77 on: May 16, 2024, 03:01:38 am »
I looked again (smallest schematic dot ever) and C129 should discharge to 6V with the zener VR2 and R29 31k6 load.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #78 on: May 17, 2024, 02:41:43 am »
Earlier I removed The VFD, and, while at it, removed the IC since it's bad.

Unfortunately the IC pads don't look like they have another de-solder attempt. One pad lifted, but thankfully it's an unused pin. A few other pads appear to be on the edge of lifting or have an extremely minor bow.

Ironically I also have one new IC remaining (although I don't have any objections to ordering more).

This weekend I'll piece together momentary push-button switches (twelve - one for each G line), resistors, etc... and begin testing the VFD. Also, I'll measure the current draw on each SEG line and filament. Maybe I'll grab a second meter and also measure the current on the G lines too.

I'll update once I get more data.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #79 on: May 19, 2024, 02:49:01 pm »
Yesterday I performed the bench testing.

Attached are the results. I had 64ohm 2W resistors, so I used four in parallel to make it 16ohms and 7V on the power supply gave the filament voltage 5V (approx. 132mA).

Also, I used 27V on the SEG lines rather than 30V (figured it's best to use lower and need to increase rather than go too high - but 27V seemed plenty).

From what I can tell, everything lit and nothing is shorted. SEG P and SEG Q doesn't light anything until it gets to G12. I don't see anything else that needs to be lit, so I believe SEG P and SEG Q is normal.

I'm working on a drawing to show which SEG lines are associated with lighting each segment.

Basically for the figure 8 pattern, A-F goes clockwise with A being the top horizontal segment and G and H are the middle two horizontal lines with G being the left middle and H being the right middle.

The middle stuff is:

I - Vertical top middle
J - Vertical bottom middle
K - Diagonal top right
M - Diagonal bottom right
N - Diagonal bottom left
L - Diagonal top left

The rest of the segments (Ch1, Freq, etc...) are listed in my spreadsheet.

 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #80 on: May 19, 2024, 09:25:17 pm »
132mA filament current is only just a little higher then DC-DC spec of 120mA but it'll be fine for a quick test.

Also good to see the segment currents are <<1mA using a 10K current limiting resistor.   Q1: Are the currents in the table the sum of both Gn and SEG_m currents?  Or just the segment current alone?  See attached; ie. A1+A2 or just A1?

Q2: If it's just A1, are the A2 currents similarly <<1mA using a 10K current limiting resistor? Don't bother with a detailed table of every combination. Just report back the largest draw.

Q3: If the Gn and SEG_m currents are <<1mA, change the grid and segment current limiting resistors from 10K to 1K and confirm that the Gn and SEG_m currents are now <<25mA.  If any Gn or SEG_m current >25mA that's going to burn-out the HV518 driver.

Q4: If the Gn and SEG_m currents are <<20mA (using 1K resistors) then switch to 100R for the grid and segment resistors and report the largest current drawn (from Gn (A2) and SEG_m (A1)).  The object here is to sneak-up on the 25mA limit. If we go over, then the display is bad because a good one will never demand more than that from a driver output pin.  Normally, there is no resistor in series with a driver output.

If the Gn and SEG_m currents are <25mA using the 100R series resistors, then finally remove the grid and segment resistors completely (each connected directly to the +27VDC supply).  Q5: What's the largest current drawn from Gn (A2) and SEG_m (A1)?
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #81 on: May 19, 2024, 11:25:18 pm »
I will set aside time sometime this week to perform the measurements you're asking. Unfortunately it may not be until late in the week or maybe the weekend.

The numbers I provided were the SEG current only. Gx was connected directly to 10k resistors (I'd clip lead the 27V to each resistor) without a current meter - I didn't have a spare current meter handy.

Attached is the 38V and 5V (38V was taken from a via next to E4 and 5V from pin 10 on the ribbon cable on the main PCB side).

Also, this is without the VFD installed and the VFD IC removed. Per what I remember seeing when the display was working with the new IC (until I turned off power and turned on power only to find out the IC blew), the +38V discharges slowly.

The scope capture shows after 12s the voltage is still 10V while Vcc has long since turned off.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #82 on: May 20, 2024, 01:57:49 am »
I pieced together the segment locations (see attached) that I noted while bench testing the VFD.

Maybe this is applicable to other VFDs, but thought to piece this together for anyone interested.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #83 on: May 20, 2024, 01:24:34 pm »
Quote
Q2: If it's just A1, are the A2 currents similarly <<1mA using a 10K current limiting resistor? Don't bother with a detailed table of every combination. Just report back the largest draw.

Since I don't quite understand how the VFD works (although I've watched videos), I wanted some clarification: Do I need to measure all the Gn lines for every SEG combination (i.e. SEG-A with G1>G12, SEG-B with G1>G12, etc...), or can I just power a single SEG line, measure all twelve G lines, and those numbers will be the same for all SEG lines?

Not sure what I have for resistors, but maybe I'll use a decade (resistor?) box. If I remember correctly, they are 2W resistors, so that may make things easier.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #84 on: May 20, 2024, 07:20:38 pm »
The intent is to confirm that at NO TIME does any Gn x SEG_m combination, that EITHER current (A1 or A2) draw is >25mA. 
First using a pair of 10K, then a pair of 1K, then a pair of 100R resistors, then no series resistors. 
1/2 watts should be fine given the brief on-time to check currents.

So, I envision two ammeters. One lead from each ammeter starts from the +27V supply.  Then the other lead (with series resistor)... first on G1, second ammeter lead (with series resistor) on SEG_A, then move second to SEG_B, .... SEG_P, SEG_DP1, SEG_DP2, SEG_COM.  Each time lookup up briefly to check if the Gn or the SEG_m currents exceed 25mA.  Don't bother documenting each reading; just note if it comes close to 25mA.
Then switch first ammeter lead to G2, second on SEG_A....SEG_COM.  Then switch first ammeter lead to G3, etc.
It should just be a couple of seconds each lookup at the ammeters to see.
I don't anticipate that the grid current will change much with each different segment lit.  I just want to be sure.
If you find that as you try every combination with 1K then every combo with 100R, and you're seeing >25mA currents then stop there; don't bother to go lower since the display is FUBAR.

If you only have one ammeter, then you'll have to do every combination of Gn x SEG_m twice for every series resistor combo; once with the ammeter monitoring the A1 current then monitoring the A2 current for 10K, then again for 1K, etc.

I don't also want to rush going from 10K series resistance directly to no resistor because it might zap something inside. Although, if any A1 or A2 current >25mA that pretty much guarantees that the display is a driver chip killer so it'll be useless anyway.

Again, we're just ruling out that the display will never draw >25mA from any A1 or A2 current when no resistor is in series.

-------
VFDs are low-voltage vacuum tubes. 
You must keep the filament (heater) constantly lit by supplying the recommended voltage that does not allow it to exceed its max. current. 
Then also supply a positive voltage (with respect to the filament) for the grid AND segment (anode).  Higher grid or segment voltages will cause more electrons from the filament to strike the phosphor on the anode and cause it to get brighter.
« Last Edit: May 20, 2024, 07:24:05 pm by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #85 on: May 21, 2024, 01:12:31 am »
I have enough multimeters, so I'll monitor both, but, since I'm at it, I'll use a third to keep an eye on filament current.

Ironically, I actually performed the measurements opposite of your suggestion. I took a perf board, soldered twelve 20k resistors on it, and then soldered a 20k across each individual one to get the 10k (I had a large amount of 20k resistors so I used them). After I put a clip on a SEG line (looking at my spreadsheet, you can probably tell the order I went - left to right on both rows), and touched each G line resistor with a clip lead.

I tried thinking of the most efficient method to make it go faster and to reduce the amount of times the VFD gets touched. This way I was only touching the VFD twenty times not including connecting all the G lines initially.

On a side note, I realized I had 82ohm resistors for the filament and not 64ohm like I originally stated, and I had five in parallel making it (as it was measured) 16.1ohms. To eliminate the chances of a resistor disconnecting and causing inrush current issues, I also soldered the resistor leads.

Also, I checked earlier, the decade boxes (that's the name I'm familiar with, but they are also called substitute boxes and resistor boxes) are 1/2W resistors - thought I had the -2W version originally. Either way, 1/2W should be more than enough. At one time I had to replace some of the resistors (they were open the day I acquired them), so even if they go up in smoke, I should have some spares.

Unfortunately I didn't expect the need to perform more measurements, so I disconnected everything. Earlier I took some time to gather everything, but most likely I can't commit myself to testing until late this week or the weekend.

Edit: did you (or anyone) look at the scope capture of the +38V taking >12s to drop? Seems odd it should take that long, almost like the zener is open.

Thinking ahead (as I did previously), I'm wondering if it's worth laying out a PCB. Since the HV518 is fed by six voltage/data lines, maybe if the VFD is found to be fine (i.e. no shorts), rather than risk damaging the PCB by soldering more HV518s, a breakout board with an IC socket would be safer.

I can jumper the six lines to the breakout board and either power the filament separately, or jumper those two lines too. Also, I could use pin sockets (whatever they are called) to fit the VFD in rather than solder. The issue would be the cost for a PCB, and, each time an IC blows, it's $8 down the drain.

« Last Edit: May 21, 2024, 01:33:37 am by bostonman »
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #86 on: May 21, 2024, 06:32:11 am »
For the current tests you don't really need to solder the VFD leads; just need to touch Gn and SEG_m with resistors on crock clips. But VFD leads are robust and can take re-soldering many times.  Just make sure the leads don't move (ie mounted on a perfboard) and don't fatigue at the glass interface. 

See attached for a SPI-VFD driver I made a couple of days ago (2*74HC595+2*ULN2003+14*10K pullups, 2.8Vrms add-on secondary filament winding+16VDC "air wired" power supply). Note the VFD taped to a foam block and strip that constrains lead movement. You can do similarly using a 0.1" thru-hole 44-PLCC socket, mounting the VFD on perfboard, and temporarily soldering leads for the SPI+DCpower+FILpower from J1.  Ultimately, it would be nice if you can manage to mount machined sockets for the VFD pins and a low-profile PLCC socket on the display board.

Yeah, I saw that it took >12s to drop.  But that was without any VFD attached so no load but the zener and a 31K resistor.
As the +5V input to DC-DC dies down, the filament is still warm and the last grid and segments that where on should still draw a few mA which should discharge the 470uF cap faster than 12s. But how fast? I dunno.

 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #87 on: May 21, 2024, 03:13:17 pm »
Quote
Yeah, I saw that it took >12s to drop.  But that was without any VFD attached so no load but the zener and a 31K resistor.

That same slow discharge rate I'm almost certain I also saw with the VFD installed and the new-new VFD Driver IC.

After installing the second new IC, I clipped onto the +38V and Vcc on two separate channels (this is why I had and still have fly leads soldered to both points). Not thinking about needing to measure discharge rate, I just looked at the voltage (probably at a fast rate) and thought the slow discharging +38V was odd after powering off the unit.

Afterwards I turned on power again and saw the +38V was now approx. +12V along with nothing on the VFD.

So don't quote me, but I'm 99% sure that slow discharge was also with the VFD installed. Also note a previous scope capture where it shows the slow discharge rate but with the blown HV518; it starts at +12V because the chip was blown, but shows the VFD didn't help the discharge rate because it was still slow.

 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #88 on: May 27, 2024, 01:08:28 am »
Wanted to let anyone following this that I haven't had a chance to perform follow up measurements.

I think Thursday night I began setting up, but I haven't had time for anything more. Possibly I'll have time tomorrow.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #89 on: May 28, 2024, 03:59:00 am »
Earlier I performed tests with 10k resistors; this time noting the current on the G lines.

Although noting all the measurements wasn't asked, thought it would make things easier to have all of them. Entering all the G line currents did get a bit tedious and decided to take the SEG measurements from my previous test due to them being very close.

Also note that all the filament currents are the same in my spreadsheet. They varied very little and I gave up taking notes for each measurement. I noticed it would start at maybe 117mA while switching G lines (i.e. nothing lit) and then drop to about 116.5mA when the G line was connected (i.e. a segment was lit).

I know and understand the trick is to sneak up on 25mA, but is it worth trying to jump from 10k to 100ohms (or 100R as it seems to be used on here) to save steps or is it too risky?

 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #90 on: May 28, 2024, 04:22:27 am »
1kΩ is the lowest I would go to work with the tube. Operated as a triode, heater to screen-grid current limit is that I find.

I don't see you checking for shorts between "G-lines" (as you call the screen grid)? You've checked between segments I think.
Another check for shorted elements is to energize one segment and see if any others will light. Same for the grids, I tie a bunch of segments together high and energize each grid one at a time, looking for drama. I was staying out of the thread so you can work with pqass, haven't followed all the tests.

Lexicon reverb common problem is the DC-DC converter loses regulation and goes too high in output voltage and blows the VFD driver IC.
Here, the HV518PJ is good to 90V so not the problem but I mention it as another VFD-related issue I have seen.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #91 on: May 28, 2024, 04:58:03 am »
Also note that all the filament currents are the same in my spreadsheet. They varied very little and I gave up taking notes for each measurement. I noticed it would start at maybe 117mA while switching G lines (i.e. nothing lit) and then drop to about 116.5mA when the G line was connected (i.e. a segment was lit).

I know and understand the trick is to sneak up on 25mA, but is it worth trying to jump from 10k to 100ohms (or 100R as it seems to be used on here) to save steps or is it too risky?

117mA filament current is good to see given that it's just under/at what the DC-DC filament output can provide.

Given how low the current readings are in your tables, it'll probably be fine to skip from 10K to 100R.  I was just being overly cautious by trying with 1K too.  Just only take a few seconds per measurement at most.   If you're way beyond 25mA it may be pointless to continue to record all measurements.   

If the round of measurements are below 25mA using 100R, then try the final round of measurements with NO resistor (ie. directly attaching GRIDn and SEG-m to +27V).  Again, If you're way beyond 25mA it may be pointless to continue to record all measurements.   
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #92 on: May 28, 2024, 05:29:38 am »
1kΩ is the lowest I would go to work with the tube. Operated as a triode, heater to screen-grid current limit is that I find.

I don't see you checking for shorts between "G-lines" (as you call the screen grid)? You've checked between segments I think.
Another check for shorted elements is to energize one segment and see if any others will light. Same for the grids, I tie a bunch of segments together high and energize each grid one at a time, looking for drama. I was staying out of the thread so you can work with pqass, haven't followed all the tests.

Eventually, bostonman will need to test all GRIDn and SEG-m pins directly attaching to the +27VDC source since that's what the driver does; no resistor in series.  Measuring the current first through a 10K, then 100R was just being overly cautious.  We talked about shorts as what to look for but really any excess current >25mA spells doom as that will destroy the driver chip. The algorithm involves connecting only one grid and one segment at a time, recording the current through each.  But, he covers all combinations so if there was some leakage current to an adjacent grid or segment then it will be reflected in the recorded current measurement.  Okay, it may still be <25mA with leakage currents, so as bostonman takes the measurements be on the lookout for multiple lit segments.

Agreed, once a round of measurements are done with NO resistor in series (with one grid and one segment on), he can then do a final multiple segment on test by shorting all segment pins together (tying them to +27VDC) and measuring the current through each GRID-n (to +27VDC); total of 12 measurements.
« Last Edit: May 28, 2024, 05:36:03 am by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #93 on: May 28, 2024, 07:15:48 pm »
Quote
Given how low the current readings are in your tables, it'll probably be fine to skip from 10K to 100R.  I was just being overly cautious by trying with 1K too.

I'm sure you also don't want to give advice that may blow up the property of others, so it's good to take caution. My next test will be 100ohms on both SEG and G lines. This time around maybe I'll just skim along without noting the currents and make a mental note of the highest one.

A few reasons why I took extra time to note the current values was to provide a clear chart on my measurements to help dissect any possible VFD issues, and for anyone else who comes across this thread who needs help. As most may know, it's extremely time consuming to stop and write each current value and then enter them into Excel.

Also, my initial current measurements included looking at the display each time to check the associated segment was lit, and I didn't see any others that were lit simultaneously.
 

Offline floobydust

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Re: Vacuum Fluorescent Display Driver
« Reply #94 on: May 28, 2024, 08:27:47 pm »
1kΩ is the lowest I would go to work with the tube. Operated as a triode, heater to screen-grid current limit is that I find.

I don't see you checking for shorts between "G-lines" (as you call the screen grid)? You've checked between segments I think.
Another check for shorted elements is to energize one segment and see if any others will light. Same for the grids, I tie a bunch of segments together high and energize each grid one at a time, looking for drama. I was staying out of the thread so you can work with pqass, haven't followed all the tests.

Eventually, bostonman will need to test all GRIDn and SEG-m pins directly attaching to the +27VDC source since that's what the driver does; no resistor in series.  Measuring the current first through a 10K, then 100R was just being overly cautious.  We talked about shorts as what to look for but really any excess current >25mA spells doom as that will destroy the driver chip. The algorithm involves connecting only one grid and one segment at a time, recording the current through each.  But, he covers all combinations so if there was some leakage current to an adjacent grid or segment then it will be reflected in the recorded current measurement.  Okay, it may still be <25mA with leakage currents, so as bostonman takes the measurements be on the lookout for multiple lit segments.

Agreed, once a round of measurements are done with NO resistor in series (with one grid and one segment on), he can then do a final multiple segment on test by shorting all segment pins together (tying them to +27VDC) and measuring the current through each GRID-n (to +27VDC); total of 12 measurements.

Last tests I did on a HP VFD, connecting a screen-grid to +HV through a 1k yielded unexpected results.
The filament stopped glowing to the right of that grid  :o  The display works fine. I'll see if I took pictures of that, had me stumped.
I figure cathode current was being pulled. The tube can be run as a diode- so screen current must be limited and 1kΩ was low enough for me.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #95 on: May 28, 2024, 11:40:05 pm »
Earlier I tested all the lines with 100ohms and I didn't see any G lines that exceeded around 0.5mA and none of the SEG lines exceeded 8mA; so I went all in with removing the series resistors.

The final tests without resistors was basically the same (I expected slightly higher current). Also, I set the power supply current limit to 20mA (although I don't know the accuracy of the over current protection), but, obviously the power supply never went into current limit.

The only anomalies were some SEG lines had either varying current (on the order of 0.2mA), or the G12 lines on a few segments began around 5-6mA, but kept climbing. Eventually it rose to approximately 8mA (hence where I got the maximum current) and stopped.

Shall I plan to measure for shorts between G and SEG lines too?
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #96 on: May 29, 2024, 12:50:14 am »
Last tests I did on a HP VFD, connecting a screen-grid to +HV through a 1k yielded unexpected results.
The filament stopped glowing to the right of that grid  :o  The display works fine. I'll see if I took pictures of that, had me stumped.
I figure cathode current was being pulled. The tube can be run as a diode- so screen current must be limited and 1kΩ was low enough for me.

First of all, VFD filaments shouldn't be seen glowing (red) at all.  It could be related to using too much voltage and/or using DC as a filament voltage vs AC which is normally used. DC produces a voltage gradient across long displays that manifests as dimmer segments on one side vs the other. AC gives a more uniform brightness.  See "5.4 DC Filament Drive" here.

I'm not a vacuum tube guy; only have a general understanding.
But "cathode current was being pulled" doesn't make sense to me.   Here's my reasoning...
the cathode heating loop doesn't change; a given floating filament voltage over a constant filament resistance equals constant current. It provides a constant source of free electrons that a second voltage loop can push out of the filament (via FIL2 lead), across the vacuum, out of the grid/segment, back into the second voltage loop source.  Any electrons from the second source replace the free electrons produced via heating.

Without a resistor in the second loop, the current is determined by the voltage applied and the physical distance between filament and grid/segment (ie. tube physical characteristics).1  Lowering the filament voltage produces less current in the filament (unchanged resistance), less free electrons, and therefore, less light at the segment phosphor.2  Same if lowering the voltage between filament and grid/segment (second loop); results in dimmer phosphor.2

1: "Plate Voltage-Plate Current Characteristic" pages 13,14,15
2: my own experiments.


« Last Edit: May 29, 2024, 01:58:54 am by pqass »
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #97 on: May 29, 2024, 01:46:55 am »
The final tests without resistors was basically the same (I expected slightly higher current). Also, I set the power supply current limit to 20mA (although I don't know the accuracy of the over current protection), but, obviously the power supply never went into current limit.

The only anomalies were some SEG lines had either varying current (on the order of 0.2mA), or the G12 lines on a few segments began around 5-6mA, but kept climbing. Eventually it rose to approximately 8mA (hence where I got the maximum current) and stopped.

That's good news.  I didn't know you had a lab supply otherwise we could have done this without resistors from the beginning.

Given that you tried all combinations of GRIDn and SEG-m (one grid and one segment on, without any resistors) and none of them tripped the 20mA current limit, you should raise the DC voltage from +27V to +38V (keeping the same filament arrangement of 5VDC, 117mA).  You should see the maximum current on any one grid or segment increase a bit more but should never go over 25mA.

Then one final test is to attach all (or at least most) segments to +38VDC (using a bare wire weaving around each of the top display leads).  Then measure each GRIDn current (not necessary to measure the total SEG-m current as that would be n*SEG-m or  >>25mA).  Again, I wouldn't expect a larger individual grid current even though multiple segments are lit.

If you get this far, and at no point 25mA was exceeded, and only one segment was ever lit (non-withstanding the last multi-segment test), then I'd say the display is good.  The driver failure has to be due to some other factor; not an over current on any one output.  We're back to the Vpp voltage lingering longer than Vdd.

Shall I plan to measure for shorts between G and SEG lines too?
If you run a test where the filament is energized (5VDC, 117mA) and leave all segments unconnected, then iterate connecting only a GRIDn to +38VDC, you should NOT expect to see any segments lit.   If you do, then you can conclude a segment is shorted to a grid somewhere.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #98 on: May 29, 2024, 03:29:49 pm »
Yes, I have a lab power supply, actually a few. Some are linear (the one I'm using for this test) and some are switching).

The one I used is three channels with one being dedicated to 5V. I'm uncertain, but believe none of my power supplies go over 32V unless I use the series mode; which means my filament voltage that is currently 6V (approx. 5V at the pin - after the series resistor) will now be 4V at the pin (assuming a linear drop) unless I hook up a second power supply.

Just to confirm the next (three) steps (not using any series resistors):

Raise the voltage to 38V on both SEG and GRID, and then go pin-to-pin again meaning, SEGa and G1-G12, SEGb and G1-G12, etc...?

Connect all SEG lines together with 38V, connect 38V to G1, measure current through G1, then connect G2, measure current, etc...

Disconnect all SEG lines (i.e. leave open) and connect G1-G12 individually looking to see if the display has anything lit

Obviously I will conduct these tests, but, before I sacrifice cutting fly leads and/or anything else meaning messing up my setup to do these tests, do you think any other tests will be necessary to prove the VFD is good?

Also, what plan can I prepare for should the VFD show it's fine (it almost seems at this point a safe bet is we can rule out the VFD - but I will still conduct the tests)?

Soldering another chip onto the display board is basically not an option as the pads don't look like they can handle another removal. The sockets I purchased should fit, however, I haven't looked to see if the IC sits above the socket when inserted (I will install a blown IC into the socket and see if it fits flush, below, or above).

If the IC sits above the socket, then using a socket isn't an option, but, also, the socket will probably touch the back of the VFD regardless which will not provide any heat dissipation for the IC (assuming the IC sits below the socket).

The only other option I can thnk of is to make a PCB, have it built at a board house, jumper wire from the four or five leads that come in from the ribbon cable, install the IC socket, sockets for the VFD (rather than solder), etc...

It's a costly experiment, and the result will be blowing IC chips.

The only thing I haven't checked is whether maybe the zener diode is open on the DC/DC. A reason must exist for the 38V to drop so slowly, but whether that can damage the VFD driver IC is another story.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #99 on: May 29, 2024, 08:13:37 pm »
Quote
Yes, I have a lab power supply, actually a few. Some are linear (the one I'm using for this test) and some are switching).

The one I used is three channels with one being dedicated to 5V. I'm uncertain, but believe none of my power supplies go over 32V unless I use the series mode; which means my filament voltage that is currently 6V (approx. 5V at the pin - after the series resistor) will now be 4V at the pin (assuming a linear drop) unless I hook up a second power supply.

Well, you could still use the 5V supply for the filament since we know the characteristics of that circuit; no chance of it going into current-limiting mode.  Just put a 1.5V AA cell in series with 5V supply to raise its max output (before the 16R series resistor) then adjust the voltage knob so that you maintain 5V (@117mA) between the FIL1, FIL2 pins as before.

Then you can use the other two supplies in series to dial in +38V (with 25mA current-limit) for the FIL2-grid/segment circuit.

Quote
Just to confirm the next (three) steps (not using any series resistors):

Raise the voltage to 38V on both SEG and GRID, and then go pin-to-pin again meaning, SEGa and G1-G12, SEGb and G1-G12, etc...?

Yes.
Confirming that no combination consumes more than 25mA in a grid or segment.

Quote
Connect all SEG lines together with 38V, connect 38V to G1, measure current through G1, then connect G2, measure current, etc...

Yes.
I figure you could use a bare stranded wire weaving around the top leads of the display (all segments but two), their elasticity keeping each segment touching the bare wire.  Again, confirming that no grid current exceeds the 25mA limit.

Quote
Disconnect all SEG lines (i.e. leave open) and connect G1-G12 individually looking to see if the display has anything lit

Yes.
Which confirms that there isn't a segment-grid short if nothing is lit.

Quote
Obviously I will conduct these tests, but, before I sacrifice cutting fly leads and/or anything else meaning messing up my setup to do these tests, do you think any other tests will be necessary to prove the VFD is good?

I'm not sure of your current setup but can you leave the other ends of the fly leads (if they're soldered to the display) open before you attempt the multi-segment lit test?

The filament circuit was tested at its rated spec. You've confirmed the current in each individual grid and segment. And you will confirm that the grid current doesn't exceed 25mA when most/all the segments are lit.  I'm not sure what else we can test the display for. 

Quote
Also, what plan can I prepare for should the VFD show it's fine (it almost seems at this point a safe bet is we can rule out the VFD - but I will still conduct the tests)?

So far the VFD is looking good. I'm not sure what comes next.
The DC-DC, the zeners and caps in the Vpp supply were chosen with the HV518 in mind.  The designers would have known the power up/down requirement of the driver chip.  I'd be second-guessing the designers if we changed the large 470uF C129 cap to something like 47uF or add a larger 10K resistor (vs relying on the existing 31K+zener) across that C129 cap to force a faster power down of Vpp.  It's worth trying though.  I hate that it costs you a chip every attempt.     A smaller C129 may result in a slight flicker in the display and a 10K (or lower value) across C129 will add to the max 38mA provided by the DC-DC which may starve the segment/grid needs.

Quote
Soldering another chip onto the display board is basically not an option as the pads don't look like they can handle another removal. The sockets I purchased should fit, however, I haven't looked to see if the IC sits above the socket when inserted (I will install a blown IC into the socket and see if it fits flush, below, or above).

If the IC sits above the socket, then using a socket isn't an option, but, also, the socket will probably touch the back of the VFD regardless which will not provide any heat dissipation for the IC (assuming the IC sits below the socket).

The only other option I can thnk of is to make a PCB, have it built at a board house, jumper wire from the four or five leads that come in from the ribbon cable, install the IC socket, sockets for the VFD (rather than solder), etc...

It's a costly experiment, and the result will be blowing IC chips.

I figured the PLCC socket plus VFD machined sockets would maintain the same distance between top of the driver chip and the underneath the VFD; just bringing the front of the VFD 1mm closer to the enclosure.  But if the traces are in that bad a shape, then yeah, make a small board to hold the PLCC socket then tack short wires to the VFD pins and J1 pins on the backside of the display PCB.

Quote
The only thing I haven't checked is whether maybe the zener diode is open on the DC/DC. A reason must exist for the 38V to drop so slowly, but whether that can damage the VFD driver IC is another story.

I thought you'd confirmed that the DC/DC was outputting the correct voltage earlier; that it produced +38VDC between J1 pin 21 and pin 23 (DCOM, GND)?  However, I don't recall anything about VR2 cathode; the center-tap of the filament winding.  It should be +6.2V above GND.  If VR2 was open then nothing would be lit since the center-tap of the filament would be at +38V. If VR2 was shorted there'd be a gradient or poorly lit/off segments.  You can confirm this now with the driver IC and display currently removed. I wouldn't try to fix the DC/DC and just replace it.


One more thing...   wondering if the HV518 could be counterfeit.  Are you buying them from a reputable source?
« Last Edit: May 29, 2024, 08:17:33 pm by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #100 on: May 30, 2024, 12:55:04 am »
Quote
Just put a 1.5V AA cell in series with 5V supply to raise its max output (before the 16R series resistor) then adjust the voltage knob so that you maintain 5V (@117mA) between the FIL1, FIL2 pins as before.

It's a dedicated 5V without any adjustment. Maybe I'll setup a second power supply and tie the grounds together so I can keep the 6V and have the 38V for the G and SEG lines.

Quote
I hate that it costs you a chip every attempt.

I'm not in the habit of burning money, but it's in the name of engineering/science, it may benefit others who come across this post, and, most importantly, I can't let this thing get the best of me.

I'd need to review my previous posts, but believe I confirmed the 38V (in fact, I posted scope pictures showing the SEG and/or G lines - I'm calling them G lines because the schematic shows them as Gx) with the original IC. This whole issue of blowing the IC began after replacing the IC. The original issue (I know most of you probably know, but better to reiterate to be safe) was digits four and five (?) sometimes not displaying correctly, multiple periods (although maybe they were commas, so don't put much effort into thinking about that), and the lower right vertical segment on the 12th digit (most right) was always on.

The counter itself worked fine and believe I left it on overnight at least once to get it warm so I could calibrate it; or use it to calibrate a generator both referenced to a Cesium standard.

I'll plan to conduct the final tests on the VFD. The current setup is a bit wonky. I'm using (I don't recall the name of them - maybe header pin adapters) pin sockets with wires. For the most part, I have wires everywhere, a situation I hate working in because that's when accidents happen, but I keep things separated and don't rush. Ideally I'd like to finalize all the tests before dismantling the setup. Although I have no issues reassembling it, I'd rather not.

In any case, counterfeit chips is always a possibility. I placed one of the bad ICs in a socket and it sat slightly above the socket, however, the pins have solder on them, so it may not have been seated well. Regardless, I think the VFD would sit on top of the socket if I use a socket on the Display Board (this will have to be a bridge I'll cross when I get to it).

I have pictures of the new IC and the Digikey bag that I'll upload in a moment. Without deviating to dig through the notes and schematics, is the original IC on the schematic a 518? I know a 518 was purchased because it was a valid "replacement" for the part number listed either on the IC and/or the schematic (or maybe the BOM).

Maybe the issue is compatibility?

Update: also, just for reference, the IC socket I purchased is: 8444-21A1-RK-TP (these have tiny plastic alignment pins on the bottom that I didn't realize and cut off so they sit flush on the board).

 
« Last Edit: May 30, 2024, 01:02:02 am by bostonman »
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #101 on: May 30, 2024, 01:53:50 am »
Never mind what I said about the 1.5V AA cell on top of the 5V supply.  We know the filament circuit will be fine with 5V as it will draw 117mA so you should be able to just directly connect the fixed 5V supply across FIL1,FIL2 VFD pins (no 16R resistor in-series needed).  You can confirm with an ammeter but I'm sure it will be fine.  I don't think you have to worry about in-rush current.

If it came from DigiKey then I fully expect the part to be genuine but...
technically (the best kind of correct), the circuit is designed with the SN75518 in mind.  Looking at the specs for that, I see it has a max Vcc1 logic supply of 15V (not 5V) and max Vcc2 (what I've called Vpp) of 60V. There is no mention of a power up/down sequence for Vpp.  It has a PNP vs PMOS on the output.  I'm thinking it's a better part; and the HV518 doesn't quite work in this design.

So if you're not footprint constrained and go with a daughter-PCB, you can use the 40 pin DIP version of the SN75518 which are available on eBay (couldn't find the PLCC version).  Yes you risk a counterfeit, but it'll likely work and get you out of this jam. I bought one from eBay and it works fine.
« Last Edit: May 30, 2024, 02:00:06 am by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #102 on: May 31, 2024, 02:18:13 am »
Believe I've completed all the tests.

When leaving all the SEG lines disconnected (i.e. open), I touched each G line individually with 37V DC and nothing turned on.

In order to tie all the SEG lines together, I used a strand of wire, however, it seems the connections weren't solid enough because I couldn't get all the segments lit. I rewound it three times and had intermittent results where segments would sometimes turn on and off. After I tried copper tape which appeared to work much better.

None of the currents went beyond around 0.5mA (without doing the math, I expected a bit higher since all twelve segments are on simultaneously).

The only anomaly I saw that was slightly concerning was the far right digit segments and part of 'u' and MHz was faintly lit.

I noticed as I ran my fingers around the G lines, I could get all twelve digits to light individually as I passed my finger across them. My guess is the G lines/segments are picking up capacitance, however, I could get the far right digit to turn off momentarily as I tinkered with the copper tape.

Maybe this is an actual problem, but due to being able to have the far right segments almost turn off, sometimes different segments in that digit would be off, then on, and sometimes fully off, I sense it's just due to the copper tape and/or weak connections.



 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #103 on: May 31, 2024, 02:26:04 am »
Quote
There is no mention of a power up/down sequence for Vpp.

I'd like to point out that when the new IC blew the first time, I removed it, soldered a new one, and, before I re-soldered the VFD, turned on/off the unit to measure whether the 38V was still 12V or back to 38V without the IC blowing.

It wasn't until I connected the VFD that the second IC blew (probably the same case as the first time).

The 38V was solid at around 40V (I think or maybe 42), the display worked, the DC/DC was cool, turned off the unit, waited about five-seconds, turned on the unit, no display, and the 38V was down to 12V again with the DC/DC getting warm.

Unless someone thinks a problem exists with the VFD due to my previous testing update, I think my next step is to measure around the DC/DC, the zener, etc...

If the conclusion is to make a daughter board to house a 40 pin DIP, I guess it will be lots of work, but the only solution. Maybe it would be a good idea to include the 44pin version too for possible future needs and/or conduct more tests that cause the IC to blow.

 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #104 on: May 31, 2024, 02:59:16 am »
When leaving all the SEG lines disconnected (i.e. open), I touched each G line individually with 37V DC and nothing turned on.

Good. No shorts from grids to segments.

Quote
In order to tie all the SEG lines together, I used a strand of wire, however, it seems the connections weren't solid enough because I couldn't get all the segments lit. I rewound it three times and had intermittent results where segments would sometimes turn on and off. After I tried copper tape which appeared to work much better.

None of the currents went beyond around 0.5mA (without doing the math, I expected a bit higher since all twelve segments are on simultaneously).

The only anomaly I saw that was slightly concerning was the far right digit segments and part of 'u' and MHz was faintly lit.

Its okay that it was a bit flaky. This test was to compare the grid current where most segments were lit to the previous test where just one segment was lit.   So what was the current on the grid?  Was it >25mA?
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #105 on: May 31, 2024, 03:42:19 am »
Quote
So what was the current on the grid?  Was it >25mA?

I only saw about 0.5mA on the SEG current and about the same on the G line current.

I was looking more at the digits to see if others lit and less on the current, but the current was very low the times I looked which didn't make sense. With a 10k series resistor the G lines were 1.5mA (doing a quick visual average) and the SEG lines were 0.15mA (also a quick visual average).

If all twelve segments were lit, and assuming 0.15mA per segment, that would be 1.8mA total; this would also be with a 10k resistor and this last test didn't have a series resistor.

Maybe since I used 5V for the filament versus 6V, this caused less current on SEG and G lines?

Update: due to lower filament voltage, the current was approx. 97mA.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #106 on: May 31, 2024, 04:39:13 am »
I'd like to point out that when the new IC blew the first time, I removed it, soldered a new one, and, before I re-soldered the VFD, turned on/off the unit to measure whether the 38V was still 12V or back to 38V without the IC blowing.

It wasn't until I connected the VFD that the second IC blew (probably the same case as the first time).

The 38V was solid at around 40V (I think or maybe 42), the display worked, the DC/DC was cool, turned off the unit, waited about five-seconds, turned on the unit, no display, and the 38V was down to 12V again with the DC/DC getting warm.

I'd expect the DC-DC to be at its highest output voltage without a load attached given there's probably an inductor inside. But your experience does hint that the failure was output current related. But so far, the display tests clearly shows no grid or segment draws >25mA so the driver should not have failed for that reason alone.  Maybe the issue occurs when combined with the lingering Vpp after Vdd goes to 0V.

Quote
Unless someone thinks a problem exists with the VFD due to my previous testing update, I think my next step is to measure around the DC/DC, the zener, etc...

If either VR1 and VR2 zeners were open, you wouldn't have seen any display output at all.  If either were shorted, you would see some dim segments (that should be off), gradient, or slightly brighter segments (VR1 shorted) that would have drawn more grid/segment current (14% more).

But you can test this now while the VFD and driver have been removed.  The DC/DC output is rated for 38mA. Connect a 1K resistor (38V/0.038A=1K) between J1 pins 21 (+38V) and 23 (GND) and confirm that it can maintain the 38V.

Quote
If the conclusion is to make a daughter board to house a 40 pin DIP, I guess it will be lots of work, but the only solution. Maybe it would be a good idea to include the 44pin version too for possible future needs and/or conduct more tests that cause the IC to blow.

I don't think it would be a lot of work to create a functional solution.  I would just use a 40x7 hole perf-board, mount two 40 pin rows of machined sockets on the long edges, solder fly wires through the inside holes next to the sockets, coming out via the short edges of the perf-board, with wires sandwiched between the perf-board and the IC above.  With the perf-board centered and (double-sided) taped to the back of the display board and between the VFD pins, the 20 wires (or so) from each short side are tacked to the VFD pins and J1 pins.

For a better job, you could make a custom PCB that would be the same dimensions as the VFD pins.  You would then solder 10mm solid wire up from the VFD pins and select J1 pins into corresponding sockets from underside of the daughter PCB just like an Arduino shield.
« Last Edit: May 31, 2024, 04:49:36 am by pqass »
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #107 on: May 31, 2024, 05:15:40 am »
I only saw about 0.5mA on the SEG current and about the same on the G line current.

I was looking more at the digits to see if others lit and less on the current, but the current was very low the times I looked which didn't make sense. With a 10k series resistor the G lines were 1.5mA (doing a quick visual average) and the SEG lines were 0.15mA (also a quick visual average).

If all twelve segments were lit, and assuming 0.15mA per segment, that would be 1.8mA total; this would also be with a 10k resistor and this last test didn't have a series resistor.

Maybe since I used 5V for the filament versus 6V, this caused less current on SEG and G lines?

Update: due to lower filament voltage, the current was approx. 97mA.

Yes, less filament current means less available electrons so less current in the FIL2-grid/segment circuit (for the same voltage).
20mA less (from 117mA to 97mA) filament current isn't much of a difference.  I'm not sure which is the correct current to run the filament with but it's in the ballpark.  What's important was the 6V+16R resistor source is probably the top-end and you never came close to the 25mA current limit of the driver output.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #108 on: May 31, 2024, 01:00:06 pm »
I'll double check the current values again with all the SEG pins shorted before I dismantle my setup.

Also, test the DC/DC with a resistor load. Looks like about 1.5W (38mA ^2 x 1k) and luckily (according to my list of resistors) I have should have 200ohm 0.5W (five in series), or 500ohm 200W (two in series).

Quote
For a better job, you could make a custom PCB that would be the same dimensions as the VFD pins.  You would then solder 10mm solid wire up from the VFD pins and select J1 pins into corresponding sockets from underside of the daughter PCB just like an Arduino shield.

A PCB with pin spacing to match the VFD was something I gave thought to sometime ago and agree, I think it would be much neater. Obviously I'll have to dig deeper once the decision is made to take that route.

Not sure if I mentioned this, but the irony is that I didn't have any intension of working on this counter. Don't remember what I was working on, but dragged out this counter to use. When I was reminded of the digits being off, I thought maybe it was something simple like a loose connection/connector. Before long, everything else got tossed aside and this became the main project.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #109 on: June 01, 2024, 12:59:18 am »
I performed the measurements again. This time I had all the G pins open rather than connected to resistor leads on the perf board (I was using the resistor leads to clip onto - initially I was going through the 10k resistors to limit the current) and none of the segments were ghosting; so it must been due to capacitance or whatever that I saw previously (my previous post).

The highest current draw I saw was about 0.8mA on the G lines and 0.5mA on the SEG lines. Unfortunately the copper tape wasn’t making full contact all the time, but I think it was solid enough since I saw all segments lit at short intervals.

Probably safe to say at this point the VFD is good.

Next step is to measure the DC/DC with a resistor load and then I guess plan on a daughter board.

 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #110 on: June 03, 2024, 01:55:56 am »
I placed two 500ohm (200W) resistors in series (overkill on the wattage, but it was the only higher wattage resistors I had) with the 38V to ground.

Attached are scope captures with the file names named accordingly.

From what I could tell, the 38V held strong under a load. The 5V into U30 is a bit noisy, but probably normal).

Also note, on some of the scope captures such as VRx, the "zero" volt line is two divisions from the bottom, but, when the power is off, the voltage sits approximately 6v above (the "zero" volt line is somewhat hard to see on the black background along with the trigger level being next to it).

 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #111 on: June 03, 2024, 05:04:49 am »
Yup, 38V is holding up at 38mA.

And U30 pin 1 has about a 42kHz ripple of 1/2Vp-p. They could have used a 470uF for C62.

The VR1 pin 1 never goes to zero because VR2 is off below 6V.  So C129 cap just keeps it there without a load. Maybe it could get close to zero while the VFD filament cools down. It probably cools faster than the load consumes so it likely breaks the power-down rule in the HV518.

The VR2 spikes to GND are also about 42kHz.

I'm curious what the waveform looks like between FIL1 and FIL2 (U30 pin 4 and pin 6) with a 50R in between (5V @ 100mA).
Attach probe channel 1 to U30 pin 4 and channel 2 to pin 6 with ground clips to GND.
Bonus if you can generate a 3rd, math channel where it's channel 1 + channel 2.

 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #112 on: June 04, 2024, 12:53:57 am »
Attached are two scope captures - files named accordingly.

Not sure if I've done a 'math' function before, and certainly not with this scope (Agilent 54831M), so I'm uncertain whether it's correct.

The phase seemed to move slightly because I kept seeing it jump, but the waveforms remained solid.

Just to elaborate on the file names: the 38V was still loaded with 1k (two 500ohms in series) to ground, FL1 went into a 50ohm resistor, the other side of the resistor was connected to FL2. Technically it was two 23ohm resistor boxes in series to reduce wattage (two 25ohm settings were about 53ohms, so I used 23) and also a current meter. The filament current was about 97mA and held constant.


Maybe due to the long le
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #113 on: June 04, 2024, 02:30:57 am »
The waveforms aren't smooth sine waves but rather (slightly distorted) square waves; the filament won't care.
As long as the FIL1 and FIL2 outputs (relative to GND) are inverted so when added average to 0V.
The yellow math channel shows this flat avg voltage but it's raised to about 6V by the VR2 zener.

I think the DC-DC (+38VDC and the +/-5V filament) outputs are fine as they can maintain their current spec. (38mA and 100mA, respectively).
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #114 on: June 04, 2024, 02:36:50 am »
Time for a daughter board then for the other TH IC?

I’ll start mentally working on it and then probably have a PCB made rather than a leaf board.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #115 on: June 04, 2024, 02:42:12 am »
Time for a daughter board then for the other TH IC?

You mean the SN75518?  Yeah.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #116 on: June 04, 2024, 02:56:33 pm »
Quote
You mean the SN75518?  Yeah.

Yes, that one.

I will probably incorporate a socket for both the 518 and SN75518 this way I can (hopefully) insert the SN75518 and see it works fine, remove it, insert the 518, and see if it blows.

My PCB layout skills are basic, any suggestions on trace routing such as size or any "magic" designs I should incorporate to reduce noise?
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #117 on: June 04, 2024, 05:30:36 pm »
From Reply#20: "Every ~976 us (~1024 Hz), there is a 32 bit word, with bits clocked at ~934 kHz."

I don't think the outputs changing 1024 times per second is going to generate much noise or pose a concern for board layout.
The SPI traces won't be very long; just up from J1 into the corners of the chip.
« Last Edit: June 04, 2024, 07:40:34 pm by pqass »
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #118 on: June 05, 2024, 02:43:02 am »
Quote
So if you're not footprint constrained and go with a daughter-PCB, you can use the 40 pin DIP version of the SN75518 which are available on eBay (couldn't find the PLCC version).  Yes you risk a counterfeit, but it'll likely work and get you out of this jam. I bought one from eBay and it works fine.

SN75518FN is the part number on the schematic (I assume you just summarized the number and they are the same).

Any suggestions on which place to purchase from on eBay? I'm seeing $3.xx up to $2x.00, all from China and Hong Kong.

I assume it's a coin flip, but figured I'd ask.

Currently I have one 518 remaining, and three sockets. When I make the PCB (as mentioned), I'll incorporate a socket. I'll be able to see if the 7551s work, and, if the do, I'll perform one more test with the 518. I will blow, but it will be in the name of science.

Quote
but it'll likely work and get you out of this jam.

I could always just install the original chip and live with bad digits. :)
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #119 on: June 05, 2024, 03:31:48 am »
I bought just one "SN75518N" from a local (Canada) seller (now sold out). It looks new old stock; not a pull (splayed pins, no solder, no marks, not faded, clean). I think I paid CA$7 + CA$5 (shipping) about a year ago.  I've confirmed that I does work driving a VFD.  I don't have any experience with Chinese sellers as I don't want to deal with customs brokerage nor the wait.

Using the original flawed chip would be a bit of a let down considering your effort.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #120 on: June 05, 2024, 01:11:39 pm »
Quote
Using the original flawed chip would be a bit of a let down considering your effort.

Very true, but felt maybe you and/or others felt a bit of responsibility for me going with a 518 and having these issues (assuming the 518 is the issue and not something else that developed into a failure). Obviously trying to fix this was all my doing, but, I was just stating at any point I could jump ship by returning my counter to the original (broken) state, so nothing would be lost. :)

I agree, waiting the several weeks for a chip from China isn't fun. Maybe tonight or tomorrow I'll do a deeper search, but my quick eBay search showed China and Hong Kong only.

A few years ago I purchased two used Agilent hybrid chips from China for my 54831 that had bad channels on 2 and 3, and so far they are working (although I fear leaving the scope on too long).
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #121 on: June 05, 2024, 01:46:30 pm »
Your adventure has been surprising given that the HV518 works as a replacement in the HP 34401 (notice pix) lead me to believe it would work for your case. 

But just checking the 34401 schematics... and it shows that FIL1,FIL2 do not come from a DC-DC, but instead from an isolated transformer winding.  How about that!  That sure does implicate the power-down rules of the chip.

Thanks for taking one (or two) for the team.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #122 on: June 05, 2024, 02:47:11 pm »
Quote
Thanks for taking one (or two) for the team.

Hahha this site and contributors have saved me more often than not. For the price of blowing up a few ICs and performing (and learning) to test the VFD, it's the least I can do.

Those scope captures I've been posting have come from my Agilent 54831M that, if not for the assistance of people on here, would have remained a two-channel out of calibrated scope. So I absolutely don't mind "taking one or two" for the team.

Technically I haven't taken anything for the team yet though. Maybe I blew up something in the unit causing the current issue and wasted the time of others. No telling unless I reinstall the original bad chip (which would be a waste) or designing the 5518 into the unit.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #123 on: June 05, 2024, 04:38:28 pm »
The schematic lists an FN, but you purchased an N.

I get far less results when I search for SN75518FN than SN75518N (the one you purchased).

According to the datasheet, an 'FN' handles 1.7A and an 'N' handles 1.25W.
 

Offline pqass

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Re: Vacuum Fluorescent Display Driver
« Reply #124 on: June 05, 2024, 05:12:50 pm »
Look at the TI datasheet, first page.  N=40pin DIP package, FN=44pin PLCC package

Yes, I purchased the DIP for one of my custom projects (not as a drop in replacement) so I had flexibility.
That's all I found on eBay, anyway.
 

Offline bostonmanTopic starter

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Re: Vacuum Fluorescent Display Driver
« Reply #125 on: June 05, 2024, 06:33:22 pm »
Oh okay, I should have realized it. I was focused on thinking both N and FN were DIP.

My attention was: new IC model 518 is the PLCC and old model is DIP; and both DIPs came in a N or FN option.


 



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