Vacuum Fluorescent Display Driver

[bostonman]

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.

[floobydust]

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.

[bostonman]

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.

[bostonman]

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.

[floobydust]

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.

[bostonman]

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.

[floobydust]

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.

[bostonman]

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)

[bostonman]

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.

[floobydust]

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.

[bostonman]

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.

[pqass]

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.

[bostonman]

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.

[floobydust]

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.

[bostonman]

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.

[pqass]

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.

[bostonman]

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.

[pqass]

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)

[bostonman]

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.

[floobydust]

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.