Fluke 8050A repair

[MikeK]

Made a bigger font for it.  Still to small, I think.

[bdunham7]

Even at that it will be much easier to read than the original.  By modern standards those original displays are pretty bad.

[MikeK]

I still like an LED solution to this.  I really like 7-seg.  Very fitting for the style and age of the 8050A.  It's limiting in what can be displayed, though.  Looks like I can get a larger version of this simple OLED (2.4" v. 1.3") for around $15.  The TFT displays seem to come in a appropriate size for less money, and it's color too.  But many more pins involved in the comm interface.  I used what I had on hand, a 28-pin PIC18F26K40.  About 20 pins get used up in grabbing the data from the Fluke processor: 4 line data bus, 5 strobes, 1 decimal point, 1 HV,  and then 3 for range and 3 for mode if those are desired.  Okay, 17...maybe I can fit a TFT and a rotary encoder.

[Swainster]

I just bought a '2.4" OLED' for exactly this purpose (i.e. restoring one of my pile of defunct 8050As). Unfortunately the single sample I bought was damaged in transit (was not boxed, just wrapped in foam). Anyway, I can confirm that the 2.42" display that I bought - doesn't - quite fit. It's close, but sadly no cigar. It actually looks like a very nice display so hopefully I'll be able to get a replacement and use it in some other project.

I suppose technically, as it's so thin, you could mount the display in front of the original aperture, but my personal preference is something that looks more a bit more "original".

[MikeK]

I just bought a '2.4" OLED' for exactly this purpose (i.e. restoring one of my pile of defunct 8050As). Unfortunately the single sample I bought was damaged in transit (was not boxed, just wrapped in foam). Anyway, I can confirm that the 2.42" display that I bought - doesn't - quite fit. It's close, but sadly no cigar. It actually looks like a very nice display so hopefully I'll be able to get a replacement and use it in some other project.

Thanks for posting your experience.  Does the display itself not fit or is it because of the PCB?  I'm thinking about separating the display from the PCB but still leave it connected.

[MikeK]

Added decimal and mode.

[bdunham7]

I just bought a '2.4" OLED' for exactly this purpose (i.e. restoring one of my pile of defunct 8050As). Unfortunately the single sample I bought was damaged in transit (was not boxed, just wrapped in foam). Anyway, I can confirm that the 2.42" display that I bought - doesn't - quite fit. It's close, but sadly no cigar. It actually looks like a very nice display so hopefully I'll be able to get a replacement and use it in some other project.

Thanks for posting your experience.  Does the display itself not fit or is it because of the PCB?  I'm thinking about separating the display from the PCB but still leave it connected.

I'm thinking you guys might be better off 3D printing a custom display frame instead of trying to match the original.

[MikeK]

I can definitely fab something.  No 3D printer, but I do have a mini-mill and plenty of hand tools.

[Swainster]

Sorry, got temporarily knocked off my feet by the 'rona but almost back to normal now.

Basically the 2.42" display cannot fit in the space where the original display holder fits. I also purchased a slightly smaller monochrome graphic STN type display which came with back-light. I think it should fit nicely, but of course it won't look as nice as OLED or colour LCD, but may have the advantage of lower power consumption.

The plan was indeed to replace the original display holder with a 3D printed modified version, and to that end I have modelled the original display holder, but haven't done the mods yet. Actually I wanted to print it out before I mod it, to check for fit, but haven't been able to get around to it yet.

[Swainster]

See attachment just in case anyone wants to try printing out the first draft 3D model of the standard F8050A display holder - please note that this is totally unchecked so may not probably won't fit. Another note, the draft clearances are assuming a resin type printer - I'd highly recommend opening them up for a filament printer. I'm not sure when I'll get around to checking it against the actual Fluke housing and PCB, so i'm posting this in the meantime. If you are brave then you could use the step file to generate dimensions for your own custom holder design.

[MikeK]

Here's some information on the Fluke's display, in case it's helpful to anyone...

* The LCD is controlled directly by the Fluke's processor, with the display data being latched with CMOS LCD drivers.
* The data bus consists of lines W, X, Y, Z; which is 2³, 2², 2¹, 2⁰ respectively.
* HV and DP lines are also part of the data.
* The BP signal is a 50Hz signal only used for refreshing the stock display.  Not needed for a custom display.
* Display data is latched with positive pulses on one of the five strobe lines (ST0 - ST4).
* Data is valid on both rising and falling edges of the strobe pulse.  (I grab the data on the rising edge.)
* Strobe 0 (ST0) latches data for U10, which handles the HV, BT, and REL indicators.
* A high on DP during ST0 indicates REL.
* The battery (BT) signal comes from a voltage dividier and PNP.  I don't use this, since my Fluke doesn't have those components.
* ST0 also latches data for U11, which handles the plus(+), minus(-), 1 (first digit), and the dB indicator.  W line is for minus(-), X is for plus(+), Y is for dB, and Z is for the "1".  Note that the plus(+) is actually two segments, so W *and* Y together turn on the plus.
* ST1 latches data for U12, which is the second digit (first whole 7-seg digit).
* ST2 latches data for U13, which is the third digit.
* ST3 latches data for U14, which is the fourth digit.
* ST4 latches data for U15, which is the fifth digit.
* U16 handles the decimals, and is latches by each of the strobes (ST1 - ST4).
* The display-only data can be taken from the display board ribbon, or rather the vias right next to it.
* Functions (Volts, Amps, Ohms...) are only available at the Fluke processor (which is why I made a carrier board).
* Ranges (2, 200, etc.) are also only available at the Fluke processor.
* The Vss pins of the chips runs at -5V.
* The Vdd pins of the chips runs at 0V.
* Display data is latched in sequence: ST0, ST1, etc.
* A strobe pulse is about 168us wide, though I observed some at 116us.  This isn't really useful anyway.
* There is about 216us between the falling edge of one strobe to the rising edge of the next.
* One cycle is pretty short, so it makes sense to grab all the data and then process it after ST4 is done.
* The whole cycle repeats about every 410ms.

Range switch logic (pins are of Fluke 40-pin processor):

	Pin 3	Pin 4	Pin 5
Range	RngA	RngB	RngC
200 ohm	H	H	L
2k ohm	L	H	H
20k ohm	H	L	H
200k ohm	L	L	H
2000k ohm	H	H	H
2M ohm	L	L	L
200 nS	H	L	L
2 mS	L	H	L

[MikeK]

Finally got my TFT display off the slow boat from China.  The 2.0" looks to be a good size for the Fluke 8050A's opening.  First step was to make sure the display works.  It does.  And it's a gorgeous display.  Gotta love the Arduino community to make these things easy.  Hardware SPI is the only way to go, as software SPI is painfully slow.  At least for the Arduino Uno.  I can run my PIC18F26K40 up to 64MHz.

Getting this setup, and porting my previous code to C, is going to take me a while...I was using a simple and uncommon compiler to get the OLED up and running that I'm sure nobody else would use.