Replacement display board for Keithley 197A

[alx2009]

Sorry for not posting anything for a while!

From HW point of view I have added an independent power supply using V+ (reusing a 7805 board I had in the parts bin). This sorted the jitter problems, no need to add anything more. I include a couple of pictures of the completed project before I closed up everything back in October 2022 (note: before closing I did re-route the black and red wires so that they are not so near the input wires). I didn't need to open the voltmeter anymore since then (SW update via bluetooth).

I have been using the voltmeter and updating the SW regularly, see my GitHub: https://github.com/alx2009/K197Display/releases. the readme describes what I have added so far: https://github.com/alx2009/K197Display#readme

I have a new release coming which I think will be the last one with new features. After that I plan to post a few pictures and/or a video to show how the new SW works.

[Brian of Romsey]

Somewhat surprised to see the earth wire also in the common mode filter toroid.

[alx2009]

Yes, you are right, that is unusual. Normally the ground wire represents the return path for the common mode noise. If it is included in the choke then the noise is not filtered anymore!

But it cannot be a mistake, there must be a purpose. Normally where a choke is used, the internal ground is connected to ground either directly or via a capacitor. Looking at the schematic, the mains ground is only connected to the transformer metal case. The trasformer itself is of dual bobbin construction, which should have very low capacitive coupling between primary and secondary. In other words, the internal ground of the voltmeter is pretty well insulated from earth, both AC and DC.

This suggests to me that the designers of the 197A did not think that the earth wire would be the main return patch for the noise. My guess is that they considered that when the voltmeter is used, there is a good chance that one of the test leads would become the return path from the noise. If not filtered, the ground wire could become yet another way for the noise to get in our out of the instrument. Hence the decision to filter it.

Well, at least this is what I can speculate. I googled a bit but I could not find anything on the subject. I have another bench voltmeter, but if I remember correctly has no common mode filter at all (but on the other end it is not able to measure as low a voltage as the 197).

[alx2009]

I have just released what I think is the first fully usable release here: https://github.com/alx2009/K197Display

The features that I find most useful are the hold mode and the graph display mode. How many times you cannot look at the display6 while you are holding the test leads in an awkward position? Now I can make the mesurement, hit the hold button, go to graph mode and the cursors allow me to go back to the measurement value.

Here are a few pictures.

Main screen:


Menu (accessed with long click on "REL"):


Statistics display mode (accessed with long click on "STO"):


And finally grah display mode (accessed with long click on "RCL"):

In this picture hold mode is also activated. Hold mode is entered clicking the STO key.

In graph mode you can activate cursors by double clicking the "RCL" key:


Clearly the text is rather small, but still legible. The SW can store up to 180 measurement points.

[alx2009]

And here are two short videos.

The first show the options menu and how to navigate the menu: https://youtu.be/tVYD4UWaDdc

The second one demonstrate hold mode & graph display mode: https://youtu.be/7X7YOPeH9es

The quality is not great, but I think it is useful anyway.

[coromonadalix]

some will hate me, 

can the roundish bezel / windows one   IE: who's not a part of the front panel could be removed to gain some pixels in height around the display ?

a bit less cramped

[alx2009]

You are right, it is a bit cramped in the vertical direction. The Camera view accentuates that a bit, but not much.

I thought about that but the front panel is actually a single piece of plastic, as you can see in this picture showing the back of the front panel.


It can be cut along the line in the front, and still leave enough space for the plexiglass window. But I think it would leave some rough edges visible. Maybe they could be carefully sanded and/or painted. In other cases I built new bezels with Tamiya "L" shaped plastic beams. It may work here as well, but I am not too keen to try, there is no way back...

At the end of the day it works good enough for me as it is, but if any of you is willing to try and it works, maybe I will follow your lead :-)

[alx2009]

Just to add that in a separate project, I have reverse engineered the way the voltmeter talk to the IEEE-488 card: https://www.eevblog.com/forum/projects/reverse-engineering-of-the-internal-ieee-interface-of-the-keithley-197a/msg5202936/#msg5202936

A library for an Arduino Uno is available here: https://github.com/alx2009/K197Control

[MathWizard]

That's a nice mod alx2009, I like the back of the board too with the copper clad island style.

I have a K197 that I just updated the buzzer circuit I made for it, and also had too add a shim under the LCD, to keep enough pressure on it to make proper contact.

I also tried adding a front light, just something I could tape to the outside of the front panel, under the LCD. I put some white LEDs in a piece of outer insulation, from CAT5 type cable. I thought maybe shinning a little line of light from it onto the LCD could work. It looks ok if it's a few inches from the glass, but up close, all I see it the reflection of the LED's.

The tube/jacket of CAT5 insulation is white, and even with it turned around so the light passes fully through the insulation, not the slit I cut, it's still not diffused enough. So it still looks like 4 LED's are inside the glass, and you can't read anything.


Now I really wish the screen was bigger too, but for now, I have a VFD unit, from a DVD player type thing.


It has 6x number/letter blocks. They are like 7-segment blocks, except with 6 extra lines, 3-top, 3-bottom.

Then there's a about another 12 special characters like "DVD", "Total", "Title", even pictures like play/pause and a movie camera. So they could be used too, I'd just need a translation guide on the DMM.


The VFD is on PCB with some function buttons, a ceramic resonator, and a big IC/driver for the VFD, if not the whole DVD player or whatever it was. The whole PCB is a very similar size to the Keithley LCD one. There's also an IR receiver PCB is still connected to this main PCB too. The IC is a CH72FG-4U54, can't find anything on it yet, I think it's by ST, if I remember their logo. Looks like the year 2003.


IDK how much power it takes to drive something like this, I think it uses some higher voltages though. But I could add some other little transformer inside the case if needed.


I don't just want to copy/paste someone else's program for driving some new display, I think it's a small enough project that I might be-able to do it on my own, as a learning project with Arduino or some other MCU as easy to use. Or at least a lot of it

IDK for sure this VFD works fully, I briefly tested it years ago with power, it does something. Aside from being a little smaller digit size, if this thing is bright enough, it could be great.

But IDK tho,  the VFD itself, is not as long as the original LCD, but slightly taller. The VFD could probably fit, but the PCB that the VFD /chip are on, might be a problem, it's about 1cm taller than the Keithley 1, and tons of traces from the IC go right to the edge.


Oh there might be a problem with the decimals tho, on the VFD there's
"0- 0: 0- 0: 0 0" , but there is "TRK" and "M R" right above the last 2 blocks that don;t have any symbol in between.

It's not ideal, but I would like to see it working, if possible. I just saved the the VFD for something to hack at. Later I can still get an OLED or other display, to try with the Keithley too.

[coromonadalix]

i don think its a problem to adapt the code of someone else,  if you do contribute the same way  (if the code was opened)

you'll add your own sauce, and share the how to ??

[MathWizard]

Yeah, but I need an excuse to try some programming again.

But unless I can find some similar datasheet for this VFD controller IC, I'll probably have to make a whole driver board, or get another chip.

I mapped out most of the PCB that the VFD and chip are on, I can narrow down the input connectors and pins that might be for the serial data and clocks, maybe chip select type pins.

But yeah I'm way over my head with that IC, the only thing I could try would be to choose some pins, and somehow like with Arduino, try sending instructions from some other IC datasheet.

It's a 64-pin IC, the standby pwr on/off switch for the whole DVD player, goes only to this IC, as do a few buttons, so it's not just a VFD driver.

But if I ever want to use this VFD, I'll just have to make all the drivers, or get a chip, and then try programming it with Arduino or somehow from PC.


Ok the chip is a Toshiba, but really , I should just close up the DMM now, replacing the LCD will be a long project.

[MathWizard]

Well I looked through my junk PCB box, and found the main PCB from that DVD player, and indeed it has the ribbon connector for the VFD PCB. I probably have the power supply PCB somewhere too.

I don;t have the hacking or programming skills for this, but I'm going to see if I can get it running, and get the VFD active. Maybe then with my scope I can see the data going to or from the VFD IC.