Keithley 175 Repair

[henmill]

Hello all,

As the title suggests, I have myself a Keithley model 175 which I picked up at a really great nonprofit in my town, called Free Geek. They divert e-waste from the landfill and have a little store front with a bunch of as-is goodies. So I saw this rather sad looking DMM and took it home for $10 USD.

It powered on no problem but all sorts of weirdness was happening on the display, and right away the buttons were very sticky.

I did a fair bit of searching and watching of videos, such as Dave's own on this 177 https://www.youtube.com/watch?v=kdCfAR06GeI. I followed along and it was a very useful video that helped me to understand what's going on inside the unit, even though mine is a different model. Of course after all the schematic tracing and poking and prodding Dave came back around to the front panel switches being pretty much the root of the major problems.

Nothing inside looks particularly damaged (will point out some questionable things though) so I set out trying to un-stick the buttons. I have some WD-40 brand contact cleaner which I did a lot of sprays with. The buttons felt great while they were wet but that stuff leaves a bit of residue after drying and buttons go back to stickiness. Do not recommend. I would also rinse with some 91% IPA but they would just not stay un-sticky (or un-dicky as Dave says). So I bought some more IPA and decided to really let 'em soak. Lo' and behold, the thing is more or less working! BUT, still after some time the buttons, especially the function selectors do not like to snap back, so today I was happy to find that my local Radio Shack had some DeoxIT D5 on the shelf, so I'm giving that a whirl now.

In my short bit of testing, it seems all the ranges on Voltage and Resistance pretty much function. But the current inputs do not give me anything, so that is what I'll focus on now.

I've gone through the A/D converter checks per the manual and the only discrepancy I've seen is the last step:


Might be hard to see in this picture but I am seeing more like 50ms pulse followed by some shorter ones. Not sure what's going on there.



But, given the volts and ohms measurements seem to be working, maybe that is nothing.

A couple other things of note. It seems like the ROM chip was probably replaced, evidenced by the flux residue underneath and general shiny-ness of the part on top:


Also, seems the area around the linear regulators has gotten toasty at some point. Or perhaps this is normal over the many years of operation. No issues with the power rails that I can tell.


I don't get a whole lot of time to work on this sort of thing, but I've become a bit obsessed the past few days. So I will continue dinking with this and see if I can figure out the issue with the current measurement functionality, and will update as I go!
Check out all my pics here, and will add to this album as I go. https://imgur.com/a/IpKBpHf

Thanks for looking, and appreciate any thoughts on what I've described!

-H

P.S. sorry for the huge-ness of the embedded pictures... Need to figure out better post formatting. But in a hurry to get this up for now

[The Soulman]

In my short bit of testing, it seems all the ranges on Voltage and Resistance pretty much function. But the current inputs do not give me anything, so that is what I'll focus on now.

Could you be more specific on what it does or doesn't do with regards to the current measurement?

[henmill]

Ok I just checked again and it was partly working! Thought it was just showing 0 last night. This time with about 30 mA the auto range on 10 amp worked but auto did not work on 2A range. Manual range sort of worked but was taking a long time to settle.

I will spend more time carefully checking every range and document what I find. Problem is I don't have precision sources so have to rig things up.

[J-R]

I say keep focusing on the switches.  Depending on how much lube/grease was in there before, they can cause a lot of weird issues and may still need cleaning.

My method is take the PCB out and remove the display, then spray CRC QD Contact cleaner on the top and in the back end of the switches.  Hold the PCB vertically with the switches down so any junk stays away from the sensitive components.  Work everything maybe 25 times, add contact cleaner, repeat a few more times.  Let it all dry, then I add a small amount of CRC 2-26 Multi-Purpose Precision Lubricant.

[donlisms]

I second the advice from J-R.  You can't trust anything about the behavior if all the signals are not getting to all the right places through those switches!

[henmill]

@J-R,

Thanks. The method you describe is pretty much what I've been doing. Since I've done a couple flushes with the DeoxIT the action of the switches feels really really good. I wonder if maybe I should have done another alcohol soak before moving to the cleaner/lubricant today. But fingers crossed things will all be good without going back a couple steps.

I've spent my available time tonight working on the switches some more as well as attempting to clean off some residues I see over on the main board. I used some FluxOff I have because it has a small little stiff bristled brush, followed by alcohol drench and spray with compressed air. Seems that this method didn't really clean much off but instead possibly highlighted the residues better haha. See the below pictures of the areas that concern me the most.

The last pic shows something I thought was interesting. Perhaps an un-done bodge at U112? You can see the remnants of leads clipped at pin 7 and the nearby through hole pad.

Also, comparing my specimen and the one shown by Wim_L here https://www.eevblog.com/forum/repair/keithley-175-repair-and-inside-pictures/
His Unit has some marked differences, like the regulator layout, really the PCB layout in general looks updated from mine, lacking all the wire bodges present on mine. I notice his selection of the higher mains voltage so perhaps his is a newer revision sent to another market (Europe?) while I have an early version that had to be hacked together. This one has stickers from the local university, perhaps these versions were offered at a discount in bulk to schools haha. My firmware version is A3 when I do the self test (only tried once).

edit: oh duh I see my PCB has the part number as 175-102-02C while Wim_L's board has H suffix. Easy to assume this is the board revision.

[J-R]

I have a 175 with the identical PCB.  The cut out component is a 15k 5% 1/4 watt resistor.

[henmill]

Small update:

I wanted to test the current ranges so started making a dual opamp current source, like this https://www.allaboutcircuits.com/technical-articles/how-to-design-a-precision-current-pump-with-op-amps/. Then I remembered I probably have a current source IC around and ended up finding some LM234s I forgot about 

So I rigged it up with a 10k pot and a limiting resistor and did a quick test of all the ranges up to 2A (limited to about 10mA with my circuit) . She seems to run just fine and pretty consistent between ranges.

It was at this time I realized my silly comment: "the auto range on 10 amp worked but auto did not work on 2A range". Well there is no auto range for current! The 10A selector is where the Auto selector is for the other functions... did I mention I don't have much free time? I've been in such a hurry in my limited time that I'm missing details such as this. Maybe I should read through the operating instructions underneath for good measure haha.

Oh and the switches seem to be doing really well 24+ hours since the DeoxIT treatment. It seems all signs are pointing to this being a fully functional unit! I need to go through a more regimented test plan just to be sure.

Funny that I spent more on the DeoxIT (almost $30!) than the DMM itself. Kind of wishing I had just bought the CRC 2-26 available at the hardware store instead, but Radioshack was closer and I like to give them business when I can. I'm super grateful to still have one in my town, they are quite rare these days! At any rate I'm still invested <$50 to save this lovely DMM and let it live out its twilight years as my garage bench multimeter.

I'll update this thrilling saga when I get around to more tests and will be doing some cosmetic work at some point too.

Cheers!

[henmill]

I have a 175 with the identical PCB.  The cut out component is a 15k 5% 1/4 watt resistor.

Oh that's interesting! I'll have to check the schematic and ponder why it is no longer there... I also found a drilled out trace near one of the JFETs. Will post more pics of the interesting bodge work soon.

[J-R]

I found a cut trace on my PCB.  It seems deliberate. (Center of photo.)  The via is also ground away.

[henmill]

Yes, mine has the exact same. It's funny how they seem to have designed in the optional path for R129 and cut/drilled the trace when they decided they wanted the gate resistor. But for Q111, they bodged in the resistor by sticking it in line with the Gate pin. Also note the schematic shows 200k but those look like 10k to me. hmmmm

Probably just means there were deadlines to meet and oversights made back in the 80's, just as they are today.

The meter seems to be working well through all ranges and functions, again with limited testing. But clearly the switches were the problem. Now if I just had a decent way to calibrate it...

Buying this meter has been my first foray into vintage test equipment, and I think I've caught the itch to start a collection... In fact, I took home quite a nice haul from work today. I'll probably start a few posts about these, but here's a sneak preview:
https://imgur.com/a/CvTIU2o

[J-R]

OK, now I see the via is ground away.  Hard to see since it's a much cleaner result than my unit.

I actually have three of the 175 models and one 175A.  Paid between $25-$45 each, which is maybe a little high but I have no local sources to feed from.
They all had one issue or another, managed to get all but one 175 working due to a broken LCD.

These old DMMs are fun to tinker with.  A lot worse things to waste money on for some idle time play.  But once fixed they lose their usefulness perhaps.  I have friends and coworkers that lightly dabble in electronics and they wouldn't even accept one for free.

[henmill]

Totally agree! I like the general ruggedness of this era and this design.

To wrap up this thread here is a final(?) update:

I decided to cut the top housing around where it had missing material to try to give a cleaner look/not cut a finger accidentally..




Also, I hastily decided to try making a voltage reference PCB using ISL21090 and OPA2182 for precision buffering/amp. It turned out ok! I might make a rev 2. The pictures show some measurements from the 175 as well as some from a calibrated 34410A at work.