Repairing a Keithley 197

[Zachrey]

Please help!

I just got a Keithley 197A from a seller on eBay.com and it is clearly defective! The seller will not take a return so I may have to try and fix this bugger! I have a little bit of experience with this and only have an oscilloscope and a multimeter to work with. Here is a Youtube video of what the unit is doing. Do you think there is any hope?

https://youtu.be/6OrLtb9O-co

Thanks!
Zac

[Wytnucls]

Here is the maintenance manual with schematics:

https://doc.xdevs.com/docs/Keithley/197/197_901_01A.pdf

[tooki]

One thing to check is the FETs that perform the range switching.

As for it switching to dB, that’s not a default for any mode, so perhaps there’s contamination on the display PCB, which the dB button lives on, causing it to detect a spurious press of the button. Give those contacts a clean. Oh yeah, and see if there’s any difference with the backlight on or off — the high voltage supply for the EL panel is also on that PCB. I don’t know if they have any failure mode that could cause noise, but it’s easy enough to test so why not!

P.S. it’s not good form to hijack an existing thread for a new question, even if it’s about the same model. It’s better to start a new thread.

[akabek]

Yesterday I received a Keithley 197a I purchased from ebay.  I took a gamble as it was listed as parts and not working.  Today I plugged it in and the screen was showing OL and the range buttons including auto would not lock in place after pressing a button.  I found a couple of repair videos on You Tube that I reviewed.   I took that cover off and removed all boards so I could look over the components.  Everything seemed OK except the buttons would not stay in when pushed.  I applied Deoxit 5 followed by Deoxit gold.  I waited 15 minutes and reassembled it.  I connected a DC power supply and tested a resister and it is working now.  With the wierd things going on I would not have thought cleaning the controls would make that much of a difference but give it a go.

[Ordinaryman1971]

I think it's a great advice, clean the switches first. Deoxit is great, exercise them after applying the Deoxit and try to take measurements again.

[akabek]

My unit was doing a lot of weird stuff.  After reviewing YT videos with similar oddities  that were corrected with Deoxit I am a firm believer in the product.  Right after doing a visual inspection of the components I would go right to cleaning and lubing switches and dials with Deoxit.