EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Zachrey on June 09, 2020, 12:44:52 am
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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
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Here is the maintenance manual with schematics:
https://doc.xdevs.com/docs/Keithley/197/197_901_01A.pdf
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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.
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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.
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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.
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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.