Electronics > Repair
RadioShack FET VOM repair has me stumped
RichardM:
If I lift the selector pcb away from the three finger contacts and then measure between K and the V-Ohms-A terminal I measure open as I rotate the selector dial until I select DCV and I get continuity. This is fine. Problem is if I reconnect the selector pcb I get Isee continuity ins Amps mode, 10M in ACV and about 1.8M in Ohms. What a nightmare.
schmitt trigger:
Could it be that D1 is leaky? Can you read its part number from the diode's body?
floobydust:
When a repair goes off the rails like this, it means one's technique is bad and there's an opportunity to learn and grow.
My usual mistakes are making assumptions and overlooking the obvious. Back up and redo things maybe.
We haven't narrowed it (leakage resistance) down between being a component or a mechanical problem.
Again, I would do a visual with a magnifier and look for anything. Maybe that phenolic is simply turning to carbon.
The rotary switch wiper could have leakage, pull it off and test it between poles.
Another technique, when you can read the leakage resistance is to apply a little hot air to heat up parts or regions to narrow it down. Not to melt things but just lift temps up to say 50-80C and see what affects the leakage resistance. It will jump dramatically with temperature.
I keep guessing it's the input clamp- D1, C1 or the JFET.
RichardM:
Thanks
Yes, I am the first to admit my troubleshooting can often head down a rabbit hole but I have learned a lot over recent years and managed to repair a lot of old test gear successfully. I generally follow the generally accepted approach to fault finding. In this instance I just couldn’t understand or explain the fault. As it turned out it was a mechanical issue with the front selector dial. Depending on how you twisted the dial and applied a little pressure you could get the 1M range working fine. The meter has had a tough life and when I pulled off the front selector dial and it was apparent it had broken in he past and a bodge was done with glue and a new central screw and bolt. The dial was always a bit how do you do. Anyway, thanks for the suggestions. I have parted it out. It didn’t owe me much.
Richard
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