Electronics > Repair
RadioShack FET VOM repair has me stumped
schmitt trigger:
I also had this same FET-VOM.
I loved it, but for a weird reason, the plastic pieces started to rot away. Disintegrating
into plastic dust.
First to go was the back case, but I could somehow mend it. That is, until the selector switch simply fell apart.
RichardM:
I have an assortment of meters:) but have been using a Fluke 87V, HP3468a and HP3478a. I also have 2 HP-410C I could try.
I have cleaned the rotary switch contacts well with IPA and a light sand with a fibreglass pen but will look at cleaning the meter more widely. Maybe the polarity switch is the culprit? I will report any changes.
Cheers
RichardM:
Ok, so it is not the polarity switch. I desoldered it and hard wired the polarity and I see the same behaviour on the 1M range.
floobydust:
How is R25, 5MEG?
With an eagle eye look for any crud, residue etc. anywhere inside the multimeter, as if a battery took a piss etc. Either you have contamination somewhere, or something is being glossed over. Look at the little JFET board too, it must be spotless. The wires along "G" as well.
A high Z multimeter i.e. HP3478A up to 3V range is 1010 ohms, you can measure the voltage drop across R25 as I keep thinking the input resistance is lower than it should be. 34401a has a high Z mode as well. Otherwise, the switch row is somehow leaky.
On Rx1MEG range, I would connect another multimeter and measure current at the test leads.
1.5V in 10MEG is only 0.15uA it's very little current in the first place.
RichardM:
I have been focussing on the little FET board. I think this may be where there is some contamination or something else. I have pulled the dual FET out for now.i have scrubbed it to death. I notice that touching and spraying alchohol around D and R25 while measuring across R19 send the resistance crazy.
R25 tests fine out of circuit. If I measure the resistance between the cathode of D1 and poitG on the other side of R25 I measure 2.7M one way and 4.8M the other way. The 2.7M is when I have the COM on the cathode. This is with the FET removed.
I will try to take some measurements ,nets as you suggest.
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