Electronics > Repair

Keithley 6485 repair


Last week during measurement I accidentally applied 2.5 kV to input terminal of this picoammeter. After accident I power on flickering numbers were on display.
So I take it apart to see if I can fix it.
The input stage contains LMC6061 and LMC662, these two opamps were first things which can be fried.
When I tried to create the schematic I also found fried 1k resistor on input  (not visible) only the value were totally out of value on package (R110).
I power it and found that on power supply +/-5V rails is voltage bellow 1V (measured on testpoints TP1,TP2).
So I try to find the lowest possible resistance on supply rails, it was around array of FETs for relays switching.
I decided to solder the wires and test which componets consume so much power and bang. It was comparator LM393.
After replacing 4 components the meter is fully working!!
Do not forget to clean all flux residues with IPA and use gloves during repair. I can recommend the bake at 60°C for an hour.
I tested it with my 2636 and it seems to be still in spec.
I hope that this will someone help to repair this meter.

It is quite useful meter with internal math function in can be in second switched to high input resistance voltmeter (I'm using 250M Caddock or 1TOhm array) or to high resistance measurement.
The internal math function can also display the proper units and prefixes ( V, Ohm, etc).


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