Fluke 75 measurement problem

[tcharron]

I just acquired a Fluke 75 cheap.  It had a lot of problems, which I have largely fixed just by cleaning it up.

I had to repair the elastometric connectors for the display and clean the contacts for the knob and reflow its solder joints just to see what was going on.  Then, it had very bad mesurements after the 0-3VDC scale (it was reading 2-3 times the input).  I fixed that by reflowing the rest of the solder joints on the board and subjected it to an ultrasonic cleaning.  Only after all of this, it's almost usable now.

My problem now seems to be some kind of stability issue.

- When first powered up (after sitting for a while), the device reads around -0.001 volts on the VDC, then over about 3 seconds, migrates to -0.026 Volts.
- Hooking this up to a bench supply, It reads voltages from 1-20V as about 0.3V high.
- Then, shorting the leads, it reads about 0.3V, but over about 2-3 minutes this decays to a 0.1V level, then over the next 15 minutes that falls to +0.04V.  It seems like it is an asymptotic decay.

There may be a calibration issue, but the calibration of the device would not affect the "0" level, and calibration would also not explain the very slow tracking.

I'd appreciate any ideas on what could be causing this.

[tcharron]

A correction to my post... After removing the bench supply, I left the leads disconnected (not shorted as I stated).