richiem at hughes.net
Fri Feb 3 01:45:23 UTC 2012
My first guess would be that the zero pot is dirty, with intermittent
contact. Try spritzing it with a cleaner -- I use LPS1 and it works
great on my old Fluke gear. Some pots are sealed like the old AB pots
ude in Tek and Fluke and HP stuff -- I just drill a small hole through
the back and use the sprizer nozzle to get the cleaner in -- works wonders.
Also, when i redid my 845, I found that even though the electrolytics
checked OK, they still needed to be replaced with verifiably low ESR units.
> My friend asked help with his Fluke 845A. It is late colour, very
> clean inside and outside. The higher ranges work fine and the readings
> are accurate (for example when the needle is on the 1V scale mark
> multimeter reads 1.002V). So the chopper seems to be working.
> The problem is instability on the lowest ranges. Noise level is ok,
> around 200nVpp on the 1uV range, but zeroing is extremely difficult.
> Even a tiny adjustment results a huge change with delay.
> Amplifier DC voltages were as expected, but we found out that
> adjusting the ZERO potentiometer affects the +/-15V power supply
> rails. The overall supply voltage changes or the ground shifts or
> both. For example the negative supply can vary from -11 to -18V. Any
> change in supply/ground changes the zero setting and we have a chain
> All the electrolytics measured like new. The +/-15V switching power
> supply works fine when we replaced the rest of the circuit with load
> resistors. Because of the global feedback it is very difficult to
> divide the circuit into parts and check each of them individually.
> Any help much appreciated.
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