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. Dick Moore > 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 > reaction. > > 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.