Hello,
a few months I've bought a 3400a, that had sometimes the problem, that the indicator needle sows a strange behaviour, but only in the 3 / 1 mV range. Meanwhile, the Instrument show this effect in all ranges, but only after about 30 to 45 minutes after power was turned on.
I did some research in the internal parts, and found out, that the LDR's (photo cells) from the optical chopper amplifier where bad. As soon, as they have been warmed up, this behaviour becomes more visible.
I have locked in the net, but the LDR's you can get today, are much to slow for the application. So I thought about the problem and came to the conclusion, to replace the LDR's with electronic switches.
Yesterday, I've drawn a schematic, that shows how I'd like to do the replacement of the LDR's from opto chopper assembly. Please have a look to the schematic and the original schematic of ths amplifier, that I also uploaded.
In general, I'd like replace the LDR's, but if no more available - what should I do ...
I hope, I can assemble the parts on a bare board tomorrow. Then I will send additional information of how it works.
Kind regards Guido
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Edit: with LDR I mean the photo cell in the copper amplifier