Preamble 1855 diff. amplifier trouble...

[ivaylo]

Got myself on eBay one of those Preamble 1855 differential amplifiers (LeCroy still seems to be selling them as DA1855A) at a price I thought is OK for a second hand working unit. Played with it, nice instrument... Trouble is, that when it warms up (~10 min) the output turns from a flat line with no signal on the inputs (both switched off) to a triangular 130Hz, 30mV pk-pk (see picture). Scope is properly terminated at 50Ohm and all. The attenuator has no effect on this. The gain when on x10 doubles it (what's on the picture is x1). I think the thing is messed up, but stand to be corrected. Any chance this to be easily repairable (not finding schematics anywhere online)? I can still return it, but in case it's something trivial. Appreciate your help...

[ivaylo]

When I set gain to x10 and the "-" channel to Vcomp it's very clearly leaking a sawtooth signal from somewhere.

[rx8pilot]

Did you ever resolve this issue? I just got two of these and one works, the other does not. The internal construction looks unnecessarily difficult.

[ivaylo]

I did not, the nice eBay seller took it back. Got another one since and that one works. What are your symptoms, similar to what I had or different? Post here (or another thread) if you don't mind and we'll have a record and hopefully fixes, these seem to be popular devices. The fact that you have a working one to compare is also to your advantage.

[ivaylo]

Never mind, noticed your other thread.