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...