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54845A Distorted Ch1 Signal

Question asked by Adrian on Feb 3, 2010
Hello all,

my 54845A has a bad channel 1. A low frequency square wave signal is distorted in a way that looks like  something is charging up after the sharp rising edge (see attached picture). This only happens in 1 Meg mode. When switched to 50 ohms, the channel is good. Obviously, Ch1 calibration fails (there are also cal failures in Ch2 that displays the signal correctly though). Deleting the cal files to force it into uncal mode didn't change anything.

I've isolated the failure to a point that I'm sure it is on the DAC mainboard and is related to attenuator module connector J6. I've swapped attenuators and A/D hybrids, so I'm sure these are good. The signal is already distorted at the attenuator SMA output connector.

As by the service guide, in 1 Meg mode, a low frequency component is split off and amplified on the DAC mainboard, plus offset added, before it is fed back into the attenuator impedance converter. The problem appears to be in that part of the circuit. I have already swapped some components (U24, U25, U31, U32, K1, CR9, CR55) on the board, but still not located the faulty one.

I would appreciate any hints what might cause the distortion.
Could you please help me with a schematic diagram of the input circuit?