[Mondial]

As far as repairing the HP 8656B, the difficulty will be getting parts since it is no longer supported by Agilent or its new successor. These generators use custom hybrid microcircuits in the output stage, so without the replacement parts repair is next to impossible. I believe this generator uses a PIN diode switched attenuator at the output, which may be where the problem is.

Probably the best solution would be to find a inexpensive parts 8656B on ebay, and use it to interchange modules to localize the problem.