I had a problem on a 6632 psu : the volts/amps meters always read close to zero. Tracked it down to a faulty AD677 ADC and have fixed it - but there was a small capacitor soldered across the input pins, and I damaged it.
I think it's a standard production mod, but the digital board isn't in the service manual so the mod probably isn't in a service update either.
Anybody know what the value should be ?