[audiomm]

OK so you've narrowed down the fault to the Disc Input Board. I notice you've replaced all electrolytics in the set? but apart from re-checking that the correct values were used, have you double checked one isn't back to front?
In the service manual, Quad recommend...."If Radio & Tape replay are satisfactory, but not Disc, try another position of the Disc Adaptor Board. If this has no effect other than that expected and another Pickup is not available, plug a radio Tuner into the Disc Input with the adaptor board in the C1 position, to prove whether the fault lies in the pickup or the Control Unit.If the latter, the Disc Pre-amplifier Board (M12019) is suspect".......
That should narrow down where the fault is likely to be.
Actually a sig. gen and a 'scope would be really handy in a case like this as you can actually see what is going on.
When setting up the balance control for equal output at usual listening levels, I use a sine wave at say 1khz into both channels(use Radio or Tape input) & monitor the output on the 'scope. Then I invert one scope channel and sum the 2 channels together. Result will be a straight line if both channels are indentical - invariably this generally doesn't happen but adjusting the balance control will make it so. Then you loosen the screws holding the connecting link to the shaft of the balance control and set the slider in the centre (without moving the actual control), make any minor adjustments to retain the null output and retighten the screws. Note that the dual gang vol. control is unlikely to have matched sections so the above adjustment will, in practice, be correct in only one spot.
Mike