AFAIK it's pretty much the same piece of kit as the 465. If it's not a B rev, the service manual is here for the 465:
http://www.opweb.de/en/model.php?id=2968 - the other versions of the SM appear to be incomplete.
Diagnostic procedure on these (I have three 465B's and am juggling them a lot):
1. Check the rail voltages using the test points on the board. The range and tolerance is in the service manual. You can check ripple with a half decent multimeter set on AC volts.
2. Check all electrolytic and tantalum caps for shorts. There's a nasty wet tantalum at the base of the A4 board that likes to explode as well as some on the vertical board that like to snuff it.
3. Check all bridge rectifiers on the A4 board.
4. Check the MC1458 op amp at the base of the A4 board.
5. Check the power transistors at the rear of the unit for shorts.
The 15v rail is referenced on the 55v rail so make sure that's working first.