The only other thing the manual suggests is to remove the P6 connector and do the same check. If you see 15V with the P6 connector removed, that means a short on another board was pulling it down. I would also suggest another experiment removing the P6 and the P25 (to D-Board) together, then repeat the plug-in and voltage check: the reason is that on the schematic, the 15V goes to both these boards. You might also, for grins, check the TP7 probe point with the same power-up test, looking for 15V. If there's 15V there, it means they definitely didn't fix your board properly.
As for the 'big caps', they are C612 and C613, discharged by applying a 1-2kohm resistor across TP1 and TP2. Don't use too small a resistor or they'll arc and drastically shorten the life of the caps.