Quoted from Crash:
Why would a bridge rectifier be problematic? It will put out the same dirty power that is fed into it from the transformer, which in turn gets its dirty power directly from the wall. This noise in general is what line power sounds like. The normal 60Hz hum plus a lot of line noise caused by power lines picking up other sources of radiation, radio frequencies, etc. The key here should be to feed the amplifier with clean DC power, and re-capping the sound board does this by conditioning the pulsed DC power coming from the rectifier.
I believe the suggestion was that the rectifier is bad/dead. In other words there is no -12 being supplied.
Quoted from castlesteve:
The sound board will work without it, but it will sound like shit and hum. Easy to test.