I've got a Trident S65 desk that's been rock solid for me for the past 4 years, but now the master channel is starting to act funny.
When I engange the "mix"-switch on the masterstrip a lot of white noise appears. The noise increases/decreases with the master fader volume, but at odd intervals - the noise is louder at some lower settings than at higher settings (e.g. louder at -20 than -15 and a few other places).
I've tried changing the master fader, didn't work! I've tried changing all the IC's, didn't work either! Is there anything I'm missing here?
well actually, you did not mention if you checked or replaced the mix switch. It happens when you engage it so, naturally it is what provokes the problem. Now I have no idea what that switch is, but if the board os more than 10 years old, you're bound to have issue with dirt/carbon buildup.
I say clean or replace the switch.
Mind you it could be other things also. I am surprised noone else has offered their expert opinion. that's why I decided to voice my "expert" opinion...
Do you have anything inserted on the masterbuss while this is happening?
Does it do this only when channels are assigned to L/R, or does it also do it when no channels are assigned?
It could be an issue with the master summing amp.
Is the noise in both the left and right channels, or just one side?
Has this console ever been recapped?
Dried out electrolytics can do weird things sometimes, like causing shorts and causing noise like you described.
A bad switch can cause scratchy or poppy noises when it's being engaged, but it wouldn't cause the issue you're having. I'm assuming the mix switch is the source switch for the monitor outs, so engaging it would send your master buss signal to the monitors, correct? That means the noise is coming from the master buss, not the switch.
Try and isolate where the problem is coming from. If the channels and subgroups are quiet (check via direct outs) then the problem is in the master channel.
Do you have an oscilliscope? You could trace through the circuit until you find the problem.
Those consoles suffer from notoriously bad solder connections on the motherboard pins. I would definitely check there and also check and possibly retension the metal tabs inside the molex connectors. Any series 65 that has not been recapped by now is LONG overdue for this and as mentioned already, shot caps cause all sorts of nastys. Good luck. BTW a recap of the power supply is also needed. The caps that were installed new were junk... sorry, I had one of these boards for years... I learned all it's shortcomings.