Soundcraft 2400 suddenly very noisy

[sircletus]

Greetings, all.  So I was mixing a project earlier today on my trusty and crusty ol' 2400.  Everything was going swimmingly.  That is, until I returned from a two hour-ish break.  I turned up my monitor controller and noticed that my console was suddenly VERY noisy.  White noise.  On all channels and busses.  Everything is passing signal properly, all sends/returns (channels, groups and masters), auxes and groups functioning normally, but noise everywhere, out of the blue.  No shot noise that I've noticed, no single-frequency oscillation, just PSSSSHHHHHHH.

The effect is cumulative; muting any single channel or group does not eliminate the noise; it seems to be everywhere in approximately equal amounts.

Something shorting to ground somewhere?  Power issues?  Any ideas where to start looking?  It would be extra nice if I didn't have to strike this almost-complete mix.

Thanks in advance for the advice!

 

[abbey road d enfer]

Can you swap PSU's? that's where I would start.
OTOH it could be just one channel dumping dirt in the whole system but I doubt it.
I suspect the PFL bus is noisy too...?
What does PFL tells you? Does it detect pre-fade noise? If not, that would mean that only bus are affected.

 

[sircletus]

I do not have an extra PSU handy, but it's been on my "rebuild" list for awhile.  Last year the -17V rail and 24V rail both gave out, and I rebuilt those, but foolishly left the +17V rail untouched.  So that could be a problem.

Today's exercise will be to fully remove one channel at a time, then one group at a time, and see if it's the "one channel dumping noise into the the mix bus" scenario, though I'm doubting that: If I mute all channels, the mix bus is quiet.  Basically, it seems that each individual channel has suddenly gotten noisier.

I'm not using the console's built-in monitoring facilities, so the PFL bus isn't telling me much of anything at this point.  I'll give that a listen.

Thanks for the advice, much appreciated!

 

[bruno2000]

I would put a scope on the power rails first......
Best,
Bruno2000

 

[abbey road d enfer]

I would put a scope on the power rails first......
Best,
Bruno2000

+1

 

[pucho812]

if it's across all channels then it's s0omething common to all channels.  Start in on the PSU

 

[sircletus]

Guess I'll be going to work to grab a scope!

 

[sircletus]

Sure enough, positive audio rail.  I had apparently ordered a pair of TIP2955s a year ago and never bothered to swap 'em, so I just did.  While I was in there I also replaced all electrolytic caps I hadn't touched yet.  And now it's quiet as a mouse.  Let's hope it stays that way...