Hi Paul,

I've had something very similar a few years ago on my S12, noise like you describe, aka crackling and intermittent "shhhh" noise started to be audible on the 2 bus.

In my case the noise was only disappearing if I muted the master program channels.

It turned out to be due to some filtering caps getting too old, a good number of them having actually started leaking on several channel's output card.

The problem totally disappeared after I spent 3 weeks recapping all the console's channels.

So maybe it is time to think about checking that on your console if it hasn't been recapped by Harrison before you got it ?

Cheers
M

Do you mean swapping every channel cards from the console for spare channels ?

What you can try is :

- get all the channels out (just deplug them from the carrier, you don't need to remove them completely of course),just keep the program cards in.
Is the noise still present ?

- if it is gone, try to put the channels back in progressively, like 4 by 4 (you need to power the console down and up to do this I think), and see what happens.
If it comes back at some point, then you might have one or several channels' output card's caps to check...

If the noise comes from the same problem that I'd had, then it is normal that is stays when the channels are muted, as I guess the mute cuts before the output card of the channel.

I would recommend in this case that you replace every channels' electrolytic caps, but beware, there are like 4000 of them on 64 channels !

Let us know what you find !

Yes exactly, the mute is actually somehow happening before the assignment/output board, so if you've got bad caps there, you're going to get noise on the bus even with the channel muted.

Look at the picture how bad things can go. See what the leakage did to the board above U42 ?

Fortunately I didn't get too many of these, and even most of them are actually still working fine after recapping, but when I recapped the channels I did found a number of bad looking caps with the green stuff on their base, all of which were in this section of the assignment boards.

And I would not have seen they were in that shape, had I not desoldered them.

Which is why I think it's better to replace every electrolytic caps on the channels if you find this is the cause of your noise problem.

Which makes me think of another test you could try : what happens if you route your channels to another pair of busses, and listen to that without going through the L-R bus ?

If you have no way to directly listen to another bus output in your studio, you can try this by simply de-plugging, on the channel rack's rear side, the small ribbon that's going to the program summing board, and then re-plugging it to the corresponding busses connector.

See what I'm talking about ? Let me know if you need more details, I can take a picture to show you the area and connectors if needed.

This way you'll be able to route your channels to for example bus 1-2, and then have bus 1-2 go through the L-R Pgm channels, which means you will actually replace the L-R summing bus by the 1-2 bus.

Which would have the double benefit to help you confirm whether or not you have the same problem that I had, AND it might give you a workaround so you can work with no noise in the meantime (it took me over 20 full time days to recap the whole channels, and I have less than you... ). I say it might, because when I figured out what the problem was, I noticed that I was having the noise problem only on the Pgm busses, not on the regular ones, the dead caps being on the Pgm part of the assignment board (which is pretty logical, as, unlike the other busses, they work all the time).

So, maybe you are in the same scenario ? At least hopefully this should help you figure this out !

By the way, what's the value on those caps? And is it the same for all those electrolytics over the whole channel board?