Hum in mix bus - PSU recap time or something else?

[kilmister]

Hey folks!

I'm servicing a clients 4k G+ Ultimation console which has a low freq hum (or brum as we say) in a mix bus but I can't find where it's coming. Obviously first thing to suspect is a PSU. I recapped control cards in it and it took noise a little away but still when I turn monitoring volume full open there is very clear bruming sound. I couldn't measure any ripple from any voltage rail with my Fluke scopemeter.

To be more specific I measuerd noise floor from Quad bus and from monitor LS with my AP System One (Z Out 40Ohm, Z In 100kOhm, BW 10 to 500kHz, RMS). Result was -56dBu from both points. Not too good.

Also if I cut signal way in patch panel by pluging dummy plug in Main Insert noise is gone. Also any stereo source nor Cue bus doesen't have this brumming noise only slight hiss which is normal to me.

All help is very much appreciated!

Cheers,
Paavo

[Matt Sartori]

never had this problem before but I am suspecting that your assumption is correct.especially because is sounds like its coming from the master buss.
a couple of things:
can you try and measure ripple right at the PSU outputs before it hits the console?
what size console is it and how many psus?
does it have a changeover units also plugged in?

I doubt the two big caps in the +-20V rail would be damaged they last forever and ever...but you never know...

Mattia.

[kilmister]

Thanks for your input Mattia.

Console has 40 channels and 1 PSU.
All regulated rails are ripple free. At the big bastards theres's ripple of 470mVrms.
I'm confused because mixbuss seems to be only noisy place. Line In to Group Out chain doesen't have hum only tiny hiss. I would belive that if the PSU is source of noise, it should pop up from another audio chains?

As a general note, Mix amp audio caps aren't feeling too well, neither some eq caps.

Cheers,
Paavo

[Matt Sartori]

Thanks for your input Mattia.

Console has 40 channels and 1 PSU.
All regulated rails are ripple free. At the big bastards theres's ripple of 470mVrms.
I'm confused because mixbuss seems to be only noisy place. Line In to Group Out chain doesen't have hum only tiny hiss. I would belive that if the PSU is source of noise, it should pop up from another audio chains?

As a general note, Mix amp audio caps aren't feeling too well, neither some eq caps.

Cheers,
Paavo

470mV it's almost half a volt...it's quite a bit! I am confused now.... as a matter of good practice (and to same money) I would start by recapping center section cards and then see what happens....interesting..

Mattia.

[kilmister]

Yep half a volt ripple at unregulated side. Regulated side 0.000Vac
Is ripple leaking somehow trought the regulation? 

-Paavo

[kilmister]

More testing...

I have a spare PSU which I plugged in. Same noise and ripple at unregulated side around 400mVrms.
As a new notice I found that when I shut down even one bay I'm gettting first sound like turbin engine which is shutting down followed by motor boating... As Mattia suggested I start to recap master cards and see what happens then.

Cheers,
Paavo

[Matt Sartori]

Yep half a volt ripple at unregulated side. Regulated side 0.000Vac
Is ripple leaking somehow trought the regulation? 

-Paavo

right, my bad! I doubt there would be a leak after the regulation, and if you have 0mV ripple for all rails on the other side then it's all good!
if there is no ripple there there must be something else in the center section playing up, I am with you suspecting caps... the fact that it goes away when you dead patch Master INS RTN it's a good indication of the fact that the hum it's either in the center section or it gets to it.
does the hum changes with faders up or down?

another thing that just popped to mind:
there are 5 axial capacitors right at the back of the PSU between the output connector and the small regulating cards,these are the last step between the regulated PSU and the console. do they measure ok (ESR)?

Mattia.

[Matt Sartori]

More testing...

I have a spare PSU which I plugged in. Same noise and ripple at unregulated side around 400mVrms.
As a new notice I found that when I shut down even one bay I'm gettting first sound like turbin engine which is shutting down followed by motor boating... As Mattia suggested I start to recap master cards and see what happens then.

Cheers,
Paavo

ok you replied while I was posting, interesting: center section it is then! 

Mattia.

[kilmister]

Noise is generated somewhere in quad bus. For a test I recapped one vca board's (82026) Power rails "Main Insert Send" signal chain which is first thing that quad bus hits at the center section. Didn't help.

I unloaded the console completly and started to put it back together channel by channel (well I vacuum cleaned inside first. 7 year dust, duh!). Now I have 24 channels back in and noise level has dropped by 10dB. The level was same all the time when I added channels in bays.

The weird thing still happens if I shut down one or two bays out of three. There's this very annoying and long noisy sound which obviously comes from uncharging channels/caps. All bays on, no problem. All bays shutted off, slight motor boating. Ideas?

Cheers,
Paavo

[StefanNowak]

Have you measured mains power to the PSU?

I used to get 100hz through a console if the power dropped below 220v here in Sydney.

You've checked the rails I assume?

S

[kilmister]

No I haven't. We have very stable mains here in Helsinki. Nominal voltage is 230VAC and can't remember it has dropper since... hmm, it hasn't :-)
edit: I've measured power rails from center section and from psu. No ripple at all.

-Paavo

[kilmister]

A bit more observations regarding "bay shutdown" noise.
I got rid of it when I unplugged S160 ribbon from fader to vca card, or ribbon to Film Pan card or drive stage 5532 (IC5 and IC6) after the pan pot. I was also able to probe noise at these ICs. Wave has only positive half side, nothing happens at negative side. Taking group amp card out didn't affect at all.

Cheers,
Paavo

[kilmister]

Finally found something.
Film Pan mod wasn't perfect. There was about 1/3 of the channels that had Power 0V wire at wrong spot thus leaving +/-18V floating at Pan card. Good thing is I got rid of "bay shutdown" noise. Bad news is there's still plenty of 100Hz hum. When I measure noise from main outs the level is -70dBu. It was pretty same when console didn't had any channels in. If I'm not totally wrong noise floor should be somewhere between -85 to -90dB. From group out I've measured -87dB. I belive there is some another problem with grounding still somewhere. Console is completly unplugged from rest of the studio equipments.

By any change does anyone have grounding schema or can anyone please explain it? Couldn't find too much info from my Service Manuals. Mix Bus 0V is little confusing.

Cheers,
Paavo

[sintech]

From Jim Lassens Archive, this is the best example of the console grounding scheme I've seen:

[sintech]

I'd be interested to know where your S3 and S4 Screens/Ribbon Drains are connected? In that diagram it suggests the PSU earth bus, but mostly they've been connected to the Bucket metalwork.