Help with Allen & Heath console noise troubleshooting (link to video)

[angelo2979]

Hello!
would you guys help me reasoning what is happening with this Allen heath gl4000 mixer?
I made a video to better show the problem. It’s a noise buzzing in all the channels and master section constantly, but with some rises at moments (if you turn up the volume you can hear it on the video).

https://youtu.be/fkKeS8yi6yM

I got this mixer for cheap, and it’s really old, but the strange thing is that I gave it an extensive try at the buyer’s place measuring every channel and function with white noise and a spectrum analyzer and it was damn quiet. The buyer said he had the ground lifted on his mains power though. When I got the mixer in my studio I found this noise showing up magically, I tried to lift the ground too and it lowered a bit but the noise is still there.
Electrical power in my room is not stabilized or filtered, but I would not think it’s impacting so much.

I am no electrical engineer but in my ignorance i was trying to figure it out. I opened up the power supply unit to see if the condensers were consumed, but they were not swollen or bad looking at all. I don’t know if that means I still have to replace them. But I’m also thinking there might be some ground loop somewhere else.

Overall the mixer looks functional, many faders are hard to move like they need some deep cleaning or replacement, but they work. Only one channel and a couple of groups are not passing signal, but everything else looks good.

Do you have any suggestions about what should I look for? It would be great to have some kind of idea or strategy before I open it up.

Thank you!

 

[ruffrecords]

Never lift the ground on a mains connection unless you have a death wish. Whatever your problem is it is not to do with the mains safety earth.

Your problem is most likely due to a lost ground connection somewhere inside the mixer itself. I do not know if this is a modular mixer but if it is, removing and re-seating modules can often help by wiping connector contacts.

Cheers

ian

 

[scott2000]

Almost sounds like dimmers being used nearby.....but maybe that would've shown itself before?/

 

[angelo2979]

I would exclude dimmers as I tried various sockets and rooms where the problem never showed up with other equipment.

yeah, It may be a ghost ground connection, like if some connector got loose during transport (?). I cautiously tried to lift the ground just because the buyer had it lifted at his place, and it was quiet there. Lifting made it a bit better but didn't solve it, and I of course know how dangerous this is.

So I will check channel to channel connectors.
would you suggest replacing the capacitors inside the power supply even if they don't look swollen?

thank you!

 

[ruffrecords]

would you suggest replacing the capacitors inside the power supply even if they don't look swollen?

thank you!

If it was dead quiet at the other guy's place and hums at yours then it is most likely something got wobbled during the journey. Caps are unlikely to have been affected by the journey so I see no reason to change them yet. Since you can get inside the PSU it might be worth sticking a scope across one of the supplies while the mixer is powered up to see if there is any ripple or noise there.

Cheers

Ian

 

[angelo2979]

Great! I suspect there might be some connection missing inside the board... I'll take it through a deep inspection, thanks!

 

[Matt Syson]

I forget which A+H model now but following complaints of it 'humming' removing the power supply to more than 6 feet (2 metres) away removed practically all the hum. The supply radiated quite badly (magnetically) and the physical latout of the bussing/grounding inside the desk is poor so allowing hum to get in. This doesn't mean that something hasn't fallen off but a couple of things to try before getting a screwdriver out. Again, DO NOT disconnect mains earths.

 

[Matt Syson]

The supply had simply been on the floor under the desk itself.