ghost power supply buzz?

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dwelle
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ghost power supply buzz?

Post by dwelle » Sun Feb 20, 2011 4:07 pm

the power supply to my soundcraft ghost (CPS 275, the beefier one) has developed quite a buzz. there isn't much in there, and the buzz is coming from the large transformer (big fucking coil) inside. it begins as soon as i power up, and stays around no matter what working condition, as long as it's on. i'm in a smaller control room now than i used to be, but i don't remember this PS making any noise in my old place (had the board for about 4 years). further, where it buzzes constantly now, it used to be quiet when powered up and develop the buzz after it had been on awhile.

so for you ghost users/smart electronics people out there, how can i clean this thing up? i don't hear it in the audio path at all, just directly from my rack where the power supply sits. but it is loud enough that is has become really annoying.

any ideas?

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Post by goldie469 » Sun Feb 20, 2011 4:18 pm

Im not an expert on electronics, but if its coming from the unit itself, it is going to be a physical vibration

Check for loose parts (screws, casing, etc)
Tighten up or dampen with something

Disclaimer: Do this at your own risk.

dwelle
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Post by dwelle » Sun Feb 20, 2011 4:58 pm

roger that.

i'm fairly savvy with electronics myself, and was hoping when i opened the box up that a simple physical vibration from something loose would be the issue, it isn't.

there is a big transformer inside, and it's buzzing like mad. i can't isolate or dampen it, this coil itself is buzzing.

this unit has a fan, and the buzz is like 4 times louder than the fan. the fan i can live with, the buzz not so much...

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Post by goldie469 » Sun Feb 20, 2011 5:41 pm

hrmm...
bear in mind i've never seen a ghost power supply, but if possible you could try unscrewing and remounting the offending xfo?

just clutching at straws really, but might be worth a shot

if that doesnt do it, im stumped.

hope you get this fixed

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T-rex
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Post by T-rex » Sun Feb 20, 2011 6:07 pm

You should contact those sound creation labs guys in nashville, they could probably hook you right up with at least some info on what it is our what it would take to fix.

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Scodiddly
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Post by Scodiddly » Sun Feb 20, 2011 7:28 pm

The bridge rectifier is about to short.

I've seen this happen in several of those power supplies. Good news is that it's a fairly cheap fix, basically sourcing replacement rectifier(s) (I think there's two?) and removing a fair number of screws to get the heat sink out.

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Post by dwelle » Sun Feb 20, 2011 7:32 pm

whoa, crazy repost there.

thanks, scodiddly! you rule, i knew someone here would have an answer.

off to find schematics...

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Post by dwelle » Sun Feb 20, 2011 7:46 pm

ok, quick question. there appear to be two, one on the +17v system, and another on the 48v for the phantom power. scodiddly, in your experience, is it one over the other that usually goes out? and is there any way to isolate which one is problematic, or should i just change out both. hell, i don't even use the phantom power from my board, do i even need the 48v side?

hmmm...

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Post by Scodiddly » Sun Feb 20, 2011 8:08 pm

It's probably the on the 17v rails. But go ahead and change them both, and use heatsink grease.

It's leakage, precursor to a full short, that's putting a load on the transformer and making it buzz. Doesn't matter if you use that output or not.

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Re:

Post by twheasley » Wed Nov 04, 2020 6:34 am

dwelle wrote:
Sun Feb 20, 2011 7:30 pm
roger that.

i'm fairly savvy with electronics myself, and was hoping when i opened the box up that a simple physical vibration from something loose would be the issue, it isn't.

there is a big transformer inside, and it's buzzing like mad. i can't isolate or dampen it, this coil itself is buzzing.

this unit has a fan, and the buzz is like 4 times louder than the fan. the fan i can live with, the buzz not so much...
I’m having what seems like the same issue with my power supply (CPS-275), and I’m wondering if you ever discovered a solution. Did replacing the bridge rectifiers help?

I’m not too savvy myself, but I did open things up to track down the location of the sound, and it was certainly the transformer. With the console disconnected, the unit runs quietly, but when connected, the buzzing is present. I checked that it was secured well to the case to make sure it wasn’t a physical vibration.

I took the unit down to the local electronics repair shop to have them inspect it and asked about the rectifiers. They looked it over and weren’t able to find anything wrong. Unfortunately I wasn’t able to bring the console down, so they weren’t able to run it with the load connected.

Incidentally, I did replace the fan with a unit that was recommended in another post (Noctua NF-A8 FLX). That has made a big difference, but now I notice the buzzing even more :(

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Re: ghost power supply buzz?

Post by Scodiddly » Wed Nov 04, 2020 1:08 pm

The bridge rectifiers are going bad.

That's the problem, even if they don't yet measure obviously bad.

I've done a few of these.

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Re: ghost power supply buzz?

Post by twheasley » Thu Nov 05, 2020 6:06 am

Scodiddly wrote:
Wed Nov 04, 2020 1:08 pm
The bridge rectifiers are going bad.

That's the problem, even if they don't yet measure obviously bad.

I've done a few of these.
Great to know. I’ll give it a shot and report back. Thanks!

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Re: ghost power supply buzz?

Post by twheasley » Fri Nov 13, 2020 8:47 pm

Success!

I just swapped out the bridge rectifiers tonight, and the buzz is essentially gone. Hardly audible if I put my ear right next to the transformer.

Incidentally, there were two different rectifiers in my unit, so I suspect the previous owner swapped one out. To be safe, I ordered two of each spec (as well as fuses while I was at it just in case). I'm using the replacement for the one that seemed like it hadn't been removed before (GBPC1502), but that's just a hunch.

https://www.mouser.com/ProductDetail/512-GBPC1502

The other (AP MB352) seems to be discontinued, so I ordered its replacement.

https://www.mouser.com/ProductDetail/583-MP352

The difference seems to be the average rectified forward current. I have the 15A installed, but I also have 35A in case that's a better option. This is out of my wheelhouse, so I'd appreciate advice. I understand the concept of a bridge rectifier, but I'm not sure which current rating is a good match.

Thanks for the help. The silence is golden!
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Scodiddly
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Re: ghost power supply buzz?

Post by Scodiddly » Sat Nov 14, 2020 6:28 am

Told ya. 8)

Current rating is basically just buying whatever has a high enough current rating to pass the amount of current you'll need, worst case. Obviously higher current rating devices are bigger, more expensive, etc. so generally you buy just a little bigger than what you'll need.

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Re: ghost power supply buzz?

Post by twheasley » Sat Nov 14, 2020 9:02 am

Great to know. Thanks again! :D

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