Locating acoustic noise.

[forrestc]

In the continuing saga of the Valhalla 2701c that I'm repairing, I've now replaced all of the larger electrolytic caps and the unit seems to be perfectly functional.

However,  it does emit a low frequency acoustic hum.  Sounds like capacitor noise or maybe a shorted turn on a transformer (but much more like some piezoelectric effect).  It's loud enough that I'm wanting to track down the source in case it is being caused by something that needs fixing.  Unfortunately it is low frequency so it isn't very directional,  and my typical tricks aren't helping find the source.   I'm also not eager to go randomly sticking my fingers on components which are in a device which has enough energy to make it a very bad day for me.

I've tried various acoustical tricks such as covering part of the circuit with foam or listening using a tube.  The sound is so non directional that I haven't been able to isolate it yet.   

As I'm writing this it just occurred to me that I might be able to tape a microphone element to a stick to form a acoustic pickup...  if I can find an appropriate transducer I'm going to try that next.

I'm asking on here with the hope that there's some tool or procedure which I'm not aware of,  like a stethoscope for this type of thing,  or something else similar.

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[Whales]

Indeed LF is a pain to track down.  It'll couple well through the whole unit, so even with a directional mic or listening stick you'll be chasing phantoms.

Can you desolder components or pull leads to disconnect parts of the device from power?  ie isolate things bit by bit?

[wraper]

I've seen this with Agilent PSUs. Basically magnetic flux from the beefy transformer causes the steel cover vibrating. Remove the cover and there is no hum at all, make cover loose and it starts vibrating like hell.

[forrestc]

I've seen this with Agilent PSUs. Basically magnetic flux from the beefy transformer causes the steel cover vibrating. Remove the cover and there is no hum at all, make cover loose and it starts vibrating like hell.

My guess here is that the "switchmode power supply" which runs the HV supply is causing an interaction in the electrolytics.  The noise sounds like it might be coming from that direction - but with LF you never know.   The severity of the noise gets worse on higher voltages.

Although in that corner there's also the transformers, which might also be the cause...

-forrest

[cdev]

a stereo microphone feeding into a scope with one channel going into x and the other into y might be helpful, or maybe similar with magnetic coils crossed? (I have a similar power supply problem and similarly have not been able to track down the source, may try this next)

[wraper]

My guess here is that the "switchmode power supply" which runs the HV supply is causing an interaction in the electrolytics.

Electrolytic capacitors won't emit any sound, even if you try them to do this. You can get high frequency noise from ceramic capacitors if there is a significant ripple current. It's nearly impossible to get any low frequency hum from parts other than transformers or big coils.