[AudioIngenia]

Here is a Studer power supply.
There is a background whistle on all console buses, tuned to 6.8 kHz.
I have tested with the laboratory PSU and the sound is clean.
The problem seems to come from the PSU, but I do not find the oscillation.
I have replaced all the condesadores, the two integrated circuits and checked the rest of the components.
Any suggestions?

https://ibb.co/gO7UuU

[AudioIngenia]

I just checked, with the original power supply , in addition to tone also a lot of background noise.
I have also checked with this:
https://szpcsy.en.made-in-china.com/product/qytmkTJxTlcA/China-48V-0-25A-15V2-5A-15V2-5A-triple-output-switching-power-supply-for-audio-equipment.html
With the Chinese psu, the sound is clean
 

[mjrippe]

Any suggestions?

Yes, get rid of the Studer supply and use the one from China    Seriously though, the last 169 I worked on it was easier and less expensive to replace the PSU.

[AudioIngenia]

Yes, get rid of the Studer supply and use the one from China    Seriously though, the last 169 I worked on it was easier and less expensive to replace the PSU.

PSU replaced by another? What brand and model?

[moamps]

The problem seems to come from the PSU, but I do not find the oscillation.

How did you test it, under the load or without?
Do you have a scope?
BTW, where did you bought the Chinese PSU?

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