Mark Bass SMPS ripple sound

[MoonsTaBass]

Hi there !

I'm new here, I'm from belgium so please excuse my English

A Costumer brought me a MarkBass 250 BlackLine that has a constant "hissing" sound in the power amp.
It comes from the power supply.... It's kind of a ripple smps sound.

First I've checked all the filter caps --> Good. Coupling cap between power transistor and transformer (C68) --> Good
Checked the oscillator resistors and caps (R74, C52, C51,...) All good
So I've tried to replace the oscillator (21531D) just in case --> It's changes the frequency sound but still there.

The thing is, when I unplug the Fan (+18v), the sound change a little bit too... strange.

I'm not a noobie but I don't have a lot of experience in SMPS and class D amp so... Do you have any ideas for me ?
In this profession, all opinions, advice and experiences are good to take to evolve

Thanks a lot !!

[MoonsTaBass]

And I forgot to say...

The sound is audible without a speaker pluged in.
You can hear the sound directly from the transformer...
So it's what i thought, maybe the transformer is dead, maybe a bad coil turn ??

[Jon Snell]

On the SMPS Transformer primary, (not shown on your basic schematic) there is a tuning circuit. Usually a pair of diodes for power factor correction and a CR network.
If there is an issue with any of that, the transformer can whistle.
As it is in a guitar amplifier, most people accept it as normal and ignore it.

[dmeek]

A few times I have found C68 1uF/250V failed in this model. Never heard any hissing though.