switching power supply problems (ADA MicroTube 200)

I'm trying to repair an ADA MicroTube 200 that goes into protection mode.

At first, I thought it was the right channel, and the amp would work if I unplugged that side.
I went ahead and replaced the power MOSFETs and driver transistors, but now it stays in protection mode even with both sides unplugged.

I'm not familiar with switching power supplies. Can anyone help me with what to look for and measure?

It uses an SG3524

Thanks!!

What makes you think it is the power supply?

If the amp is going into protection mode then the supervisory circuitry is working, hence the power supply is working.

To make sure that the problem is the power supply, you have to isolate it completely from the rest fo the equipment and test it on its own.

Getting a voltage reading on the output without a load can be completely misleading. Connect a suitably rated bulb and see if it is lighting up the way it should. Then read the voltage. If there is a significant voltage drop or no voltage then the psu is the problem. And with the absence of the schematic, all I can advice is to start checking the components one by one, making your way from the output to the input. First actives, then passives. Don't interfere with the inductors, nothing goes wrong with them unless they are tampered with.

Be aware of the high voltage discharge of the caps hence electric shock.

Thanks sahib.

Is there an simple way to test MOSFETs in the circuit?
(Such as the diode tests for a transistor)

Found a bad diode in the reference circuit, but the replacements go bad.

D8 (1N4148) tested bad. I replaced it and the unit worked fine for a while, then gradually went bad.
D8 and D12 (another 1N4148) were both bad this time.

I replaced them with 1N914A, which has very similar characteristics (100V, 200 mA, 4ns fast recovery).

Is it important to order the correct diodes (1N4148), and not something similar (1N914)?

Could there something else wrong?

Unfortunately, there's a lot of wires and stuff on the board, which makes it difficult to lift it and check the electrolytics.

I suspect the fast switching  frequency (I calculate 125 KHz) may be a challenge.

Schematics are here
http://www.adadepot.com/adagear/gearpages/poweramps/ADA-Microtube-200.htm


Thanks!!

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