Krell KSA-50 problems

Hi

One of my friends has an original Krell KSA-50. There was a contact problem with the heat protection sensor.

After repairing the bad sensor contact, I decided to look at the square waves on both channels.

When feeding the amp with a 20khz square wave, I had a big surprize. I never saw something so bad as what I saw on both channels square waves.

This amp seems to oscillate or it is really sick. Placing a 1uF in parallel with the load shows that it doesn't like capacitive charges at all.

As this amp was made in 1983, someone told me that the output transistors and the power supply caps are worn.

What is the problem. On the picture, you see 20khz square wave with 8 ohm load. Both channels are showing that.
 

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