Slowly clipping off the top.

[Axtman]

I am still working on the GK 200MK keyboard amplifier. I thought I had it repaired until this cropped up.

I put a sine wave into the input and dummy load with scope on the speaker output. The sine wave initially looks great but after the amp warms up for about 2 minutes the top of the sine wave slowly gets cut off. The flattening out of the top slowly starts creeping downward.

What are your thoughts about what could be causing this behavior?

[Enzo]

The output cannot source current into the positive half cycle into the load. When it is in this state, just disconnect the load and run without, does it still clip it off? Or does the sine wave restore to full cycle?

[J M Fahey]

Agree plus: POST-THE-SCHEMATIC

[dmeek]

I would guess the driver Q8 TIP29C or the 10 ohm resistor R224 to start with, assuming the power supply is OK.

[eschertron]

bias drift?
Is the scope set to AC or DC input?

[Mick Bailey]

Check if the positive rail drops to determine whether it's a supply or amp issue. If it is the amp then maybe localize the fault with freeze spray.