Trace Elliot AH250 smx gp12 - oscillation on the gate of 2sk135 mosfets

[martyman4050]

Greetings..
I recently was playing my favorite Trace bass amp when I noticed a drop in the output.
The amp still worked but it just did not sound right and the heatsink became quite hot to the touch. I opened it up and did not see and obvious burnt components. I powered it up on the bench with a suitable dummy load and started probing around the output with a 1khz sine wave at the input. The DC on the output was about 40mv which did not appear excessive. The amp is working and the signal is getting amplified but when I looked at the gates of the 2sk135 output MOSFET I noticed a high frequency signal riding on the sine wave when the output is turned up just a little.(which I assume is causing the hot heatsink) The HF does not appear on the output or on the gates of the 2sj50's. What could cause this? I figured I would just replace the output transistors....any ideas? - thanks in advance - Martin

[loudthud]

Got a link to a schematic anywhere?

Are there resistors in series with the gates of those FETs very close to them? Do you see the HF signal on the sources of the FETs? Your scope probes could be causing the HF oscillation. You might try probing the gates with a 220 ohm resistor in series with the scope probe.

Is the bias current excessive? Are the FETs sharing the bias current equally?