Interlude IL60

[Beauvais]

qwertdyukg

[jener8tionx]

Posting to help anyone else trying to fix their IL60 plate amp. While I will admit this basically just follows what the service manual says, here were my steps:

Power Board:
1. Replace the two IRF740 mosfets. I also went ahead and replaced the 22 ohm resistors and the 422 ohm resistors. On one amp I also had to replace the in5242b diodes. Replace the IRF640 mosfets. Visually check the rest of the board for damages components and replace as necessary.
2. Test the power board as described in the service manual and verify an output of ~18-19v.

Linear Board (Output board)
3.First, screw both bias pots closed (counter clockwise).
4. Remove the linear board and test each mosfet for shorts. If any have a short, replace all four. (2 x IRF9540 and 2 x IRF640). For the IRF9540, the part is obsolete, but I found some "Bojack" brand ones on Amazon. I do not recommend mosfets from unverified sources, but they are working for me. Your mileage may vary.

Testing
5. Be sure to use some thermal compound when you put the boards back together and use the transistor insulators where they were at disassembly.
6. Follow the bias procedure. I had 0.0mV when the pots were both counter clockwise. You will see that they are very sensitive, so "eyeballing" this doesn't work. If you are biased to high, you will blow the new transistors and have to start over (ask me how I know!).

Good luck!