Macrom 42.10 repair

[Mote]

This amp has been previously repaired. There are signs that 2 of output transistors failed and at least 2 PS FETs failed.

There are two TIP35Cs and two TIP36Cs per channel. Two banks of FETs each consisting of 5 FETs. Mine had one bank of BUZ71As and one bank of P16NF06s. I have another amp like this which works, confirmed that correct replacement for FETs would have been BUZ71A.

Broken amp had no gate voltage on either banks of FETs. PS chip is KIA76494P. Legs 9 and 10 had voltage on them, so I assumed that chip works and produces output. I replaced 4 transistors which were connected to legs 9 and 10 of 494, they were not shorted but they seemed to be open. FETs themselves seemed fine and gate resistors were also all in tolerance: 46-48 ohm.

Anyway I got the amp to power up and play clean audio for a few minutes and then I turned the volume up, amp started to draw current and stopped playing music. I had 25W current limiting lamp inline with B+.

Gate voltage from the bank with P16NF06s was gone. One of the small transistors which I just replaced had shorted.

First thing that comes into my mind is that I should probably get rid of the P16NF06s and use correct replacement? Or is there something else that could have caused FETs driver to fail?

[Perry Babin]

The P16NF06s aren't a good choice as a replacement for the originals but they shouldn't fail at low power. What was the original driver and what was the replacement driver you used?

[Mote]

Drivers were A1015 C1815 pair for each bank. I used also A1015 and C1815.
P16NF06s survived, only driver shorted.

[Perry Babin]

I've seen FETs that had intermittently shorted gates that could cause this sort of problem but that's rare. It's more likely that you had a defective driver.

I'd suggest replacing the drivers and power supply FETs with all new components.

[Mote]

Only two of BUZ71As in stock, have to order from Germany.
Meanwhile Ill try replacing drivers again and see what happens.

[Mote]

Put in new A1015 and C1815 from different batch (I have ordered them a few times) and amp worked for 5 minutes at low volume, then I cranked the volume up so far as the current limiting lamp let me. Amp worked for half an hour, then I removed current limiting lamp cranked the volume even higher and still no problems.

I guess it was a defective C1815.