NAD 7240 PE Multiple Transistors Dead

[Rocks]

I haven't abandoned the thread yet. Just a few comments and questions while I wait for my transistors. I think they are literally on a slow boat from China.
- I noticed in the schematic that my R461 and R463 are not 3.3 ohm resisitors but just jumpers. I assume the jumpers are more recent since the location is marked for resistors. Any idea what these are for?
- I'm planning to build a dummy load so I can test it without blowing my speakers if I get it working. I've seen some posts saying to mount the resistors on a heat sink but don't they already have a heat sink?

 

[Rocks]

This is an example of where comparing the signatures of the good channel with the bad one using a Huntron Tracker™ might be very productive.

I haven't repaired a lot of DC-coupled SS amps with blown channels like this, but the few I have seen have leakage or other "one signature doesn't look like the other" subtle problem, typically in the transistors at the very beginning of the amplifier stage.

I don't doubt what you're saying, but this is out of my budget. If I decide to take this hobby more seriously, I'll think about it.

 

[Rocks]

Latest. Lucky I bought several transistors. After again checking with my dim bulb tester, powered up ok. Checked power supplies (lower than nominal but normal), I decided to connect power cord directly and measure some voltages. Lasted about 30 seconds and then same as last time. Q417, Q419, Q421, Q423, Q425, Q427, Q429, R459, R467 toast, but this time I noticed C421 also looked swollen and sure enough it was shorted. OK. replaced everything again. C421 replaced with a 50V one.

This time I powered up with dim bulb and checked all voltages on the schematic. All voltages were good. Only thing is that emitter of Q428 on RH channel seems rock steady with a small ~2Hz ripple on it. Emitter of Q427 on LH channel is a little more erratic but the jumps are in the order of a few millivolts. Is this because I haven't adjusted bias current yet?

Decided to try a 60W bulb which increased the high voltage power supply from ~43V to ~55V. Everything still good. Decided to plug the amp in without the dim bulb tester but first removed Q431 and Q433 to try and save some transistors. All still measured good.

I realized I had installed a 2SD1062 in Q433 per schematic on the initial repair, versus BD911 per the parts list. Q431 is a BD912. I though they were equivalent but data sheets show they are very different. More transistors on order....

I think this will end with me throwing this thing off a rooftop, but hey, I'm having fun. Yes, I am aware of the definition of insanity...

Outstanding questions if anyone can shed some light:
- Schematic shows a 3.3 ohm resistor in series with o/p transistor base (R461, 463). My PCB has jumpers but they look factory installed. Are these important?
- Could my mismatch of Q433 and Q 431 (2SD1062, and BD912) have any significance?
- I can't find other versions of the schematic should I be using Q431, 433 per schematic or parts list? The BD911/912 seems to have better ratings.
- erratic offset at emitter of Q427? I tapped and moved pots for offset and bias adjustment to see if things would go unstable but this doesn't seem to have any effect. They are 10 turn trimmer pots and resistance measures good with an ohmmeter. Maybe I'll try a fixed-resistor replacement just to se if it stabilizes.
- My amp is serial number A740AK 26306, manufactured in Singapore if that helps.