Nad 310 Gone Wrong

[Kajman]

I have got a faulty NAD 310 which has given me sleepless nights.

Firstly the amp blew a mosfet (It was blowing the fuse on one channel)which I replaced and it worked fine and I run it in for a week without any problems. I sold the amp to save for a new only to be told it was not working when connected when the guy who bought it took it home.I was not worried thinking I would find the fault easily. How wrong I was.

The problem I have now is that the green LED comes on for a few seconds then off. The amp is not blowing any fuses at all. I am getting a weak and distorted output from both channels. I have checked and replaced many components even if they test ok. The only faulty ones I have found so far are R244 2.2OHM AND R220 3.3K. The only anomaly I can find with a meter is that the voltage on Q102 and Q104 base pins is 23V and not 10v and 23V again instead of 11.3V on the emitter pin of Q104. I have removed and tested the 2 transistors and they are ok. I replaced them with new ones anyway but still no joy.

Its not economical to have this repaired but I have virtually all the replacement parts so can still use this as practice for troubleshooting.

Any suggestions are very welcome. I have some schematics attached

[anatech]

Hi Kajman,
R244 has a choke across it, should have measured short in circuit. I am surprised it's defective. Check R242 (should have a solder short, may have been left open after setting bias) and Q214.

Q102/104 are in the preamp section, R126 (3K3) may be defective.


-Chris