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NAD 2200 problems

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Hi

I have a NAD 2200PE I am working on. The right channel is faulty and I am starting to chase my own tail , so any assistance is appreciated.

The current state is as follows:

1) the amp had been working fine and came to me with a very distorted right channel.
2) The amp have been recapped (by me) some time ago and has worked fine for the last 2 or 3 years.
3) When I was diagnosing the fault- which was on the right main amp board I found a faulty 2SA970 and thought “bingo”- however when I replaced the transistor (I used a KSA992)- suddenly the amp slams the ammeter on the isolated power supply- (it usually initially peaks at about 1.25A and then settles to 0.4A with no input) and blew the 4A fuse in the isolated power supply.
4) so I start checking all the resistors in circuit and anything that looked suspicious had one end lifted and they are all good
5) next the diodes- same process and outcome all good
6) Next the transistors- all of these have been tested out of circuit with a peak transistor tester- and all test good- I have replaced them with new alternatives (which I have used with no problems before).
7) I started replacing the transistors (except the output transistors) from input stage (up to Q212) and then moved through the circuit- all is well (in that the amp fires up, protection operates then clears and (new) relays click until I insert Q308 (see attached circuit) and slam! – the current draw goes through the roof again (this time I was very quick on the power switch, so no blown fuse).
8) So with all transistors (including the outputs) fitted except- Q308 and Q322/Q324 (selector for section switching) and Q236/Q328 (switching section)- the amp comes out of initial protection and all the voltages up to Q212 are correct as per the attached circuit.
9) I have tested Q312, Q316,Q320 out of circuit with the peak tester and they all test good.

So I am perplexed as to what to do next- so any assistance or thoughts would be greatly appreciated-and apologies for the overlong description.

The attached circuit is for the NAD2600- and is labeled as the MAR593 C revision of the board, however R393/394 and R392/391 are not fitted in the NAD 2200PE

Thanks

Peter

Attached Files
File Type: pdf 2600 amp board.pdf (627.0 KB)

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