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
 

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  • 2600 amp board.pdf
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If you short the collectors of Q302 and Q304 together that will put the output stage to full cold bias. Then try installing Q308 and if it draws normal current you know your problem is in the bias circuit.

Q205 is in the LEFT channel and I don't think that's part of the OL circuit.

Craig
 
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