NAD 2200 problems

[pwdiya12]

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

 

[llwhtt]

Which transistor was the 2SA970?

Craig

 

[Mark Tillotson]

Sounds like the bias circuit (Q334 and its pot) is completely out, leading to massively high idle current.

 

[pwdiya12]

Craig- it was Q205- part of the overload detection circuit- before I replaced it the Overload LED was on, after it is not.


Mark- I was wondering about that- I will replace both Q344 and the pot and report back-



thanks to you both

 

[llwhtt]

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

 

[pwdiya12]

oops craig, I meant Q206.


I will try the short- does "cold bias" effectively mean no bias?

 

[Mark Tillotson]

Yes, no bias voltage, so class C operation at low signal levels.

 

[llwhtt]

Yep, no bias, predrivers, drivers and outputs fully OFF and the output stage should should draw no current. Q336 and Q338 are the OL detectors. Q206 was probably CAUSING an OL condition and replacing it killed the OL condition.

 

[pwdiya12]

Craig and Mark- with Q308 installed and collector of Q302 and Q304 shorted- it draws normal current and comes out of protection. I will replace the pot and Q344- the original was a 2SC1953 and the replacement is a KSC3503


thanks so much


Peter

 

[pwdiya12]

unfortunately, when I remove the short from Q302/Q304- it slams again-so if iot is the bias circuit and R350 and R348 are ok- and there is new pot- where should I direct my attentions?

 

[pwdiya12]

so ZD302/ZD304- measured as diodes, but not as zeners- so I have replaced them (10v 0.5watt) and checked D310/312- both of which are OK- still slams. I will lift each of the other components in the bias circuit and test and report back.


Thanks for the diagnostic help


Peter

 

[pwdiya12]

ok solved- Q334 was in backwards( mea culpa)- I read the circuit/layout incorrectly- which when I thought about the "cold" bias, and Mark's comment about massively high bias- made me check this again.


Thanks so much for the lesson on cold bias- I have learnt something very useful


with gratitude


Peter