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NAD 2600 In Protection and General Troubleshooting Advice

S

SansuiPui

New Member
I recently purchased a NAD 2600 that appeared to be in beautiful condition - it was never worked on and cosmetically looked 10/10. It was clear the amp was rarely used and babied. However, once I plugged it in and powered it on, the protection light came on immediately and stayed on. This happens with or without any speakers/inputs connected.

What I have done: I took off the cover and checked all the fuses - each appears to be intact.

I looked at all the components and none seem to be burnt out (none of the electrolytic capacitors are bulging and no resistors are burnt.)

What I have looked into: I have read up on the protection relay issues and do not this this is the problem because protection stays on. From other forum posts I have read, it seems like checking voltages is the next step.

My questions: Is there a good guide for how to safely power on an amp and check voltages hot? I have HV electronics experience but nothing on amps. Are there components that are known to go bad with age on this amp that I can check? I have a bench top multimeter and scope on hand. Has anyone had this issue and knows where to start?

Any insight or references would be greatly appreciated.

Thank you,
Josh
 
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Is this a 2600 power envelope? That's the SM I'm referencing from hifiengine.com.

TP101 (left chan) and TP102 (right chan) are the test points before the speaker protection relay. Measure between the test point and chassis ground. Values should be 0V (+/-5mV).


Joe
 
Hi Joe, thanks for your response.

It is a 2600 power envelope. I have downloaded the service manual and am comparing it to your directions and have what might be a very stupid question. I see TP101 but it has two pins... does it matter which I measure from? Also, how do you know from the manual that it should be 0V? I see the "0V" callout at the output, but not at the TP. Thanks for the help and I will get back soon with test results.

Josh
 
Actually the SM says TP101 and TP102 in the instructions but the board diagram indicates these as JP101 and JP102.
Is this actually correct in your amplifier, JP instead of TP?

Ah the measurements are between the 2 pins, my mistake.
Not pin to chassis ground.
 
Ah ok that makes more sense. I only have TP201 and TP202 on my boards that I can find. I located the output (white wire labeled "OUT") as well, and was planning to measure between that and ground. I can't see anything on the left or right channel boards that even start with 1_ _. I am looking on the top (component) side. Are there more points labeled from beneath?
 
Well if you got the OUT wire then I'd measure the DC offset (between OUT and GND).

I'm not certain what level of DC causes the protection to kick in but any reading over 100mV is needing attention.
 
SansuiPui said:
I only have TP201 and TP202 on my boards that I can find.......I can't see anything on the left or right channel boards that even start with 1_ _. I am looking on the top (component) side. Are there more points labeled from beneath?
Click to expand...

I'm only using the SM for my reference

TP101 and TP102 may be "implied ". Meaning the emitter resistors are probably ceramic and will have pin extensions so as to connect test connectors.
 
SansuiPui said:
Right channel out is ~94V. Left channel out was 0.7V and rising...
Click to expand...

Seems like one of the output devices in the Right ch. is a dead short, 94V is the power supply.
One (or more) of final transistors is shorting the power supply to the OUT node.

The left channel is failing as well (700mV), so some components failed or failing as well.

Well at least you know the protection circuit is working.
 
Is this an easy fix? I assume the next step would be desoldering and testing each output transistor individually. I could find the shorted one with a continuity test using a multimeter but I don't know how to do much more than that. Would I need a transistor tester?
 
@SansuiPui- I think you posted on DIYaudio as well- I have responded there- I ma fairly certain as @jhjove states that some of the output devices are blown. You will need to remove them form the amp to test
 
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