NAD 2100 power amp won't come out of protection

bradfro

bradfro

Active Member
I've got an NAD 2100 power amp, when turned on, the protection light remains lit, and the speaker relays never click on. After checking all the AC input voltages, I checked the DC power supply outputs and they all seem fine. The problem is there is 26v on the test point to measure idle current. This is on the left channel. The right channel works perfectly. I was able to adjust the idle current for the right channel to around 15mv
I’ve attached a schematic
If I follow back the 26v(red circle on the right side of the left channel) I come to a point where there is 0v indicated by the capacitor circled on the left. This seems to me to be the origin of the problem, but I can’t find anything wrong.
The point where I’ve indicated 6.4 and 7.5v on the right channel (good channel) measures nearly zero, maybe something like .6v.
I’ve replaced C401, I’ve pulled and checked D401, d403, Q453 and q454 with an ohm meter and they test good. Q401 and Q403 are part of an op amp, I think. Anyway, I’ve swapped these with the right channel and the problem remains on the left side. R401, R403, R405 seem to measure good in circuit.
Can anyone point me in the right direction on this, I’m stuck
Thanks
 

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I don't think that cap is the "origin of the problem" as you think. I think you have voltage there because the voltage is going "backwards" from the problem point.

So, do you have 26 V on all that line where you typed "26" ?

I'd check, at first, output transistors... Q439, Q437, Q435, Q433. If the transistors are bad, check their respective emitter resistors, and driver transistors, they could be burned too.

But no relay click usually points to shorted output transistors... (Q439 and Q437)
 
Last edited:
elnaldo

Yes, 26 is 26 volts at that point. I'll check all the transistors and related parts tonight and report my results.

Thanks for your help
 
Often people focus on faulty transistors when Dc offset is present in the out ...Not always correct .If transistors fail at 99% they go short so expect full rail voltage in the output not just 26 volt.

Anything might take a NAD out of balance ...focus on fusible resistors of the opposite side of the polarity you measure but also make sure that aux power supply works properly since there is chance the fault come from the input .

Is kinda late and i will look in to a proper schematic tomorrow and let you know in more details .

Kind regards
Sakis
 
I pulled the four transistors Elnaldo suggested and bingo....Q435 is shorted. I haven't checked the emitter resistors and driver transistors yet. I'll go ahead and order replacments for both channels and I'm pretty confident that this will fix it

Thanks Elnaldo
 
So Q435 is damaged, but Q439 is OK ? That's weird to me... But it could be that Q439 was more robust and survived to a short...

Check voltages WITHOUT the transistor installed, specially at the transistor base and collector. You should have the right voltages or very close when the transistor is not installed.

This will tell you if something else is damaged. I'd change it's driver transistor (Q431) as well, to be on the safe side.

Check R443 and R441 too, they are connected to the burned transistor

You should change the complementary Q433 if your replacement is different from the original.

If transistors fail at 99% they go short so expect full rail voltage in the output not just 26 volt.
Click to expand...

That's a good point, I never thought this way... Perhaps in this case the output transistor is OK, and the Driver transistor is shorted, so that's the reason for this readings... I don't know, I can't read the circuits that deep.
 
Last edited:
Elnaldo

All I can tell you is that Q439 measures good with an ohm meter out of circuit. The voltages that I measured on Q435(before I knew it was shorted) were 58volts on the base, 63 volts on the emitter (rail voltage) and 21v on the collector. I'm not skilled enough yet to look at that and conclude the transistor's shorted. I'm learning.

I'll check voltages tonight and see where they're at for the missing Q435. I've ordered the replacement transistors and they are exact replacements
 
East electro

Please, please, any advice would be welcomed. I'm trying to learn this stuff, and I need experienced help, like yours

Thanks
 
check ytube on how to measure a tranistor ...it only takes a dvm

measure voltage drop on resistors around the driver area ...verify voltage drop with the working ch
 
Petehall347

Thanks for the link, very comprehensive. a good refrence to have

Um, just for the record, I do know how to check a transistor with a DVM. I'm confussed by East Electro's response, as if I was doing something very wrong. I don't know what he means. I was hoping he would tell me why i'm proceding in a wrong direction
 
I'm not skilled enough yet to look at that and conclude the transistor's shorted.
Click to expand...

Basically, outside the circuit, I shouldn't be any continuity between collector and emitter with the diode test of your DMM, in any sense.

The only oltage drop you should read is between base and emitter, and base and collector. In one sense and not in the other. PNP and NPN have opposite current flow (as described in the link before), but any of them, if you read voltage drop in one sense, you must read Open circuit in the other sense.
 
a transistor will test like 2 diodes connected together ... funny thing is you cant make a transistor out of 2 diodes ..
a snippet of understanding ...........
with an audio amplifier the driver stage will amplify the voltage ..the output transistors add current ..
 
Q435 is replaced and the amp is fixed. It's always a great feeling when those protection relays click for the first time. I readjusted the idle current for both channels and it's playing music.

Thanks to all who offered help
 
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