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NAD 7100 Troubleshooting

S

Samuel STEIN

Member
Hello!

My dad gave me his NAD7100 and it worked great for a while, then I came home one day from work and the fuse was blown in my living room, with the NAD no longer in service. It turned on and the central resistors close to the heat sink started getting really hot and no sound would come out.

I checked a few diodes that I assumed was a blow-out, turns out to be a faded black stamp. After returning them, I added a 75 watt bulb to test out how it's working, and it immediately shut down. I removed F503,F504,F505, and F506 and it turns on, but the transformer is clicking and the readings of F501 and F502 are either at 21V+/-.5V with an occasional click that makes them go all the way down to 2V.

I am checking if the folks here think this is a breaker or if I am having transformer problems.

Your help is much appreciated!
 
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In the current configuration, with F501 and F502 installed, F503 - 5056 removed... Once the unit turns on, does it stay on? Or does it try to power up then shut off? Sounds like you have a 75 W DBT installed. Does it light up bright or is it dim or not lit at all? Are you measuring from F501 or F502 to ground, or from F501 to F502? AC volts, not DC at that point.

The service manual with layouts is available here: https://www.hifiengine.com/manual_library/nad/7100.shtml . Can you be more specific as to what resistor is "the central resistors close to the heat sink"?

I didn't do so well on the last unit, are you sure you want me to help troubleshoot or should we wait for someone else? :)
 
The unit stays on, but I think it is trying ot shut itself off! the 75 Watt bulb stays on dimly. I was trying to describe R468, R464, R467, R463, seamed the hottest
 
Samuel STEIN said:
I was trying to describe R468, R464, R467, R463, seamed the hottest
Click to expand...
These resistors provide base drive to the switching transistors that switch the outputs from lower voltage to higher voltage. From that, I would go along with the others and suggest you remove and test the output devices. But that doesn't explain the odd behavior with F503-6 removed.
 
Ylli said:
remove and test the output devices. But that doesn't explain the odd behavior with F503-6 removed.
Click to expand...
To clarify, do you mean Q452,458,444 and the corresponding left transistors for the "output devices? Because these readings are all just DMM readings.

My receiver vocabulary is pretty new, so please understand some slowness!
 
The "output devices" are Q437 and Q439 on the left channel, and Q438 and Q440 on the right channel.

But I'm confused as to what you are doing. If you have F503-506 removed, there is no power going to the amplifier output stages. There is nothing to measure voltage-wise if there is no power applied.

I understood that with F503 - 506 removed, but F501, F502 still installed that the unit powers up but is clicking.
 
I have F503-506 removed, and I checked the voltage of the output for kicks. I was getting a voltage of about .03V-1.6V, which I think was an error on my part.

I am now removing Q 437-440.
 
Did you check collector-emitter for shorts?


See attached clip of a portion of the Preamp board. Please measure and report the voltages on the following points,
Negative meter lead to chassis ground:

P521 (should be about +30 volts)
P522 (should be about -30 volts)
P524 (should be about +18 volts)
P525 (should be about -18 volts)
P901 (should be +5 volts)
P902 (should be +12 volts)
Screenshot_1.png
 
Nothing was consistent, it would jump all over the place, but the peaks were as follows:
P521 was 14
P522 was -14
P524 was 9.5
P525 was -9.5
P901 was 1.4
P902 was 4.6

Does this have anything to do with the dim lit 75 Watt bulb on the floor?
 
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