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NAD 7100 Troubleshooting

So the second time, the bulb lit up brightly, then dimmed and the unit stayed on? That is exactly what should happen. If that is where we are at, we can continue so see if/why the protection circuitry is cycling that output relay.
 
Before. Let's check the DC offset.
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Measure the voltage on the left side of R617 and R618. Be extra careful here, you *DO NOT* want to short anything here. Hopefully the voltage at these points will be less than 0.1 volts. If that is the case, then measure the voltage at Q605 pin 6.
 
Go ahead and reconnect the lifted side of R625. Turn it on and listen for the speaker protection relay to click in. If it clicks in and stays, then double check the output offset by connecting the voltmeter to the speaker terminals. If less than 0.1 volts, go ahead and connect the speakers. Are we there yet?
 
I think we may be. I have one question, is the voltage limiter whats turning the stereo off when I start it up? Because when it's cold it keeps cycling off before it will work. I am afraid of taking it off the regulator.
 
The receiver is acting really lethargic. It froze on one of the turn ons today, but it is playing music. Also the radio function is taking enough power to make it auto-shut off. But it's working!!!! Thank you so much for all your help! Do you have any more recommendations for it?
 
So it takes a while to start up, but it plays music, and probably needs a tuner tuning. However, I left it plugged in for a couple of hours, and Q551 got hot. The input when the machine is off is 26V, which feels off. I tested the inputs while off and yellow was at 6.3V and 6V, and red was at 12.6V and one side of D552 is -28.4V. I'm afraid i shorted something that is powering things that are not supposed to be powered!
 
Samuel STEIN said:
So it takes a while to start up
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They all have some delay on power up. It may take 5 seconds of so for the system to stabilize and the protection circuitry to say "OK". More than 10 seconds would indicate a problem though. If you'd like, we can look at the protection circuit inputs and see if we can discover the 'slow poke'. Let me know.

Samuel STEIN said:
Q551 got hot
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How hot is hot? If you can hold onto it, it is not too hot. If it burns your fingers, something may not be right. Q551 is a 15 volt voltage regulator that provides power to the tuner/standby sections.

Samuel STEIN said:
The input when the machine is off is 26V
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Input to Q551 I assume. Since the caps in the circuit are 35 volt caps, and manufacturers didn't tend to derate parts too far, this is probably right. Nothing is listed on the schematic though, so that would only be a guess.

Samuel STEIN said:
yellow was at 6.3V and 6V, and red was at 12.6V
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I have nothing that references wire colors. I'm guessing the 6/6.3 volts is the secondary feeding P553/P554 and the 12.6 volts is the secondary feeding P551/P552.

The secondary connected to P553/P554 is not directly referenced to ground, so measuring it to ground may be misleading. 12.6 vac at P551 would typically produce about 18 volts DC at the outputs of D551 and D552. Not sure where the +26 volts you measure at the input of Q551 is coming from. Might be something to look at more closely. Let me know.

Samuel STEIN said:
one side of D552 is -28.4V
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That is consistant with the +26 volts you measure at the input of Q551. The circuitry around D552 is a functional mirror of the positive regulator.
 
That is reassuring, it takes about five seconds to start up. Q551 was not burning hot, but the machine was off, which is what scared me. If I had been cranking it, or even if it was on, I'd understand, but hot when off means I have to unplug! Also P551 is at 0V when Q551 is 26V. That is peculiar, right! I went to D552 because it was backwards on the diodes (even if it goes through the ground), and abs(D552)>Q551!
 
Samuel STEIN said:
Q551 was not burning hot, but the machine was off, which is what scared me. If I had been cranking it, or even if it was on, I'd understand
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This section of the receiver is on as long as the unit is plugged in. It provides for the ability of the receiver to be in a 'standby' condition and for it to be able to receiver remote control signals. The power dissipation here is not at all dependent on whether or not you are 'cranking it'.

Samuel STEIN said:
Also P551 is at 0V when Q551 is 26V
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P551 would have an AC voltage, no DC.

Samuel STEIN said:
I went to D552 because it was backwards on the diodes (even if it goes through the ground), and abs(D552)>Q551
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D552 takes the AC voltage at P551 and rectifies it to a negative voltage. This negative voltage is regulated by Q553 (and associated circuitry) and provides -15V to those circuits that need it. The negative voltage at the anode of D552 is a bit higher than the voltage at the cathode of D551 because the current draw from the positive side is higher than from the negative side.
 
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