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NAD C320BEE Amplifier Input Selection Not Working

DustinB

Nov 15, 2021
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Hi all

I've pulled my old audio amplifier out of storage and the input selection is no longer working. The amplifier has power but no other buttons work. I have limited electronics knowledge so really hoping someone can help. The service manual can be found here: https://audio-circuit.dk/downloads/nad2/NAD-C320BEE-int-sm.pdf

I believe I have identified the problem, with the +18V rail measuring only 0.7V at D412. I have also investigated the power board, and it looks like without the main board connected, the +37V and -37V rails are only measuring +29V and -29V as measured at D424-D427 or CZ46. I'm assuming this is a problem.

I have also checked the majority of components on the power board.

Any help greatly appreciated. Thanks.
 

Harald Kapp

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with the +18V rail measuring only 0.7V at D412.
Sounds suspiciously as if you had measured the voltage across the diode which would, of course, be only 0.7 V.
the +37V and -37V rails are only measuring +29V and -29V as
This issue could be caused by old electrolytic capacitors in the power supply that have lost their capacity (a typical effect with electrolytic capacitors). This will not necessarily be obvious by visual inspection. You'd have to remove the main electrolytic capacitors from the circuit and test them using a capacitance meter (or an ESR meter). If you don't have one, replacing the main capacitors (the "big" ones) even without testing the old ones is usually a good start to solve that kind f problem.
 

DustinB

Nov 15, 2021
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Thanks very much for the reply Harald. I'd managed to check all the other caps apart from the 2 big ones. I'll go ahead and replace them.
 

bertus

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Nov 8, 2019
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Hello,

As @Harald Kapp said, old capacitors will lose a lot of the capacity.
This page shows you the effect of different capacitors after de rectifier:
unit12_capacitor change.png
I have also attached the complete PDF where this page comes from.

Bertus
 

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DustinB

Nov 15, 2021
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That's awesome bertus, thankyou. I'll have a good read of that over the next few days.
 

DustinB

Nov 15, 2021
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Hi all, I tried replacing all the capacitors on the rectifiers circuit apart from C435 but still not working.

The current situation is that the +/- 46V and +/- 37V rails are both at +/-41V. Does anyone have any ideas please? The 5.6V rail is fine.
 

Harald Kapp

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Find out where the 18 Vare (supposed to be) coming from and check the voltages along the way. It seems there should be an 18 V regulator wich is either faulty or is itself not supplied by the proper input voltage.
 
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