NAD C356BEE (Missing -18 Volt rail)

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lazio2000

Member
Joined 2016
#1
Hi Members,

my new NAD adventure with this amplifier that seems to have very fragile traces and solder points has started.
The unit turns on fine, amber LED on then click, click Blue LED stable.....good news.
DC Offset is on the high side, one channel 50 mv other one 100 mv after 10 minutes (not able to adjust it at all).
BIAS Current is good (6 mV) and fully adjustable.
I suspect that somewhere in the feedback circuit something is wrong.
Upon inspection all voltages are present except the minus 18 V and -26 V rails.

I pulled the LM337M and tested it separately in a small circuit to verify that it was working.

Refer to attached pictures, appreciating any feedback.

Thanks
 

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#5
Did you check any voltages starting at the connector point on this amp board (-55V at the point of R46 + R42 ) ?
Follow the connector wires to the power board?
Have you verified the +/- 55VA supply from the power supply board (D56 (diode bridge), C523/C534 ripple filters)?
 
#14
I'm agreeing with Ylli above.

Open R414 or shorted C48 would present voltages you're seeing. With power off, what is in-circuit of resistance or R414 and R414//R485//C48? With circuit active, what is voltage across R410? Should be only a few mV (Base current * R410).
 
#15
From the top of the board, check the voltage on each lead of R414. I want the voltages directly on the resistor leads, and not the solder pads.

It appears the negative regulator is not seeing a good output from the positive side, so it is shutting down.
 
#16
Thanks for your feedback I will check again this evening R414 & R415 were pulled one leg and tested for resistance......ok
Last night I pulled the LM337M out and left the circuit open with pads only to see if I would get the -26 V on the emitter of Q42..........no such luck.
The negative regulator as mentioned previously was tested on a separate circuit with few components and trimmer and it was adjusting VOUT properly. I will solder it back in place and post a complete voltage check of the supply circuit nodes for you guys, thanks
 
#18
Will do, as you may know the traces & pads are very fragile & thermally stressed but they have been tested (not every single one) for continuity.
Another example of NAD trying to save money on components and blow-out a decent amplifier design.
 
#19

Ylli &​

BSST I will have to congratulate you for finding the problem, R414 (6.8Kohm) was hidden in front of the Q41 heat sink and suffered from thermal stress. The resistor was partially charred and open, that was the cause of the problem. Reinstalling a new resistor not only fixed the problem but made the DC offset within very few millivolts. Since the supply circuit is largely underdesigned I will replace the network of resistors with larger dissipation & metal oxide for better stability. You can already see slightly burned paint and noticeable heat smell........Thanks again Giorgio 🙂​

 
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