Question about NAD C370 2-Channel Amplifier

1 Answer

NAD C370 in protection mode

Hi Friends,

My C370 is not working -- the power LED turns red soon after
power on (it is green for a few seconds), before the speaker relays
ever get engaged.

I found your earlier posts, checked the capacitors. One was open
(c785), the others seemed ok. I replaced all of them except c784
(47u/50V), but the problem remains.

The voltages on the 8 pins of the upc1237 are like the following:
pin 1: 0.6 mV
pin 2: -15 mV
pin 3: 0.7 mV
pin 4: 4.2 V
pin 5: 0 V
pin 6: ~22 V
pin 7: ~30 mV
pin 8: 36.5 V

I suspect the problem is that pin 6 is too high. Shouldn't it be
around 1-2 V? Please enlighten me.

Thanks much and best regards,
DH

Posted by on

  • SmallMonkey Feb 14, 2008

    Hi Dariusz,

    Thank you very much for the advice. Luckily, the big caps on
    my unit are not white, they are purple/light blue.

    Following your advice, I also replaced C784. It helped a little
    but did not solve the problem. Now pin 7 of upc1237 changed
    from ~30mV to 0.65 V. Guess the old C784 had a big leakage,
    and most importantly, pin 7 has an internal short so it cannot
    hold the voltage -- according to the NEC datasheet, pin 7 should
    have about 2.* V.

    I also checked R777 to see if it is broken. No, it is still 57.* K.
    When R777 was disconnected, i measured the resistance
    between pin 7 and ground (pin 5 is connected to ground), it
    was like 1.2 K which seems to explain the 0.65 V.

    Please, if you have a working unit, help to measure the voltages
    on all the pins of the upc1237 and tell me.

    Thank you again and best regards,
    DH

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  • 4 Answers

With closed eyes -replace all capacitors on protection PCB All of those caps are overheated and dry out (they lost there capacidance).Note that capacidance meter will not help you at all. Now unit will work. BUT!!! Most important!!! Check four big filter capacitors in the middle of unit!!!! If they are white (NAD 10000uF/80V) DO NOT USE UNIT AT ALL!!!! FIRE HAZARD!!!!! Contact me... Dariusz
Dariusz70@verizon.net

Posted on Feb 13, 2008

  • Dariusz Feb 15, 2008

    well my friend:


    one of your thermal sensors(on heatsink) is open!
    pin 8 on upc1237 should be about 2V not 36!!!
    so your problem is that unit is thermal protection. change one of the defective thermal sensors (one for each channel) or Q728 (2sc1815) that is thermal sensor switch (on main PCB)
    and enjoy
    a-ha my first suggestion (about all cap's) is still valid and good you have change them all.

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