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1980sjohn 5th April 2014 06:49 PM

NAD C320 fixed (another protection circuit problem)

Finally fixed my mate's NAD C320. This had the common problem that it took ages to come out of protection (power LED stays red rather than turning green). I foiund the circuit diagram and read various similar problems here, although I replaced a few components I believe the fix was to replace C421. This connects to pin 4 of IC44 (a uPC1237, protection IC), and pin 4 is used for the AC mains detection. The design is to feed pin 4 from the mains transformer via a half wave rectifier and a resistor, the IC detects when the mains is switched off and goes into protection. Same IC is used in loads of amps, seems to be a common chip, so other amps likely to issues with the capacitor on pin 4.

Initially I though this capacitor was OK, as pin 4 originally measured about 1.5V on my DC multimeter. However as suggested on other threads I removed C421 (4.7uF 50V) and it only measured 1uF. When replaced with a new cap pin 4 measured 1.9V. However I think previously there must have been a lot of ripple on pin 4 hence the IC saw the voltage drop below threshold (0.9V) and so thought the AC was off and triggered the protection.

I also think there is some interaction on the IC between pin 4 and pin 7 (switch on delay). Pin 7 was stuck at around 200mV rather than the required 2V, so I thought originally the problem was with the associated capacitor for pin 7 (C420).

I also replaced C420 and C422 but they measured only 30% below spec and were probably OK.

Much earlier I had replaced IC44 and D412 but I don't think they were faulty.

Lesson is to think whether a DC voltage that seems OK with a multimeter could have a lot of ripple and actually fall below protection IC threshold, in which case a scope would be needed to check.

I saw on Youtube someone repaired a C320 by replacing all the caps, I think this is overkill (but would probably have saved me a lot of time!)


sbrads 5th April 2014 09:26 PM

Sounds exactly like a fault I had with a Harman Kardon sub amp. A mains detection circuit cap was only 10% low in capacitance causing shutdown.

starcrouz 8th May 2014 01:04 AM

NAD C320 fixed too
Thanks a lot for the tip about C421 on the protection circuit (pin 4 of IC44). I was stuck on this problem because my measurement of pin 7 of IC44 was, like you, very different of what I expected, according to the service manual.

By the way, this amp belongs to a friend. He has fallen down. And the first breakdown I found was a broken track in the +/- 18v circuit. Moreover, I'm surprised a fall can lead to a big change in a capacitor value, reason why I was not searching in that direction ! 2 breakdown to repear, but it's done thank to your experience ! Thanks again !

SelwynAn 23rd July 2018 11:41 AM

Thanks 1980sjohn. I've just been working on a NAD C320 which wouldn't come out of protection. Unfortunately I shorted one of the driver transistors while testing voltages but it was a blessing in disguise - when installing the replacement I noticed all but two of the drivers had heat-damaged traces, and using the schematic worked out somebody had installed one backwards! So some trace repair and everything seemed fine, but it still wouldn't come out of protection. Found your post, tested C421 which had only dropped to 3.5uF, but had high ESR and 19% Voltage loss (those Mega Testers are really useful), replaced it and the amp now comes out of protection. Thanks for the great information!

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