NAD C320BEE distortion

Hi, been a long time since I posted, but here goes.
This amp appears to work normally, but music through both channels sounds mildly distorted at all volumes, sounds like low-level clipping as if a component is not getting enough voltage. As the fault is identical on both channels, I was inclined to suspect faulty supply rail or associated components. Fuses check ok and the soft clipping switch makes no difference. In fact I isolated the issue to the power amp stages by removing the pre-main links and feeding input direct to the main amps, starting with a low volume and stepping it up - result the same,distortion at all levels..
Measuring the rectified power supply to the main board, it comes in at a stable +/- 51V, which is a little above the 47V spec.
But measuring across the speaker terminals I get an fluctuating DC offset of 0.27 to 0.3V on both, obvs far too high but apparently not enough to trip the protection.
Any steer as to what can be causing this? Is the distortion linked to the DC offset?
I did squirt some deoxit around the trimpots and gave them a tweak back and forth (equally , to arrive back at the original set point) but this seemed to have no effect.
I don't have a scope, just a pocket meter , a soldering iron and a schematic.
It's a good amp and It would be nice to get it back.
kind regards, folks, keep up the good work
 
Hi, I think I have found the problem. I checked the 18V supply tothe preamp stages and the -18V rail was down.
Tracking back to the supply circuit I found a 33 ohm fusible resistor which supplies the -37V rail voltage to the transistor that regulates it down to -18V had gone open circuit. The transistor and associated components seem to be OK as far as I can measure in-circuit. Fusible resistors seem hard to come by, am I taking toom much of a risk to replace it with an ordinary one? I suppose whatever might have cuased it to fail in the first place my still be lurking. The resistor itself d
shows no external sign of overheating, so maybe it was just duff.
 
Hi, I think I have found the problem. I checked the 18V supply


You mean I told you where the problem was :p

#standard NAD fail point :);)

The 649 pass transistor can fail also, I'd check all the smaller caps in that psu area, some will def be (age fatigued) fried.

0.5W/33r fusible's can be found and this is what I'd always strive to replace with, usually metal film, but specifically specified as fusible and/or flameproof.


The risk is yours if you don't source the correct component, you need to spec same wattage in this case 0.5W, never higher. It's a safety critical component, they don't put them in for fun.
 
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I am indebted to you Mike for pointing me in the right direction, it undoubtedly saved a lot of time. I will try to source a 33ohm .5 watt FR, but at the moment the best I can find is a 22ohm .33W. If I run that in series with a 10ohm resistor, that should work, according to my basic ohms law calculation. I'll also replace the elcaps in the circuit while I'm at it, as you suggest. Many thanks for your help; I will post again when I've made the repair.
 
Hi, just a sign off to say the amp is now fixed. I couldn't find a 33R fusible anywhere, but got a 10 and a 22 in series. While I was removing the bad FR I found another resistor in the circuit had broken a lead, so that may have caused the original FR to do its job and open. So I replaced that too , a few caps and all is sweet music again.
Thanks again Mike P