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NAD 304 Integrated Amp Post-Repair Question(s)

KilgoreCemetery

Apr 12, 2017
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I had an NAD 304 integrated amp that was stuck in protect mode. I measured about 14vdc in the signal path at the speaker relay and backtracked to the power amp section. Nothing looked physically damaged and, long story short, I found an open resistor. It wasn't burnt or cracked. I'm assuming it was supposed to be a fusible resistor, but I can't find anything else wrong with the amp and I'm not sure why it went open in the first place. Before I do much of anything else with the amp, I'd like some input as to what may have caused it to fail or if there are other components that I should check before putting it back together.

I do get a clean 1kHz sine wave on the scope with no noticeable distortion. DC offset dropped to about 98mV, and I was able to calibrate it down to about 1mV. I pretty much expected to put in a new resistor and have it smoke immediately, but it doesn't even get hot. Side note: the same resistor on the other channel was way out of tolerance and I replaced it too.

Open resistor was R333, a 47k 1/2W 5%
 

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Harald Kapp

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I se no high power load on this resistor. As the other resistor was "way out of tolerance" this is possibly a quality issue with these parts, Happens.
other components that I should check before putting it back together.
Since you have the amp open, check all electrolytic capacitors, even the not obviously broken ones. You'll have to de-solder them from the PCB for checking capacity and if possible ESR.
I use one of those "multi function component testers" for such purposes, Surely no match for a quality instrument, but good enough for purposes such as this.
 

KilgoreCemetery

Apr 12, 2017
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Thanks for your input Harald! It's such an odd failure, but I suppose that would explain it. I've been running the amp with no issues other than a sketchy speaker relay that I'll probably just end up replacing. I'll check capacitors too while I'm in there. I have an ESR Blue meter that's done well for me in the past. It doesn't tell the whole story, but it good for comparing left and right channels, and that sort of thing.
 
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