I have a NAD 761 5.1 surround receiver which suffered a strange fault last summer. I was playing a stereo signal, amp in stereo mode, on a very hot day when it shut down. After cool-off and startup, I noticed a distortion issue which is tough to describe - with a normal music signal input it sounds as expected, but on reverb tails, piano chords fading out etc, the signal becomes distorted as the sound drops away into silence. With louder signal I can hear no distortion at all.
On testing, this phenomenon occurs on all 5 amplifiers which makes me think it's a PSU problem. A visual inspection shows no obvious damage.
The service manual is here for reference.
I'd very much appreciate any tips or advice. The amp isn't used for serious surround purposes any more but is an excellent and enjoyable device which I'd hate to dispose of.
On testing, this phenomenon occurs on all 5 amplifiers which makes me think it's a PSU problem. A visual inspection shows no obvious damage.
The service manual is here for reference.
I'd very much appreciate any tips or advice. The amp isn't used for serious surround purposes any more but is an excellent and enjoyable device which I'd hate to dispose of.
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Odd. Your description sounds like severe crossover distortion, but that's not caused by the power supply (but rather dead transistors or power resistors in the power amp). Bad output relay contacts can cause similar issues, too - and knowing NAD's reputation for parts quality, I wouldn't be surprised to find some. But why would all 5 outputs be affected, suddenly and equally? Relay driver issues (bad solder joints perhaps)?
The things I might expect on a very hot day is maybe a power amp going up in smoke, but it doesn't look like that's what happened.
Do you have something else you could connect to the NAD's pre-outs in order to isolate the fault? Maybe another power or integrated amp?
The things I might expect on a very hot day is maybe a power amp going up in smoke, but it doesn't look like that's what happened.
Do you have something else you could connect to the NAD's pre-outs in order to isolate the fault? Maybe another power or integrated amp?
OK, I spent an hour or so with the amp today. Preamp output is AOK, and the amp plays well with FM radio signal. However, when I apply a signal to the power amp inputs, especially when the signal and contains a lot of transients the distortion is clearly audible - it doesn't sound like an amp clipping but a constant low-level hash, signal peaks sound clean enough. The problem is present on all 5 amps, to the same degree and nothing I do seems to impact it. Well perplexed here!
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