NAD 705 strange behaviour, intermittent fault? - diyAudio
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Old 20th May 2012, 01:42 PM   #1
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Default NAD 705 strange behaviour, intermittent fault?

I have recently acquired another NAD 705 receiver. Power it up, jump the pre-outs and the power amp inputs with an rca lead, amp seems to be running fine. I turn it off and set it aside for about an hour while I move onto other stuff. I get back around to the NAD and decide to load test it and measure its output before clipping.
I get it all set up with 2X 8 ohm resistive loads, a multimeter set to AC volts, and an oscilloscope to view both channels etc, so far so good.
I feed a 1K sine into it and I can see the waveform on both channels is horribly distorted, at low power only half the waveform was present (just the top half of the sine) and when I increased it I would get a full waveform but very misshapen, and nothing was consistent. I opened it up and began tracing through the circuit with the scope, and found the signal on the collector of the input transistor to be be quite distorted, but the waveform on the emitter looked okay.
I went further into the circuit taking voltage measurements to compare to the schematic, and the all the voltages were within +/- .2 volts of what was listed which was not bad.
I noticed the waveform was horribly clipped and turning the volume up would cause the protection to cut in, and reduction of the volume would then make it come out of protection.

But what was very strange was as I was probing around, all of a sudden it cam right and was functioning normally. I measured 50W per channel before clipping and the service manual rates the power at 44W (probably with slightly lower THD but I am not set up to measure distortion) and I left it sitting there at full power for an hour, turned it off, and turned it on again later and it was still working fine.

At that point, I have no idea where to check as I can't get it to play up again. Both channels had a similar looking output when the amp was faulting, and the preamp's output was clean. Does anyone have any ideas? Any known common problems with these amps? I really would appreciate any help with this, as I don't know where to begin as its working fine currently. I have included a schematic of the power amp so you can see what's going on. Thanks for your time
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Old 20th May 2012, 02:10 PM   #2
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Try tapping around the amp board with the plastic handle of on old tooth brush or similar insulator to see if you can bring the fault back on .

Could you post more of the protection circuitry schematic.
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Old 21st May 2012, 01:21 PM   #3
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I already tried tapping the board and components to bring the fault back on, to no avail. Please find screenshot of protection circuit
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Old 21st May 2012, 05:09 PM   #4
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Were all the protection transistors giving the correct voltage because with an output overload of voltage or current the protection is designed to clip the input as you described but with one important difference it should clip both pos and neg equally......so maybe you had an erroneous overload condition on the neg cycle side only .........i would quote the transistor Nos involved but i cant quite read them .

Last edited by epicyclic; 21st May 2012 at 05:35 PM.
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Old 21st May 2012, 05:53 PM   #5
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Reading your post again ......maybe you had an erroneous overload condition that was affecting both cycle sides but only showed up half cycle with a low enough input .
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Old 22nd May 2012, 01:44 PM   #6
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I turned the amp on again today and it was doing the same thing again, so I took a photo of the oscilloscope, this is from the speaker outputs. Different input levels didn't change the shape of the output waveform, just the amplitude. If I turned it up too high it would click into protect mode. Then it started to behave normally again, so I went to load test it again to check its stability. It was fine for a couple of minutes then started clipping, but the clipped portion of the sine was on a a decreasing angle as opposed to the normal flat line along the top starting from the peak sloping down, leaving a flat spot on the side of the apex. I have included a small picture to better explain the waveform, and then it went back to doing the same thing it was before (refer photo of scope).
I appreciate your help, I'll be looking forward to making it work!
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Last edited by flyingtele; 22nd May 2012 at 01:55 PM.
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Old 22nd May 2012, 08:35 PM   #7
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Your receiver has soft clipping control , the sketch is showing this soft clipping in operation so you were probably driving it near its limits to show this waveform so that bit appears to be OK and nullifies my thoughts so far .
Probably best not to drive the amp into protection mode as this comes in as a result of high heatsink temperature.

Would you post a scope photo with the oscillator output on one scope channel and the resulting distortion ( not the clipping waveform) on the other so we can see how it correlates .
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Old 22nd May 2012, 10:10 PM   #8
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First you have pencilled -0.8 volts at the -15v pin of the protect ic.Pin 5?

As mentioned above, i would suspect the clipping detection, but 0.9v at protect input (Pin3) seems to be quite high. is this DC or AC?
check output relays. oxidation of contacts can give a diode effect..
Is there any offset voltage on the output, or even the input?
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Old 23rd May 2012, 08:52 AM   #9
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Modifying my statement about protection mode..........the final phase of protection is high heatsink temperature shutdown.......not i think what you have been doing .

Going back to your original post .....its odd that the signal looked OK on the emitter of the first trannie Q101 as thats where the global feedback attaches .
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Old 23rd May 2012, 12:46 PM   #10
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Thanks for your reply. Epicyclic, yes I thought that was odd as well. The distorted sine wave I drew a picture of is not the soft clipping, when the amp was working I tested the output with the soft clipping on and off purely out of curiosity and all it did was round off the edges of the dc portion. What I drew a picture of is completely different and I have come across it on other amps but not sure what it means.

@ whizgeek- Ignore all the pencil marks on the schematic, as this was already there. Someone must have scanned an original paper document that had the writing on it and not erased it, but its not mine. Sorry, I should have mentioned that.

Next time it plays up, I will take more photographs of the scope at various points throughout the signal chain. I will measure the voltages throughout the protection circuit, I checked the voltages in the amp and they were all okay. I have a feeling the problem may lie in the protection circuit simply because its one circuit acting on both channels, and both channels show the same fault behaviour at the same time.
Check back on this thread, as I will upload more scope photos as mentioned. I truly do appreciate your help and it is appreciated.
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