NAD3020 Startup thump repair

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My NAD3020 has started making a loud buzz sound as it starts up in both channels. After that the amp seems to work fine. Obviously I’m using crappy speakers incase its harmful to them. The problem is not affected by the volume control level.
Here’s a recording of it: YouTube
I’ve isolated it to be in the preamp stage, so that rules out the large 2200uF electrolytics. I believe that the second power supply section with the regulator (seen on the block diagram) could be to blame. I’ve read on this and other forums that this sort of issue is probably related to power supply capacitors so thats where I started my search.
I’ve tested C905 and 906 out of circuit and they measure about 0.14ohm ESR, 1.3% vloss and around 330uF. A bit of googling suggests this isn’t far off.
I’d rather not replace capacitors for the sake of it but I’m not sure where to look next, the schematic seems to get a lot more complicated after that :p
None of the caps are showing any visible signs of wear.

If anyone has any suggestions on where to look next, a hint would be greatly appreciated.

I’d change the title from thump to buzz but I can’t
 
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What did I miss here? Did some parts get replaced? We hear that NAD did not know what the thump is caused by (hard to believe that!). Did the OP replace some capacitors or just identify the one's he was looking for?
The problem is not affected by the volume control level.
Great, you have just confirmed that your problem is in the amplifier stage.
I’ve isolated it to be in the preamp stage, so that rules out the large 2200uF electrolytics.
This is not the logic I would recommend...but is it fixed now? If so, what part of the circuit was faulty?

RA
 
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From posts #2 and #3, these are filter caps for the small and hopefully clean DC supply that biases the JFET preamp muting switches (Q509, 510) on - after a brief start-up delay. Any noise or disturbance on this DC supply to the JFET gates would be injected straight into the amplifier input along with the audio signal and that would not be good.

NAD schematics on the net tend to be old and crappy and this no exception. It's of model 3020B which seems to agree on parts numbers with the OP description.
 

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3120

Hi everybody it has been a long time...
Need a bit of help with this amp and my problem is closely related to this thread so here it goes:
Main problem is that the sound doesn't pass the preamp stage, checked most of the amp and what I sound is that the fets 509/510 aren't conducting, the voltage get as far as diode D507 (1n4148) on the fet side only get 0.3V and yes the diode is fine but I replaced it anyway.
Power amp stage is fine with around 10mv of dc at the outputs and bias is stable just below 30mv.
I am at a lost here and would welcome any help
 

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This is a start-up muting circuit. Replace the electrolytic capacitors C531, C532, C533 first.
Use 47uF/50V or 47uF/63V for all three of them. Watch their polarity. It's very unlikely that both
of the fets are bad. Can you hear the relay SW-1 close?
 
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SW-1 is a separate DC switch mounted on the front of the main power switch.

The indicated -20.8V should be +20.8V (you'll see it if you check some of the other online 3020 schematics), and, in my experience, C531 is the usual culprit in this situation. I use a 100V rated cap for C531 now.
 
Ok, power stage is working fine, injected square and sinus signal strait into the "lab" and "normal" input and have a nice and clean signal at the speaker terminals.
So the problem is the preamp stage but can't find anything wrong, just rebuild the preamp power supply with fresh capacitors the original ones looked very tired and now have correct voltages after the regulators but still now sound...
To StewartH83, there is no "sound" at all I misspelled "found" in my original post.
 
Grüße sgrossklass, was trying to figure out what's going on and it a brick wall.
By the junction from c531,r547/8/9 and d507 I have -50v ( anode -50, cathode 0.3v) diode was tested and replaced.
According to manual all voltages are correct within tolerance, (we have here 240v mains) so voltages on amp are slightly higher then stated in manual.
After voltage doubler I have -65 (manual -62) and so on
After some more testing I found that I have signal at the Source of the Fet J111 (509/510) but nothing at the Drain ( voltage at gate is 0.3v).
 

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Let me get that straight, -50 V at the C531 / R547 / R548 / R549 junction? Either SW-1c has been flooded with contact cleaner and is highly leaky even when open (proper cleaning required), or C531 is toast as mentioned several times. You should be seeing up to +17.5 V in this spot.

If C531 has already been replaced, check its polarity. **** happens.

I'll second the recommendation of a 100 V rated part for C531. Use a quality part with reasonably low leakage, Panasonic FC or similar should do. Low ESR not required.
 
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Thanks for all the advise
Amplifier is a NAD 3120 (3020B without tone control same service manual) and was unmolested apart from the capacitors on the power stage.
Preamp power supply was rebuild with Panasonic 100V, 105c caps and the same for the voltage doubler for the muting circuit.
Checked every capacitor with the markings on the board and circuit diagram/schematic and everything is correct, I did however found an unbalance voltage split for the 509/510 Fets.
 
Hi everybody (again)
I don't like to give up but this amplifier is testing my patience...
Did some more testing and this are my findings:
Injected square wave on all input RCA sockets and signal is amplified and is present at the "source" pins of fets 509 and 510.
Again signal injected on "lab" and "normal" inputs after preamp and signal (and sound) present at the speaker terminals.
So problem is limited to the "muting" power supply and driver fets J111.
Power supply rebuild with Panasonic 100V and 105c capacitors and are in the correct position according to the service manual and markings on the board.
All diodes and resistors tested and are all inside tolerance but for some reason at the node of R547, R548, R549 and C531 I read -50V instead of the -20V from the service manual and can't figure out what is going on...
Any ideas before I transform this amp into a brick ?
 

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