NAD C326BEE Stereo Amp with occasional POP!

[David_AVD]

Oh, something I meant to say earlier was that occasionally bias and offset trim pots will go noisy. Sometimes the noise will be a random crackle, other times it will be sensitive to tapping the board.

[Xenon]

This is exactly the same as on my Marantz SR5008, so it is not a NAD thing.

In my case it is the center channel only. A few minutes after powerup it starts with white noise slowly getting louder, and then a loud pop. No more pops after this, it occurs only once after powerup.
I have noticed that if I bang the case when the noise is just starting to become audible, it ramps up quickly and triggers the pop much earlier. So I suspect a bad interconnect between boards which I will investigate.

It could be something similar in your NAD so I found it worth mentioning.

[cincin]

I had to put this on the back burner for a bit, but I'm back at it now.

I went through the schematics and listed all the transistors and caps on the left channel, past the volume pot. And that's a quite a few parts to check! I'm trying to put luck on my side and not have to actually test all of them before getting to the problem, and again could use your help. So I don't test the most sus bit last.

Is there an area, other than the area showing heat discolouration on the board, that I should check first?

Like, would it be more likely to be on the pre-amp board, on the power amp board? Should I check the transistors that are on heatsinks first, or last? Should I check the bigger parts first (I usually check the bigger caps first, does this apply to transistors)?

Are there specific parts that are know to fail?

Here's a lit of the transistors that I have to check:

Pre-amp:
Q104 2SK932
Q105 MMBT5087
Q106 MMBT5087
Q107 MMBT5551
Q108 MMBT5551
Q109 MMBT5551
Q110 MMBT5401
Q159 MMBT5551

Power-amp:

Q123 2SA1015
Q124 2SC1815
Q129 MMBT5210
Q125 MMBT5087
Q126 MMBT5087
Q127 MMBT5210
Q128 MMBT5087
Q130 MMBT5087
Q131 KSA1220
Q132 MMBT5551
Q133MMBT5210
Q134 NI
Q135 NI
Q136 MMBT5210
Q137 MMBT5087
Q138 MMBT5087
Q139 MMBT5087
Q140 NZT660
Q141 NZT560
Q142 KSA1229
Q144 KSC2690
Q145 NJJW0302G
Q146 KSC2690
Q147 NZT560
Q148 MMBT5551
Q149 NZT660
Q150 KSA1220
Q151 NJW0281G
Q152 MMBT5551

[andy3055]

If you can, take a plastic chopstick and tap on the transistors one by one. If it is one of them, you will be able spot it. Keep the volume pot somewhat low.

[cincin]

Good call. I keep forgetting to start with the chopstick test.

I went around and tapped on all the transistors and caps on the left channel, with the headphones plugged in so I could hear any feedback, but got nothing at all.

The amp does pop in the headphones when I power it on, but ever more dramatically, it does this crazy needle-scratching-off-an-LP noise like this, but a bit slower, after I turn it off:



That sort of sounds like a cap discharge to me. But a failed transistor or diode could be letting that discharge on the audio channel too I guess.

A lot of those components are tiny SMD caps and transistors with very small values. Are they as likely to fail as the bigger ones?

Cheers!

[David_AVD]

A lot of NAD amps run quite hot in various places. That and SMD electro caps is not a good combination.

I have a vague recollection of replacing all the SMD electros on a NAD amp in the past to solve various issues, but I don't think popping was one of them.

[shakalnokturn]

As others have said I wouldn't go replacing all the elecyrolytics. (Nor playing with bias... too bad if you have.)
If the amplifier was behaving well until you moved it when changing your setup double check the solder, NAD are terrible in that respect.
If the connector you mentioned is seriously heated check interconnect resistance, clean if necessary, replace or solder directly in doubt.
Even if it has no link with the symptoms you have, unless there is a big difference in PCB copper surface on cathodes there is no reason that D134 should get much hotter than D135.
Beware of the interconnect to thermal compensation transistors Q160 and Q360.
I also think the most likely (after solders) is a flaky transistor. I'd start with the differential pair Q136/Q137, try swapping those with Q336/Q337 on the right channel see how it behaves after.

[cincin]

For posterity: It was a bad solder.

I put the amp on the back burner for a while, again. When a sweet stereo microscope fell on my lap, I took the opportunity to do a close inspection and re-solder on the main board. I redid everything that didn't look like the rest. All the joints looked hazy and cold to me, but that's probably the lead-free solder they use now. Still, among those hazy joints were a few definitely suspicious ones (even hazier, with lines that could be cracks, etc). After doing that close pass, the POP disappeared. Happy camper.

You were all correct, all along.

One more thing: The amp sounds great, BUT I did redo the bias as per the service manual, which many of you said was a mistake. Some mentioned that those trim pots get brittle, and that the danger is in breaking them, but that didn't seem to be the only concern. So I have to ask, for my own peace of mind, what damage did I potentially do by redoing the bias? I don't think any of the trim pots broke. I did the adjustment carefully with a fairly new Fluke 115. So, what should I be weary about as a result of me having done that?

Cheers and many thanks!

[andy3055]

If you did the bias as per the service info and was able get them as specified, there should be nothing to worry. Over a period of time there would have been dust particles on the carbon tracks. If so, you could have scraped the tracks. Cleaning them with IPA would have been the first thing to do but now it is done anyway.

[cincin]

Thanks for explaining that.

Although..... the amp just popped again... so I guess that wasn't the fix. It did go for several hours without popping.

Back to the bench!

[Heinrich]

Hi, I see the post is quite old. Did you manage to find the fault? I have a similar issue with white noise on the left channel that comes and goes and at times triggers the protection circuit. I narrowed the fault down to be on one of the vertical buffer amp boards as I swapped them over and the fault moved to the right channel. I swapped all electrolytics on that board and also Q131 and it did not help, also all diodes. There is no bad solder joints on the board as I carefully inspected the board an reflowed all solder connections, and also not temperature related as it sometimes happens when still cold. My next move will be replacing the smd transistors Q125 to Q133, Only issue with that is that the original part numbers are obsolete and I would have to go with subs which can bugger the gains and matching a bit.

[cincin]

Hi,

I got as far as you did. I swapped the vertical boards and the problem switched channel with it, so I started swapping and checking parts on that board with no success. I reached out to NAD via Lembrook and it turns out those vertical boards are $12 CAD ea (+$15 CAD shipping)... cheap enough that I ordered 2. I figured might as well refresh the weak spot on both channels.

The boards arrive in 4 days.

The best place to reach NAD support for parts is: Parts-USA@lenbrook.com (even from Canada, apparently)

Cheers to clean sounding amps!

[andy3055]

Can you post pictures of the front and back of the faulty board?

[cincin]

I will, when I pluck it out of the amp to swap them.

[cincin]

Here you are.

I replaced both boards, which (obviously at this point) fixed the problem.

Yay!

[andy3055]

Jumper 146 looks like a bad solder joint close to the edge of the board.  There may be others. I would re solder the transistors also and check.

[Audiorepair]

It would only take 5 minutes to resolder EVERYTHING.

[cincin]

I almost did take the time to re-solder everything, but the board was $10usd... The amp sounds great now!