NAD C326BEE Stereo Amp with occasional POP! - Page 1
Forum section: electronics / repair

author: cincin
author_rank: Regular Contributor
timestamp: April 25, 2022, 01:08:20 am
content: My home stereo amp, a NAD C326BEE, started popping occasionally after I've changed home theatre PC. The pop is quite loud and doesn't appear to be affected by the volume knob. It's only on the left channel. It sounds like a static release, or a cap discharge of some sort. It has a bit of a static sounding ramp up to the pop.On some occasions the pop will activate the amp protection circuit, and I'll find the amp with the power LED on red, and unresponsive. The manual states that this protection circuit activates in case of "serious abuse of the amplifier, such as overheating, excessively low loudspeaker impedance, short circuit etc".I've confirmed that it's not the source. The pop occurs with no source connected, so it's not the new PC. I've also changed the speaker cables. The system performs otherwise normally.I've opened the amp for a visual inspection and found the plastic connector next to the D134 diode to be warped and yellowed by heat. The diode itself looks okay. Everything else seems clean.In the service manual (https://www.manualslib.com/manual/1441478/Nad-C-326bee.html) the part for that diode is a DIODE ZENER 5.1V 3W 5% DO-41 1N5918B (RoHS), part# 04-001-0059-90.My questions are:- Should I replace that diode or is it normal for those to run hot?- Could this diode be causing the POP? (I can't quite tell if D13 is only part of the left channel or not)- What else could be causing this POP?At first I had a faint suspicion that it might be caused by me coiling the extra cables together (causing induction or what not): power cables, patch cables, and speaker cables, near the power surge power bar. That's one thing beside the PC that changed since the pop started. I uncoiled most of it but it's still happening.Any help is greatly appreciated.Thanks!

author: coromonadalix
author_rank: Super Contributor
timestamp: April 25, 2022, 02:13:56 am
content: Before doing anything, can you check the ALIGNMENT PROCEDURES chapter and confirm if its okay ?on the speaker terminals, say volume at zero, do you have an dc voltage, even in the milli volts range ??A pop could be an imbalance between the push pull section of the amp.... preamp or the output transistors ...

author: andy3055
author_rank: Super Contributor
timestamp: April 25, 2022, 05:15:17 am
content: That could very well be a semiconductor intermittent failure. Can be a transistor or a diode, etc. Something after the volume control.

author: bdunham7
author_rank: Super Contributor
timestamp: April 25, 2022, 06:02:58 am
content: That sounds like a defective transistor. D13 is in the right channel, so it isn't a factor. If you can't reproduce it easily, you may have to shotgun it. This is not an uncommon problem. I didn't see any part numbers that jumped out at me, but I don't have much experience with NAD. If you can get hold of a grizzled audio tech veteran that has seen lots of these amps, they may have a better idea which ones are suspect. If not, I'd start by looking at at Q22/23/24/25, Q201/202/203/204, then Q26/27/28/29.Edit: EEVBloggermzachariashas a lot of expertise, but I don't know how active he is. It might be worth a PM to see if he's around.

author: FIXITNOW2003
author_rank: Regular Contributor
timestamp: April 25, 2022, 06:17:00 am
content: I would check the ESR of the caps, these units are know for it

author: David_AVD
author_rank: Super Contributor
timestamp: April 25, 2022, 06:46:10 am
content: I've not worked on that model, but have seen a lot of NAD amps with cracked solder joints around the areas that get hot.Sometimes they are so bad I've been amazed that the unit was still working at all !They also suffer from dried out caps, especially the ones near the heatsinks, though that doesn't usually cause popping.I would carefully resolder any cracked joints before moving on to transistors and caps.There's also a service bulletin for that model to do with the volume control motor supply, etc. Hifi Engine should have it.

author: cincin
author_rank: Regular Contributor
timestamp: April 25, 2022, 11:21:58 pm
content: Thank you all for the help. I appreciate the guidance, I'm pretty new to all of this as you may have guessed.And.... I'm afraid I've been careless taking notes. The amp is actually a C326BEE. Service manual here:https://www.manualslib.com/manual/1441478/Nad-C-326bee.htmlThe diode nearest the overheated connector is actually D134(the4was under the leg post), listed as: DIODE ZENER 5.1V 3W 5% DO-41 1N5918B (RoHS), part# 04-001-0059-90I've edited the original post to reflect that in the title. My apologies for the confusion.Quote from: coromonadalix on April 25, 2022, 02:13:56 amIn the C326BEE service manual, I'm looking at the ADJUSTMENT POINT DIAGRAM on page 11. I'll be able to do the Idle Current Adjustment, but not the ISC Adjustment as I don't have a function generator. However I'm already confused at step 1..."For left channel, adjust VR103. Voltage across TP1 & TP2 3 - 3.5mV"TP1 and TP2 have 2 pins each. Am I supposed to check across the pins of TP1 and then across the pins of TP2, and tweak VR103 until both TP1 and TP2 are within range? or, if I'm supposed to measure across from TP1 to TP2, then which pins should I use?I have 1mV on the right channel and 0mV on the left, with the amp ON and the volume at 0.Quote from: bdunham7 on April 25, 2022, 06:02:58 amThanks, given that I was confused and that the amp is a C326BEE, and the hot diode is D134, does that change your recommendation? (apologies for the confusion) Which transistors does that translate to on a 326?Cheers!

author: bdunham7
author_rank: Super Contributor
timestamp: April 26, 2022, 12:01:08 am
content: Quote from: cincin on April 25, 2022, 11:21:58 pmI tend to doubt that a zener diode would cause those symptoms. As I said, that static-pop event is a common issue with transistors in a wide variety of audio amplifiers, not just NAD. I'll have a look at the C326BEE manual when I get a chance and see if there are any of the usual suspects in there.

author: mzacharias
author_rank: Frequent Contributor
timestamp: April 26, 2022, 02:42:26 pm
content: I'm not specifically familiar with this model but I do think I would start with bad solder connections, and bad caps in baked areas of the board, both of which are notorious NAD issues.

author: cincin
author_rank: Regular Contributor
timestamp: April 26, 2022, 03:04:31 pm
content: Thanks! I will do a full inspection.Noob question: How do you test the adjustments with the 2-pin test points? All test points I've dealt with so far have one contact, and I can't seem to find a clear answer by searching the web... Am I supposed to check the voltage across the 2 pins? or from TP1 to TP2? etcCheers!

author: David_AVD
author_rank: Super Contributor
timestamp: April 26, 2022, 08:55:49 pm
content: Yeah, it's another odd NAD thing. On that model each of the two pin connectors is really just one connection. I confirmed this by looking at the schematic.So put one meter probe on either (or both) pin of TP1 and the other on TP2.

author: cincin
author_rank: Regular Contributor
timestamp: April 26, 2022, 09:21:13 pm
content: Oh geeze, yeah I guess I could've figured that out by looking at the schematics. Thanks though. Hoping to become less useless as I learn...

author: cincin
author_rank: Regular Contributor
timestamp: April 26, 2022, 11:23:08 pm
content: Alright, so the voltages of the Adjustment Points procedure were off by just 1mV. They were both at 2mV, now double adjusted to 3mV.My guess would be that's not the cause of the pop. But I wouldn't bet any money on my guesses when it comes to electronics.So I'll do a full careful inspection of solder joints, and look closer at the caps and transistors in the area that looks overheated.Then if I don't come up with anything, I should replace caps that are past the volume control or near the baked area, shot-gun style? followed by transistors if caps don't do it?Sounds like a sound plan towards better sound?Cheers!

author: andy3055
author_rank: Super Contributor
timestamp: April 27, 2022, 12:54:58 am
content: It is either a transistor or a bad/over heated dry solder point. Leave the capacitors to the last.

author: cincin
author_rank: Regular Contributor
timestamp: April 27, 2022, 03:39:13 pm
content: I did a pass of reflowing anything that looked suspicious on the main board. I'm assuming the problem is not likely coming from the control or power board (please correct me if I'm wrong). I didn't find anything truly cracked, just some less-than-well wetted joints. There's a slight discoloration of the PCB in areas that I assume got hot, so I looked extra carefully in those areas. Plugged it back in and the POP is still there, so, moving onto transistors.Are there specific transistors I should start with? More likely to fail?Start with the ones in the hot areas?I guess I can somewhat test them while on the board.

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2022, 04:24:21 pm
content: Quote from: cincin on April 27, 2022, 03:39:13 pmIf the pop noise is regular enough, you can try putting a bit of ice in a baggie and then carefully (don't pop the bag!) icing one transistor at a time. Just identify the transistors in the left channel after the volume control and try each one.

author: cincin
author_rank: Regular Contributor
timestamp: April 27, 2022, 05:33:07 pm
content: Quote from: bdunham7 on April 27, 2022, 04:24:21 pmIt pops every hour or so. I'm not sure I'm that patientGood ice trick though! Cheers!

author: andy3055
author_rank: Super Contributor
timestamp: April 28, 2022, 01:34:32 am
content: If you are up to it, interchange the transistors in the identical positions from Left to Right ch. If the pop changes to the other channel, you know which is bad.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: April 28, 2022, 10:36:48 pm
content: Ice in a plastic bag around Electronics or (Hot) electronics even worse.The stuff to try is freeze spray one transistor at a time before changing themor interchanging them also one at a time.Jeff

author: CaptDon
author_rank: Super Contributor
timestamp: April 29, 2022, 12:38:53 am
content: Do Not start tweaking adjustments!!!!!! I don't know why screwing with the pots is often the first suggestion that comes up??? Look for broken solder joints FIRST. Resolder ANY solder joint that looks even a tiny bit suspicious!! I have had to fix more than just a few amplifiers where they started tweaking the bias pots and the pot broke while adjusting and the amp had to be placed in 'The Smoking Section'. I can't ever remember an amplifier coming into the shop that was failed because the bias slipped. That adjustment usually only needs to be made if some components were replaced. Sometimes I think this section of the forum should be called the 'Impair' section were people will help you fix your unit to death. Or my favorite, If your unit isn't broken you haven't tried to fix it. Cheers mate, best wishes for getting the amp cured and sounding good again!!

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 29, 2022, 01:30:27 am
content: Quote from: cincin on April 27, 2022, 03:39:13 pmWithout seeing them I can think of reasons for cracked joints on both the control and power boards.

author: David_AVD
author_rank: Super Contributor
timestamp: April 29, 2022, 10:14:56 am
content: Yes, bias (idle) trim pots can go brittle over time. An experienced tech will usually spot them and make measurements first in case they break. I've replaced quite a few, mainly on gear that's 20-40 years old but there have been a few where the pot was cooked due to poor layout choices.I've seen some quite high (and sometimes almost zero) bias currents over the years. Sometimes it's the result of a prior fiddler, sometimes the trim pot has gone bad and occasionally someone's replaced a transistor in the vicinity and not bothered to check the bias afterwards. I always check the bias and offset (or mid point voltage if applicable) even if we don't have a service manual with specs. You get to know what sort of bias current is reasonable based on the design.Cracked joints are pretty much always the number one thing to look for with single sided (non plated) boards. Don't start replacing parts at random. Start with the basics first. After a soldering check, are the correct voltages from the power supply present? Is there any visibly damaged parts?I really despair when a repair comes in that someone's "had a go at". It takes so much longer as you've got to check that they haven't introduced more problems (wrong parts, poor soldering, etc). Sometimes the job has become a write-off due to their efforts and would have been a simple repair if they just knew their limits.

author: cincin
author_rank: Regular Contributor
timestamp: April 29, 2022, 04:36:44 pm
content: Quote from: andy3055 on April 28, 2022, 01:34:32 amAre those sorts of transistors expensive? If not I'd probably just swap them for new ones. Less work, new parts, etc.Quote from: Jeff eelcr on April 28, 2022, 10:36:48 pmI've done that before, with an upside down can of air duster. However, since it only pops every hour or so, it might get a little expensive in freeze spray. Also, my patience.Quote from: CaptDon on April 29, 2022, 12:38:53 amToo late. I've already adjusted the trim pots. They were a little stiff, but they didn't snap off on adjustment. They are now sitting at 3mV, which looks to be within spec. I'll do it again after the part replacements.I did all the solder joints that looked suspicious on the main board but my problem remains. I'm looking for cracked solder joints specifically right? Characterized by a ring around the pin? It looks like lead-free solder in there, everything looks dull and cold.Quote from: BrokenYugo on April 29, 2022, 01:30:27 amI'm sure that could happen, but would it cause a POP in the left channel only if the problem was on the control or power board?Quote from: David_AVD on April 29, 2022, 10:14:56 amI'm admittedly going outside "my limits" here, but I'm doing it to learn. And with the help of this fine forum I think my chances of succeeding are pretty good. My soldering skills are okay, I'm certainly more careful than most I think. I use a static-free mat and static discharge wrist band. What's the potential risk I should be weary of in reflowing some joints and replacing a few parts? I'l do some voltage checks before replacing anything.So do we all agree that my next steps should be:- Check voltages on the various rails, maybe in areas where the channels are split?- Since the POP only occurs once an hour or so, there's no way for me to test transistors with temperature. I can test the gates while they are on the board, but only somewhat reliably. So I should identify all the transistors that are part of the left channel only, and replace those one by one, starting with the ones nearest the areas that show sings of overheating?Sounds like a plan? or what else should I try first?Thanks for all the help!

author: bdunham7
author_rank: Super Contributor
timestamp: April 29, 2022, 05:11:05 pm
content: Quote from: cincin on April 29, 2022, 04:36:44 pmStart by looking up all of the transistors and see which ones you can get exact replacements (same p/n, not from NTE) for and which ones you need to make substitutions for. Also, if you can, remove and test some of the capacitors near the baked area. Those are probably pretty cheap and easy to come by, so it wouldn't be silly to swap all of them first.

author: andy3055
author_rank: Super Contributor
timestamp: April 29, 2022, 09:25:54 pm
content: If it happens only once an hour or so, this is going to be testing your patience anyway! As I suggested, if you interchange a transistor, you will have to wait till it pops again to observe which channel is popping now. Another thing is that once you try this method, it might not do the popping for days! When the heat gets in to the component, it sometimes will slow down the problem. Be prepared to have a lot of patience.The only thing that is certain is that the problem lies somewhere after the Volume control.

author: David_AVD
author_rank: Super Contributor
timestamp: April 29, 2022, 10:06:41 pm
content: 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.

author: Xenon
author_rank: Contributor
timestamp: April 30, 2022, 11:47:52 am
content: 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.

author: cincin
author_rank: Regular Contributor
timestamp: July 17, 2022, 07:30:14 pm
content: 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 2SK932Q105 MMBT5087Q106 MMBT5087Q107 MMBT5551Q108 MMBT5551Q109 MMBT5551Q110 MMBT5401Q159 MMBT5551Power-amp:Q123 2SA1015Q124 2SC1815Q129 MMBT5210Q125 MMBT5087Q126 MMBT5087Q127 MMBT5210Q128 MMBT5087Q130 MMBT5087Q131 KSA1220Q132 MMBT5551Q133MMBT5210Q134 NIQ135 NIQ136 MMBT5210Q137 MMBT5087Q138 MMBT5087Q139 MMBT5087Q140 NZT660Q141 NZT560Q142 KSA1229Q144 KSC2690Q145 NJJW0302GQ146 KSC2690Q147 NZT560Q148 MMBT5551Q149 NZT660Q150 KSA1220Q151 NJW0281GQ152 MMBT5551

author: andy3055
author_rank: Super Contributor
timestamp: July 17, 2022, 09:48:10 pm
content: 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.

author: cincin
author_rank: Regular Contributor
timestamp: July 18, 2022, 09:24:08 pm
content: 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!

author: David_AVD
author_rank: Super Contributor
timestamp: July 19, 2022, 11:42:23 am
content: 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 20, 2022, 10:20:31 pm
content: 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.

author: cincin
author_rank: Regular Contributor
timestamp: December 20, 2022, 09:40:15 pm
content: 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!

author: andy3055
author_rank: Super Contributor
timestamp: December 21, 2022, 12:07:56 am
content: 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.

author: cincin
author_rank: Regular Contributor
timestamp: December 22, 2022, 07:34:18 pm
content: 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!

author: Heinrich
author_rank: Newbie
timestamp: January 19, 2024, 04:24:22 am
content: 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.

author: cincin
author_rank: Regular Contributor
timestamp: January 19, 2024, 05:23:05 am
content: 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!

author: andy3055
author_rank: Super Contributor
timestamp: January 19, 2024, 05:54:59 am
content: Can you post pictures of the front and back of the faulty board?

author: cincin
author_rank: Regular Contributor
timestamp: January 20, 2024, 12:09:34 pm
content: I will, when I pluck it out of the amp to swap them.

author: cincin
author_rank: Regular Contributor
timestamp: January 25, 2024, 11:17:44 pm
content: Here you are.I replaced both boards, which (obviously at this point) fixed the problem.Yay!

author: andy3055
author_rank: Super Contributor
timestamp: January 26, 2024, 07:44:39 am
content: 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.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 26, 2024, 10:25:15 am
content: It would only take 5 minutes to resolder EVERYTHING.

author: cincin
author_rank: Regular Contributor
timestamp: January 26, 2024, 03:50:59 pm
content: I almost did take the time to re-solder everything, but the board was $10usd... The amp sounds great now!