NAD C326BEE Stereo Amp with occasional POP!

[cincin]

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!

[coromonadalix]

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 ...

[andy3055]

That could very well be a semiconductor intermittent failure. Can be a transistor or a diode, etc. Something after the volume control.

[bdunham7]

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:  EEVBlogger mzacharias has 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.

[FIXITNOW2003]

I would check the ESR of the caps, these units are know for it

[David_AVD]

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.

[cincin]

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.html

The diode nearest the overheated connector is actually D134 (the 4 was under the leg post), listed as: DIODE ZENER 5.1V 3W 5% DO-41 1N5918B (RoHS), part# 04-001-0059-90

I've edited the original post to reflect that in the title. My apologies for the confusion.

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 ??

In 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.

That sounds like a defective transistor.  D13 is in the right channel, so it isn't a factor....  If not, I'd start by looking at at Q22/23/24/25, Q201/202/203/204, then Q26/27/28/29.

Thanks, 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!

[bdunham7]

Thanks, 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?

I 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.

[mzacharias]

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.

[cincin]

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? etc

Cheers!

[David_AVD]

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.

[cincin]

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...

[cincin]

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!

[andy3055]

It is either a transistor or a bad/over heated dry solder point. Leave the capacitors to the last.

[cincin]

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.

[bdunham7]

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.

If 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.

[cincin]

If 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.

It pops every hour or so. I'm not sure I'm that patient  Good ice trick though! Cheers!

[andy3055]

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.

[Jeff eelcr]

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 them
or interchanging them also one at a time.
Jeff

[CaptDon]

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!!

[BrokenYugo]

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).

Without seeing them I can think of reasons for cracked joints on both the control and power boards.

[David_AVD]

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.

[cincin]

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.

Are those sorts of transistors expensive? If not I'd probably just swap them for new ones. Less work, new parts, etc.

The stuff to try is freeze spray one transistor at a time before changing them
or interchanging them also one at a time.

I'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.

Do Not start tweaking adjustments!!!!!!

Too 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.

Without seeing them I can think of reasons for cracked joints on both the control and power boards.

I'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?

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.

I'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!

[bdunham7]

Sounds like a plan? or what else should I try first?

Start 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.

[andy3055]

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.