NAD C320 woes

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Hi

Yesterday I was listening to a CD when someone came to the door. I put CD on pause, and forgot all about it. I noticed today and turned CD player and NAD C320 off.

Shortly thereafter I turned both back on and CD played thru speakers for about a minute. Then no audio. CD player was still playing.

Then I noticed the power LED on C320 was red. I turned volume down (but it was not high) and power cycled C320. Same thing, 30-45 seconds of audio and then LED goes from green to red and no audio.

I've since:

  • disconnected everything, speakers and all inputs, power up and same result
  • turned soft clipping off, power up and same result
  • on power on try each input in turn, but same result
  • disconnected pre-out from main-in (both), but same result

If it matters, I am in Germany (where me and amp have been for many years).

I opened case and dont see any obviously blown capacitors, there's no bad smell, and it doesn't seem particularly warm anywhere (as compared to say inside of a PC).

Suggestions on how to either fix, or narrow down on the issue here, would be greatly appreciated. I've had amp for 20 years + --> Its like an old friend.
 
If the delay is always 30 to 45 seconds no matter how long the power has been off then I think it is less likely a thermal issue. It’s possible there is a small dc offset on an output and the protection circuit’s low pass filter cauusee the delay.

Typical ageing faults are dried out electrolytic capacitors and pcb trace cracks.
Good luck
 
Thanks for responses. I've ordered multimeter to check things and I will make short video using thermal camera on next power up (once multimeter arrives).

I should say I'm relatively inexperienced here and, well, I'm ultra cautious with electronics.

"Its possible there is a small dc offset on an output and the protection circuit’s low pass filter causes the delay."

I do think it's thermal issue causing power cut off, as it never goes directly to red. How to verify the above?

"Check for amp output DC offset before the output relay (either end of R250 would work).
Should be no more than a few mV or so."

Er, I will try to figure out how precisely to do that?
 
Multimeter set to read mV, leads in appropriate plugs.
Black lead into negative speaker output.
Red lead to either end of R250 take measurement.
Red lead to either end of R150 take measurement.

Looking for below 30mV is generally fine.

Be careful around the mains voltage - it is lethal. So too are supply capacitors, even after power is off.
Get clip on probes for your meter - that way you can set it up and turn power on with your hands out of the amp. Also minimises risk of slipping with a probe and letting smoke out!

Let us know how you get on.
 
I've had a look at the service manual.
The protection circuit looks to be triggered by 3 situations:
- Excessive, sustained dc offset on either speaker output
- Over-current in either output stage
- Loss of power, probably AC power, to the amp.

To stop it being dc offset triggered by normal music, it has a low pass filter. This means that it takes some time for the protection to trigger. Based on the component values in the schematic, I estimate that if one channel has zero dc offset and then the other has to have about +160mV OR -44mV to cause the red light after 35 seconds. The thing actually sums the outputs of both channels so the time delay depends on what both dc offsets are.

You can attach voltmeter probes to the speaker output on one channel and switch the amp on with no input. Observer the dc offset. It should stay within +/-30mV. Switch off and repeat with the other channel. See if either exceeds this range or changes a lot over the 35 second or so until the red light comes on.

If the dc offsets look ok, then it may be the over-current protection. This is harder to observe. The over-current appears to trigger if more than 10A is drawn. You'll have to take the lid off and measure the voltages across TP1 & TP2 for one channel and TP3 & TP4 for the other. Do not accidentally short these pins together.
The voltage should be less than 10mV (I estimate) with no input. If the voltage gets above about 1.2V the protection will kick in. If so, it could be that something is heating up and causing the output stage bias current to rise out of control in one of the channels.

It might be the AC detection but less likely than the above. Check the other two first.
 
So, multimeter arrived and I did some measurements.

The amp has also been off for several days, but when I turned it back on problem remains thought it took a bit longer to go to "red" state. I made video with thermal camera as it powered up if that will help anyone.

I measured the various resistors and got

  • R453 220
  • R151 10
  • R150 1.9
  • R452 220
  • R251 10
  • R250 1.0

[ It was pretty hard to measure the R150/R250 due to their placement on the board and the cables/probes I have with the multimeter. The other were a bit easier to measure. ]

As to "attach voltmeter probes to the speaker output on one channel and switch the amp on with no input ..."

I did this after amp had been powered up to make the thermal video, but while it was off again. I saw ca: 200 mV but that number was steadily decreasing across both speaker outputs, and broadly the same rate and I presume that means something is discharging?

Anyways, then i turned amp on and measured R and L in turn (switching off in between).

Right - number steadily increased to 1.3V and then the red state (with audible "click" from some component

Left - number was steady around minus 10 mV until the red state. Note it was minus even though I had red/black on red/black.

what does that tell us?

thanks for all advice, RT
 
attaching photo from relevant area of board with R150/R250/etc.
 

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I also measured across TP1/2 and TP3/4 as I powered on until red state. It oscillated pretty wildly, but I saw nothing more than around 30 mV or so. Same sort of oscillations on both pairs, and in both cases often more that 10mV but never in the hundreds.

And I need to get some clips as with the probes that came with multimeter, bad eyes, and shaky hands it's not easy :-(
 
So, multimeter arrived and I did some measurements.
Anyways, then i turned amp on and measured R and L in turn (switching off in between).

Right - number steadily increased to 1.3V and then the red state (with audible "click" from some component

Left - number was steady around minus 10 mV until the red state. Note it was minus even though I had red/black on red/black.

what does that tell us?

thanks for all advice, RT
You have a dc offset drift problem on the right channel. The click is the output relay disconnecting the speakers to protect them. There is a single protection circuit for both channels.
I'll take a look at the schematic again.
 
Here's the right channel power amplifier stage schematic. I'm posting this so others can more easily suggest culprits.
For a start you might want to visually inspect C116 and other electrolytic capacitors for leaking fluid on the circuit board. The electrolytic capacitors are usually metal cylinders with plastic wrappers.
 

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Thanks. C116 looks OK, in fact all the caps look OK (to my untrained eye). Image attached.

btw, I measured R150 several more times and could not get a consistent reading. It was between 1.0 and 1.9, but varied a lot. The other resistors were much more consistent but also easier to get the probes onto.

Does rapid variance when measuring across TP1/2 and TP3/4 tell me anything or is that expected (or maybe user error?).
 

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Don't worry about the value of R150. That's probably measurement error. Multimeters aren't always good at low resistances. Disconnect speakers of course.

I would expect some initial, wild variation in the voltages across the test points after power up. How long does it take to settle down and what range does it settle to? I don't know the NAD bias setting but I would expect the voltage to remain less than 20mV.
 
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Always check the supply voltages are correct as a first step. C15 and C16 should have close to the full supply across them which looks to be -/+46v. Also check the voltage across the two 4.7 volt Zener diodes D11 and D12. Should be around 4.7 give or take.

If those check out OK then you might have success using freezer spray used very sparingly to see if anything does seem thermally defective. Its quite a complex design and although there are a multitude of tests that can be done to localise the fault, you would need to have a good working knowledge of the circuit and how to interpret the results. Freezer just might be a quick route for you.

(A can of cheapo air duster which is butane based will work well as freezer if you invert the can. Aim to dispense literally a single drop at a time onto components such as the small signal transistors)
 
Can you measure the voltage across R123? I want to see if it also rises to 1.3V or does something else. This will narrow it down.
Be careful with those probes not to slip.

(PS: In the schematic R123 is 47k but in your photo it looks like 39k. So a production tweak at some point. This doesn't matter for tracking the problem down.)
 
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I have thermal attachment for my iPhone so can make thermal videos of boot up (but mp4 attachment limit is quite small here). And also attach thermal images. Problem is its quite hard to see whats what on those image.

Mooly: I am a little bit nervous with "supply voltage" and don't quite know what its suggested I do with the freezer spray (whicjh anyways I dont have for now). What to expect when all is OK and what to expect when all is not OK?
 
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Supply voltages are the key operating voltages at various points in the circuit. There is the main -/+46 volts DC plus the other derived voltage rails that I mentioned.

Checking supplies is always the first thing to be done.

Freezer spray would be used while you monitored the DC offset. Cooling a good part should have no effect, a bad part or thermally defective part would see the offset alter dramatically. The secret is to apply the freezer spray sparingly. If you blast a whole area and the fault is fixed then you have no idea what you actually cooled. Careful control of the can and its straw nozzle allows dispensing a drop at a time. Small semiconductors would be the first parts to try that on.
 
traderbam: I measured R123 and its is 39k (well 38.7k). On powerup it does increase to 1.3V before the cutout click.

Mooly: I am inexperienced and therefore cautious. Anyways for now I dont have freezer spray, and in Germany on a Sunday everything is closed. I tried to make measurements as asked. For C15/C16 I cannot access/see the "legs" to touch with my probes. I did not dismantle the whole unit yet in fear I'd not be able to put it all back together. For both D11/D12 I am seeing 550mV, either +/- depending on red/black orientation. To be clear I am touching probes to legs on both sides of D11 and measuring, while power is applied. Then same with D12.

Both: I really do appreciate the help!!
 
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