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nad c326bee integrated amplifier with defective pre amp circuit help?

T

TrevorBarten

Well-Known Member
According to the previous owner it was working but suddenly they could hear pops from time to time via the speakers until one day there was a very loud pop and it stopped functioning properly. I tested it and it seems like the problem is limited to the pre amp as the power amp is working fine if I use a seperate pre amp for testing. If I do try the pre amp it outputs static and a very low volume audio signal for one of the channels. So does anyone have any experience with similar issues? My first thought was checking for shorts between the collector and emmitor for the transistors but could this actually be the issue? And if it is what other components should I check and replace?
 
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Disclaimer: I am not an electronics technician and am fairly ignorant compared to many on AK. My only knowledge of this integrated comes from buying a malfunctioning unit cheap and "fixing" it with a couple of Deoxit applications. Responding mostly because your post has sat unanswered for two days. I believe this unit has been discussed before on AK. If you can't find these discussions with the AK search, try google.
Judging from what I've read about NAD the first thing is to look for bad joints or traces. Don't blindly replace parts, though I've been lucky enough that ignoring this advice has only cost me about $25 (don't ask about the time involved in replacing perfectly good parts, I don't track this for sanity's sake. )From my limited experience, don't be shy with the Deoxit. The service manual is available on hifiengine.com in case you might be interested.
 
Guido57 said:
Disclaimer: I am not an electronics technician and am fairly ignorant compared to many on AK. My only knowledge of this integrated comes from buying a malfunctioning unit cheap and "fixing" it with a couple of Deoxit applications. Responding mostly because your post has sat unanswered for two days. I believe this unit has been discussed before on AK. If you can't find these discussions with the AK search, try google.
Judging from what I've read about NAD the first thing is to look for bad joints or traces. Don't blindly replace parts, though I've been lucky enough that ignoring this advice has only cost me about $25 (don't ask about the time involved in replacing perfectly good parts, I don't track this for sanity's sake. )From my limited experience, don't be shy with the Deoxit. The service manual is available on hifiengine.com in case you might be interested.
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I have obviously already checked traces and cleaned the potentiometers and switches. Of course a trace could still be bad but I am not assuming it is that considering the age of the amp. Also I am not blindly going to replace parts I was planning on measuring for shorts on the transistors but I was just asking if anyone knew whether this is even worth my time.
 
The pre-amp is the worst part about the 326.

I have built Nelson Pass B1, a DCB1 (no coupling caps), and an ACP+ headphone amp/preamp and they all have massively improved the sound of the 326.

It is a great amp, but the only advantage that the built-in preamp has is convenience. At our house, I am the remote control....
 
Hammett said:
The pre-amp is the worst part about the 326.

I have built Nelson Pass B1, a DCB1 (no coupling caps), and an ACP+ headphone amp/preamp and they all have massively improved the sound of the 326.

It is a great amp, but the only advantage that the built-in preamp has is convenience. At our house, I am the remote control....
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Fair enough, I would still like to get it working though. I can decide if I want to use it after.
 
Do you have an oscilloscope? The proper way to troubleshoot this is to input a 1KHz sine wave and probe through the circuits in the preamp to see where the signal is lost.
 
TrevorBarten said:
Fair enough, I would still like to get it working though. I can decide if I want to use it after.
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Those two vertical boards: preamp-in module + preamp-out module are the "achilles' heel" here.

I have a C325BEE and these looks similar. These boards run hot and the caps fail.
Best to remove them from the main board, replace the caps, check the other devices on-board for possible failure.
The C325BEE used a combo of radial and SMD caps. I replaced the SMD caps with polymers for better reliability....and these may be on the C326 as well.

If you plan on keeping this unit examine the main power supply board for stressed components as well. Also many of the components for the +/-37V, +/-18V and 12V supplies get real hot too. I replaced out of spec parts and/or elevated these components off the board for better thermal control.

Also with the age of this unit the power amp stage, even though it works, will need adjustment along with the ISC section.

It's a worthy amp but being a NAD needs a little TLC. With proper components and a little physical placement adjustment will last longer than factory installed.
 
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