Hi everyone,
This is my first post on this forum, so your patient help is greatly appreciated!
So, this is the story of my problem: I bought a working NAD3240 with the idea of doing some minor upgrades (capacitor and thermal paste replacement, fixing broken speaker terminal). After I had done all of these steps, I wanted to do the voltage/current alignment. Going through the procedure in the manual, I discovered that there is no bias current on one of the channels. The only other debugging I did was to check all the voltage points on the schematic, but these all measured roughly correct.
In the meantime, I also ordered an 8-ohm load resistor to do some more debugging.
Please share your suggestions on what the most likely culprit would be. Where should I start debugging from?
About me and what I have available: I am an electronics graduate, however I have little experience in analogue circuits. I am interested in learning more about analogue amplifiers (at this stage the NAD in particular), so if you have any materials that will be relevant for me, please don't hesitate to share them! I am using the lab at my workplace, so I have at my disposal a scope and DMM.
Thanks for your help in advance. I hope that with your guidance I can resurrect this amplifier.
Regards,
Boris
This is my first post on this forum, so your patient help is greatly appreciated!
So, this is the story of my problem: I bought a working NAD3240 with the idea of doing some minor upgrades (capacitor and thermal paste replacement, fixing broken speaker terminal). After I had done all of these steps, I wanted to do the voltage/current alignment. Going through the procedure in the manual, I discovered that there is no bias current on one of the channels. The only other debugging I did was to check all the voltage points on the schematic, but these all measured roughly correct.
In the meantime, I also ordered an 8-ohm load resistor to do some more debugging.
Please share your suggestions on what the most likely culprit would be. Where should I start debugging from?
About me and what I have available: I am an electronics graduate, however I have little experience in analogue circuits. I am interested in learning more about analogue amplifiers (at this stage the NAD in particular), so if you have any materials that will be relevant for me, please don't hesitate to share them! I am using the lab at my workplace, so I have at my disposal a scope and DMM.
Thanks for your help in advance. I hope that with your guidance I can resurrect this amplifier.
Regards,
Boris
Welcome to diyAudio Boris 
Firstly I would advise you use a DBT (dim bulb tester) for all fault finding as it can limit damage if things go wrong.
First thing to check is whether the voltage across the vbe multiplier (Q17/18) increases as you turn the bias preset.
You need to reach around 3.5 volts before the output stage would begin to conduct.
If any of the original semiconductors have been replaced you may find the bias is out of range and would need a component tweak to bring it back.
Firstly I would advise you use a DBT (dim bulb tester) for all fault finding as it can limit damage if things go wrong.
First thing to check is whether the voltage across the vbe multiplier (Q17/18) increases as you turn the bias preset.
You need to reach around 3.5 volts before the output stage would begin to conduct.
If any of the original semiconductors have been replaced you may find the bias is out of range and would need a component tweak to bring it back.
First of all, I found the manual with a schematic here:
http://www.rsp-italy.it/Electronics/Audio%20Schematics/NAD/_contents/NAD%203240%20service%20manual.pdf
Looking at the schematic we find a typical VBE multiplier with the bias adjustment pot in the usual place, where, should the pot fail, go open due to corrosion, the bias falls and not rises. This is standard practice. If the problem is indeed a pot failure, you may be able to temporarily clean and reset the pot, but you should think about replacing it and the other one for the other channel.
http://www.rsp-italy.it/Electronics/Audio%20Schematics/NAD/_contents/NAD%203240%20service%20manual.pdf
Looking at the schematic we find a typical VBE multiplier with the bias adjustment pot in the usual place, where, should the pot fail, go open due to corrosion, the bias falls and not rises. This is standard practice. If the problem is indeed a pot failure, you may be able to temporarily clean and reset the pot, but you should think about replacing it and the other one for the other channel.