Bias & Volume troubleshooting on my NAD '88 JCM800 2205

Silverburst

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I finally found the culprit with a fresh mind this morning! :fever:

For the sale of future problems other members might come across, here are my notes of my n00b amp mind going through the process lol: As @Pete Farrington mentioned, check voltages and where it is off in fault mode it should be easy to find. So i figured (it is the first time I went through this more in depth amp diagnostics exercise :erk:): well then indeed the heater voltage is not getting into that V4a triode, resulting in missing voltage amplification (188v in stead of 306v in working mode) in that triode. The tube visually seemed to word fine because the other triode was lighting up. So I started wiggling the heather wire specifically and it revealed an oddly movement on pin 4! :idea: jup, a loose solder joint, like a needle in a haystack for a rookie amp tech. I could perfectly recreate the effect now just with that pin 4 contact, which of course now just makes perfect sense hindsight. Check the hidden MF-er!

Z6te2ua.jpg


Now that this is in my brains visual library, it is gonna help me to spot these things faster in the future. But this has been a very good exercise for my so far. No way better than getting out your DMM and dive in there (safely! draining filter caps, or while amp enabled one hand in my pocket).

I'm a bit proud here lol :woot:
Thanks for all your help and patience!
 
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XTRXTR

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You have a lot of corrosion in that amp, like it was in salt air near an ocean or something, as stated before you should reflow worn joints, maybe clean it up with a detailed cleaning. The way you approached Its good experience for your future troubleshooting. Cleaning it may have been the first step you could have taken. You stuck to it, found your own pace, didn't give up, and looks like you found the issue. Its definitely not always obvious. Good Job.
Glad you found the problem.:yesway::)
 

Silverburst

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If i remember correctly the platevoltage
on my v3b is fairly higher because it is
connected before the first dropping resistor,
more like 480 to 490v.

Anyone could confirm this? @RickyLee , you used to turn your 2205 inside out if I remember correctly and used to have voltage charts I believe. Could you chime in? Thx!
 

Tatzmann

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Thats not corrosion on the pins,
its flux residue. Whitish coat is
normal oxidation of over 30 years.
Since you found a bad solder on
a socket i would resolder all sockets
while cleaning up the messy wiring
in those amps, but since your a beginner
resoldering should be sufficient for now.

But before considering this play through
it for a couple of weeks, see if more gremlins
appear.

Yours is a Nick 2205, or isn't it?
Must be a Nick...
 

Tatzmann

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Circuitboard assembly and wiring
done by Nick, there usually are little
stickers on the pcb somewhere with
a name on it.
 


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