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signal after power switched off
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Old 5th August 2014, 09:10 AM   #1
rettedm is offline rettedm
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Default signal after power switched off

I have a curious problem with a 30+ year old NAD 3140 integrated amp.
Power switch on and panel lights glow, speakers give a thump, normal signs of life. But no signal. can hear very faint input source with volume cranked up but no amplification until the power switch is turned off..and then, normal amplified signal is heard but dies away as capacitors drain.

Any ideas? Power switch?

Thanks in advance !

Dave Rettew
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Old 5th August 2014, 09:17 AM   #2
Mooly is offline Mooly  United Kingdom
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signal after power switched off
Wouldn't be my first suspect if the power lights are on.

Check the rails (and to the preamp). It sounds more like a common derived bias voltage for the two channels is incorrect and passes through a working point as the rails collapse. Interesting fault... shouldn't be to hard to pin down but it needs measurements.
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Old 5th August 2014, 09:26 AM   #3
udok is offline udok  Austria
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Could be the output protection circuit. If there is an offset the protection circuit switches the output off. When you switch off the amp, the offset fades away and the protection circuit says: ok... for a very short time...
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Old 5th August 2014, 10:43 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by udok View Post
Could be the output protection circuit. If there is an offset the protection circuit switches the output off. When you switch off the amp, the offset fades away and the protection circuit says: ok... for a very short time...
possible.
Good thinking, too many coffees this morning !
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Old 5th August 2014, 02:18 PM   #5
shredhead is offline shredhead  United States
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Power supply is what I would check first.
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Old 5th August 2014, 02:57 PM   #6
rif is offline rif  United States
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signal after power switched off
When you first turn it on do you get amplified signal that then dies away (normal signs of life) just like when you turn it off? Is it both channels? I don't know much about this, but could it be the fuse on the input mains? Maybe the fuse holder is loose/incorrectly seated?
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Old 5th August 2014, 03:46 PM   #7
rettedm is offline rettedm
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There is no amplified signal on turn on, just on shut down. The protection circuit kicked in a few times intially but does not kick in anymore.

Un amplified signal can be heard faintly in both channels with the volume control at 100%.

I have rebilt a few tube amps and can work safely but am learning as I go, seeing most things for the first time....I appreciate simple troubleshooting advice and probably won't follow more complex ideas just so you know where I am.

Dave
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Old 5th August 2014, 03:52 PM   #8
rettedm is offline rettedm
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Also, this amp has dual transformers equal in size (most similar NAD amps I have seen have one) I am assuming one per channel but I guess it could be one for A speaker output and one for B. I'll chase that down a bit as it may help diagnosis.
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Old 6th August 2014, 01:43 AM   #9
rettedm is offline rettedm
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I checked the left and right offsets and set to zero. No change.
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Old 6th August 2014, 01:48 AM   #10
sregor is offline sregor  United States
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One possibility - had one with one channel low- problem turned out to be the soft clipping (or SLC not sure which switch) switch needed cleaning and a workout. YMMV
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