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Old 3rd September 2005, 09:55 AM   #1
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Default Here's a good power amp fault

This is the output waveform (time triggered) of an AMC 5 channel amplifier model (I think it is a copy of a NAD?)

One channel exhibits this large spike (as per attatch) thru the speaker about once every half hour or so. Totally makes you jump out of your skin!!!

Does anyone have any ideas on what would cause this fault?
It occurs regardless of a speaker load connected or not.
I know it is not the power supply as the other 4 channels don't have this fault at all.
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Old 4th September 2005, 02:13 AM   #2
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Do u have any schematic???
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Old 4th September 2005, 07:32 AM   #3
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Most likely a dry joint somewhere on the board. Do a blanket resolder and see if that helps.
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Old 4th September 2005, 05:49 PM   #4
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Or clean and wiggle all the connectors. Also tap on the protection relay if there're some.

good luck.
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Old 4th September 2005, 08:49 PM   #5
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I vote for an intermittend leaking electrolytic cap. Or, less probable but possible, a semiconductor (diode or transistor). I don't think it is a solder joint. Forget the output relay. As the man said, do you have a schematic?

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Old 4th September 2005, 11:46 PM   #6
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Oh nice! I used to service these things. Resoldering the board will be lots of fun! They normally blast the outputs right to heaven (or hell depending on your outlook).
This may be a bad solder joint or an intermittent transistor junction. Either way the amp is unsafe at the moment.

There was, at one point, another board added to the heatsink that makes getting to the screws for the outputs difficult. Draw or take pictures and do take your time.

I will also say I've never seen one of these with this fault. They were normally blown by the time I saw them. A variac may be handy here.

-Chris
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Old 4th September 2005, 11:48 PM   #7
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I should add. Not a copy of an NAD. Possibly a good thing. Is it a 25100?

-Chris
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Old 5th September 2005, 12:02 AM   #8
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looks like semiconductor problem to me most likely of vas origin, you cant pin down a fault without doing some tests first and you dont mention any tests
youve done so my suggestion is highly generalised,good luck

cheers
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Old 5th September 2005, 09:21 AM   #9
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Thanks for the pointers guys.

Yes, it is the AMC model 2N 100 (5 channel amplifier)
It employs dual rails +/- 40 volt and +/- 70 volt rails.

(were AMC making stuff for NAD or not??...maybe there is a
parallel to the Nad circuits?. I know some early CD players were.

I have the schem in paper form and will try to beg, steal or borrow a scanner to get it online before the thread runs out...
pcb pick.

Regards
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Old 5th September 2005, 09:34 AM   #10
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And some tests that have been done.

Tests so far were:
test 1 loaded 8 ohms with 1K sinewave test (0.5 hours)
test 2 loaded with a speaker load (a very old cheap speaker)and music playing (12 hours)
test 3 with the OPS loaded / and unloaded,amplifier idling and no driving signal applied to the input.The trigger sensing shows waveform (as per first pic). The Spikes show up regardless of loading the OPS.

I will redo test1 and see if the spike shows up when it is drawing some load current.

The pops occur about every half hour or thereabouts so maybe it is a capacitive timing thing. This is a good one!
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