Here's a good power amp fault

andjur78 · 3rd September 2005, 09:55 AM

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.

Leolabs · 4th September 2005, 02:13 AM

Do u have any schematic???

pinkmouse · 4th September 2005, 07:32 AM

Most likely a dry joint somewhere on the board. Do a blanket resolder and see if that helps.

audio3000 · 4th September 2005, 05:49 PM

Or clean and wiggle all the connectors. Also tap on the protection relay if there're some.

good luck.

jan.didden · 4th September 2005, 08:49 PM

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?

Jan Didden

anatech · 4th September 2005, 11:46 PM

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

anatech · 4th September 2005, 11:48 PM

I should add. Not a copy of an NAD. Possibly a good thing. Is it a 25100?

-Chris

mastertech · 5th September 2005, 12:02 AM

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

andjur78 · 5th September 2005, 09:21 AM

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

andjur78 · 5th September 2005, 09:34 AM

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!

4th September 2005, 07:32 AM	#3
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Most likely a dry joint somewhere on the board. Do a blanket resolder and see if that helps. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

4th September 2005, 08:49 PM	#5
jan.didden diyAudio Member Join Date: May 2002 Location: Great City of Turnhout, Belgium Blog Entries: 6	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? Jan Didden __________________ /Yes! Its out: Linear Audio Vol 5! I'm not an "accademic", just a plodder who loves a challenge - Ian Hegglun

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4th September 2005, 02:13 AM	#2
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Do u have any schematic???

4th September 2005, 05:49 PM	#4
audio3000 diyAudio Member Join Date: Jun 2005 Location: Canada	Or clean and wiggle all the connectors. Also tap on the protection relay if there're some. good luck.

4th September 2005, 11:46 PM	#6
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

4th September 2005, 11:48 PM	#7
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	I should add. Not a copy of an NAD. Possibly a good thing. Is it a 25100? -Chris

5th September 2005, 12:02 AM	#8
mastertech Banned Join Date: Mar 2005 Location: Australia	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

5th September 2005, 09:34 AM	#10
andjur78 diyAudio Member Join Date: Mar 2005 Location: Aussie	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!