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Old 12th February 2005, 02:50 AM   #1
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Default Yorkville PA powered head distortion

My daughter's group sent their PA head home for me to look at. The output is very low and very distorted. If you crank up the volume it will start to play properly for a few seconds and then fade into distortion again. If you hit it again it will start to play loud and clear but then fade again.

Any idea what componant may be causing this? I checked the output with a volt meter and it is at 3.5 mV. I don't have scope. The ripple measures OK at the filter caps as well.

Any help will be greatly appreciated

Blessings Terry
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Old 12th February 2005, 08:04 AM   #2
Enzo is offline Enzo  United States
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Is this a "box mixer," looks like a guitar head with preamp/mixer controls on front and speaker jacks on the rear panel?

Take a spare guitar cord and plug it into the MAIN OUT jack - there musy be something like that on the front probably lower right. Plug the other end of the cord into the POWER AMP IN jack. If this restores the sound, then the POWER AMP IN jack has bad normal contacts inside. They are the contacts that open when somethig is plugged into the jack. When the jack is not in use, the contacts close and carry the signal past the jack. Bad contacts - no signal passing.

If this is not it, isolate the problem. PLug a signal into the power amp in. Does it steadily come out the speaker? We are verifying the power amp.

Run a line from the MAIN out jack to some other amplifier and listen to it. Is that signal steady and sounding good? We are verifying the preamp/mixer part.

I expect there is a basic graphic equalizer in there too. It ought to have IN and OUT jacks on the front panel too. The inout jack on it could be faulty same as the power amp in jack we described above.

The signal comes in through the preamp channels and are mixed together. then it goes through the master volume stage. The MAIN out is that point. Form there it routes through the graphic and then on to the power amp(s). There are front panel "patch" jacks between those sections for versatility sake. ANy jack there that is an input will have normal contacts that can fail.

It may be something else, but your decription of the symptom is classic bad jack.
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Old 12th February 2005, 12:08 PM   #3
jaycee is offline jaycee  United Kingdom
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or more likely bad capacitors in the signal path.... especially electrolytics
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Old 12th February 2005, 01:15 PM   #4
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Hi Enzo,

You nailed it. I had already discovered it last night after I posted this thread. I had done just as you suggested and inserted a patch cord into the effects/return jacks and everything worked great. I then tested both sides of the contact on the return jack and found it to be bad. A little cleaning and it is working fine. Thanks so much for your insight. Had I not found the problem you would have led me right to it.

This place is awesome!

Blessings, Terry
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Old 14th February 2005, 04:31 AM   #5
Enzo is offline Enzo  United States
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Jaycee - I had the advantage of servicing those things for a living. Bad shunt contacts on patch jacks is maybe THE single most common problem. Bad caps is a reasonable thing to look for though.
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