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Old 10th July 2008, 08:52 AM   #1
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Default Faulty amp - diagnosis?

I'm too skint to take this to the local repair shop, so if anyone can suggest the source of this problem I'd be right grateful!

Sorry for being such a n00b, but I don't know much about power amps and I could do with getting this fixed sooner rather than later.

The amp is a solid-state 50 Watt PA amp, I'm currently using it as the power stage in my guitar setup.

Symptoms:

Treble is there, but very little else! I've confirmed the signal from the preamp is fine, so it's either the power amp or the speaker. It's a single driver so there's no crossover involved.

There's no unusual humming or buzzing noises.

Used it for hours one day, it was fine. Switched it on the next it was up the spout. Next time I switched it on the sound was fine for about half a second before all the mids and lows just fell away again. This kinda suggests the speaker will produce the right sound when fed the right signal, so I'm assuming the amp is at fault. (I don't actually have a second speaker or second amp to compare, unfortunately!)

Both left and right channels on the amp are doing it. The power supply is the only common element so I'm guessing that's where the problem lies.

Transformer? Rectifier diodes? Capacitors? Which failure would likely cause a major loss of low response? How can I test?

If you can shed any light please post!
Thanks.
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Old 10th July 2008, 10:28 AM   #2
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I'd suspect a broken ground connection somewhere in the input/pre-amp section.
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Old 10th July 2008, 10:41 AM   #3
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Sounds like a coupling capacitor has opened up.
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Old 10th July 2008, 12:44 PM   #4
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>Sounds like a coupling capacitor has opened up.

Not if its doing it on L AND R channels simultaneously.

Try this test: connect audio to only ONE input (L Or R, but not both).
Do you get sound from BOTH outputs (L and R), albeit distorted bass-less audio? If YES, then it's certainly an open ground.
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Old 10th July 2008, 01:37 PM   #5
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You are correct. I missed the Left and Right part of the message.
I should have read more carefully.
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Old 10th July 2008, 04:09 PM   #6
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Quote:
Originally posted by Steerpike
Try this test: connect audio to only ONE input (L Or R, but not both).
Do you get sound from BOTH outputs (L and R), albeit distorted bass-less audio? If YES, then it's certainly an open ground.
I'll try that this evening and see what happens. Thanks!
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Old 10th July 2008, 05:12 PM   #7
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Default Post pictures for us man!


Will be nice.... let us inspect the unit.

regards,

Carlos
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Old 11th July 2008, 10:52 AM   #8
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Can't post pictures as I have no camera, sorry.

Tried feeding audio to only one input, only got output from that channel, other side gave no sound - actually just a very very small amount if I put my ear to the speaker, but I think that's just normal bleed-over.

I managed to borrow a DI box with 40dB of padding, was able to connect the amp's output into the line-in on my PC. Fed the amp some white noise and looked at the spectrum using Voxengo SPAN and it looked flat as a pancake - so it seems like the problem is with the speaker after all.

I'll proceed on that assumption for the minute, if it turns out not to be the case I'll come crying back here.

Thanks for posting guys, really appreciate it!
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