audio output distortion that corrects after 20 minutes

Feb 14, 2007 9 Replies
M

This is a piece of music gear, a Roland MKS-20 piano module, which has an audio output problem where the volume of both channels is low and very distorted when the unit is first turned on.



Gradually, over the course of 15-20 minutes, the volume surges up and down, and the distortion becomes less and less. After the 15-20 minutes the distortion is gone, and the volume stabilizes too.



Both the left and right 1/4" jacks and the stereo headphone jack experience the same problem, but the problem corrects usually quicker at the headphone output.



Can anyone suggest where I might start to look for the cause. I've thoroughly cleaned all the outputs already, even though that doesn't seem likely to be the cause.



I'd appreciate any suggestions.



Thanks, Mikeh



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C

Push-pull amplifiers can suffer from crossover distortion. It is a bias issue ... some modest forward bias is required to reduce this distortion. A simple check: is the distortion much worse at low volume levels? If yes, then it is probably crossover distortion and you will have to investigate the forward bias circuit for the output transistors. If no, then it could be any number of things such as a bad electrolytic capacitor, a transistor, and so on. Not an easy one to fix, by the way.

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A

I wouldn't have expected an electric piano to have contained a power amp. Is this a power issue we are talking here, or low level audio output to an external PA amp ? Either way, it should be easy enough to track the problem down with a hair dryer and a can of freezer.

Arfa

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M

On Wed, 14 Feb 2007 23:05:19 +0000, Arfa Daily Has Frothed:

It's a rack mount piano MIDI module.

Pierre Salinger Memorial Hook, Line & Sinker, June 2004 COOSN-266-06-25794
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D

Since it's common to both channels I'd first look at the power supply rails feeding the audio amps - sounds like one leg starts off low then gradually comes good, so likely a smoothing or decoupling capacitor.

*Why is the man who invests all your money called a broker? Dave Plowman dave@davenoise.co.uk London SW To e-mail, change noise into sound.
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E

Sounds like the power supply to the audio output to me.

Graham

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M

Thanks, distortion is just as bad at all volume levels. And at all outputs too: there's a headphone, L/R Hi-Z and L/R XLR. Sounds like this one might be a bear.

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E

No, it's simple. Find where the volts are dropping.

Graham

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M

Thanks, it does sound like a good place to look first. Lucky me, I found a source to get the service manual.

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