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audio output distortion that corrects after 20 minutes

M

mikeh

Jan 1, 1970
0
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
 
C

Charles Schuler

Jan 1, 1970
0
mikeh said:
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.

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.
 
A

Arfa Daily

Jan 1, 1970
0
Charles Schuler said:
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.

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
 
M

Meat Plow

Jan 1, 1970
0
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

It's a rack mount piano MIDI module.
 
D

Dave Plowman (News)

Jan 1, 1970
0
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.

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.
 
E

Eeyore

Jan 1, 1970
0
mikeh said:
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.

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

Graham
 
M

mikeh

Jan 1, 1970
0
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.

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.
 
E

Eeyore

Jan 1, 1970
0
mikeh said:
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.

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


Graham
 
M

mikeh

Jan 1, 1970
0
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 [email protected] London SW
To e-mail, change noise into sound.

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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