Peavey KB3 keyboard amp problem (distorted output)

[pavthefiddler]

Hi, I have a keyboard amp by Peavey, model KB3.

http://www.peavey.com/products/keyboard/kb/index.cfm/item/72483/KB 3.html

I have it for a couple of years now and it developed a strange issue. After about 2 hours into the use (i.e. in the middle of the gig ) the sound gets really distorted. Also the monitor input starts generating a white noise. I gather this must have something to do with the output transistor or other element overheating or something similar.

I did a test and played music through it quietly for a whole day and couldn't replicate the issue, but with volume turned up the distortion came back after about 30 min.

I do have experience with DIY electronic equipment but never serviced power amps before so I don't really know what to look for.

What components could be causing the above issue?

Thanks!

 

[KingKai]

I used to play with a keys player with an older version of this amp. His amp would make all kinds of noise after playing for a while. Turns out the send / return jacks were noisy when vibrated by playing. The easy way to check this is to put a short patch cable in the send / return loop and see if that helps. if so then clean all the 1/4 inch jacks with cleaner and a brush. Actually cleaning the jacks is never a bad idea!

Hope this helps - Irv.

 

[JohnRoberts]

Hi, I have a keyboard amp by Peavey, model KB3.

http://www.peavey.com/products/keyboard/kb/index.cfm/item/72483/KB 3.html

I have it for a couple of years now and it developed a strange issue. After about 2 hours into the use (i.e. in the middle of the gig ) the sound gets really distorted. Also the monitor input starts generating a white noise. I gather this must have something to do with the output transistor or other element overheating or something similar.

The scenario sounds heat related, but I am unclear of a specific mechanism that acts like that. I guess it may be helpful to get a better description of the nature of the distortion.  Things that can change with heating are thing like amplifier class A bias, but this doesn't cause audible distortion as much as release of smoke for too much, or crossover distortion for too little, but heat usually causes more class A current.

I did a test and played music through it quietly for a whole day and couldn't replicate the issue, but with volume turned up the distortion came back after about 30 min.

I do have experience with DIY electronic equipment but never serviced power amps before so I don't really know what to look for.

What components could be causing the above issue?

Thanks!

the keyboard amps are similar to a bass amp but probably flatter...  If you don't have a scope, maybe you can make some before and after measurements of DC operating voltages for the output stage with a VOM,.

perhaps make a sound sniffer, to probe around in the the circuit and see if the signal gets distorted before the amplifier or in the final amplifier stage.

Don't dismiss looking for something out of place... like something loose from the heat sink.

JR