Peavey KB100 KB 100 Distorted Output

[misterc57]

This KB100 has low volume and bad distortion when using any of the three input channels.

If I plug a signal into the "Power Amp Input" on the back, the amp sounds normal with good power and no distortion.

I would conclude the issue is in the pre-amp section but not sure where to proceed to root-cause the issue.

As always, thank you for helping me to learn! Mark

[Amp Kat]

Sounds like an op-amp maybe is gone bad. First you could check the +/- rails on them and see if your getting what around +15 and -15. Then you could run your signal in the input and use a scope to see the output waveform at the output of the op-amps. If you get some distortion on the output that should be the culprit. You should get a nice clean Peak to peak output out at moderate volumes if it's working right. Of course this depends on you're signal source. Either way you should see the distortion on a scope. If you don't have a scope you may just have to swap them until you get the right one if it's only one. Theres other problems also but I don't have the schemo but I know who does.

[Enzo]

Amen, check the +/-15vDC supplies. After that it is something the channels have in common. Go down the ICs and look at the output pins of each op amp. if yo9u find several volts of DC on one, it is likely bad.

[misterc57]

not finding the +15VDC or -15VDC

I am using a schematic found in another thread here regarding the kb100.

I do not have a scope.

The board is not labelled at all so with my inexperience it is tough to correlate the schematic to the physical board.

I looked for +15V and -15V at all pins 8 and 4 of all op-amps. Was only finding very low (fraction of a volt) voltage on each pin. Similar readings at what I thought to be CR32 and CR33.

I was not sure if I needed an input signal so I connected a FM radio signal to input 1.

Same low voltage readings. However now the amp is operating normally.

So even with the signal running through the amp, and the amp functioning at what appears to be normal operation, I still cannot find the +15V or -15V.

Am I testing this correctly? My meter (set to DC volts) had the common lead clipped to the chassis and I was probing with the red positive lead.

I have tried the amp a couple times today with a keyboard. It is operating fine. I suspect the initial issue will appear again at some point.

Thank you. Mark

[booj]

chopstick it. It's probably a bad connection if it works sometimes.

[Euthymia]

You probably "fixed" the bad connection while you were probing around with your meter. It may stay good for a long time, or not.

If the opamps are socketed, give them a good push with your thumb to make sure they are all the way in.

[Enzo]

Measure a 9v battery just to test your meter.

If the amp works, you definitely have some voltage at pins 4 and 8 of the op amps. Are you sure you are counting pins correctly.