Peavey KB 300 Distortion on Channel One

[misterc57]

I picked up a KB 300 with known issues across all channels. I cleaned all pots and re-soldered some input jack legs.

This got Channel 2 and 3 working properly. When using channel one 1/4" high Z input the sound is distorted.

All testing done using a synth keyboard piano sound.

Before I pull the board out again I wanted to get some suggestions. I have not been able to find the proper schematic. This unit has the XLR input and does not have level pull gain pots nor does it have the comp. led.

I have attached a front panel pic from the manual of the KB 300 I have.

I was going to swap IC's next from another channel to see if that changes anything.

2nd possible issue: I also thought my dual piezo was not working as I could not hear them. But testing them separate from the 15" I did have sound. The volume balance between the 15 and the piezos is such that I can barely hear the piezos. In reading some other posts folks talked about a resistor in series with each piezo, I do not have any resistors.

Thank you

[Jazz P Bass]

Here is the proper schematic.

Make sure that the EQ out jack contacts are clean.
There is a normally closed contact internal to the jack.

[Enzo]

Resistors would lower the piezo output, not raise it, so missing resistors is not a cause of low output. The piezos only add sizzle to the highs, not a substantial amount of sound.

Channel one comprises just two ICs, and they are most likely the source. First check I do is look at pins 1 and 7 of each for unwanted DC offset. Usually DC ther is a bad sign. U1 and U2.

You can further isolate with the insert jack. Plug your signal into the insert half way - to the first click. Does that sound distorted? We bypassed the input stage. You can test the input stage by playing into the input, then plug a cord from insert into power amp in. clear or not?


And you can always get the Peavey schematics from customer service there.

[misterc57]

Channel one comprises just two ICs, and they are most likely the source. First check I do is look at pins 1 and 7 of each for unwanted DC offset. Usually DC ther is a bad sign. U1 and U2.

Would I be measuring DC Offset across pins 1 and 7, or from each pin to ground? If DC is there should I try swapping the IC from channel 2 or 3?

Thank you

[The Dude]

Measure each to ground. The only pins NORMALLY speaking, that have any DC are the supply pins. If there is DC on other pins, remove the IC and see if the DC is gone from the pads where the IC solders in. If the DC is gone, the IC is likely bad.

[Enzo]

The 8-leg IC is two op amps in one IC. The power pins, 4 and 8, are common, otherwise each section is independent. Thus there is no need to l=measure 1 to 7, each is doing its own thing.

[misterc57]

I cleaned all the jacks before proceeding and it is working fine now.

Thank you for all the help!