KB 300 reverb question

[kk4msa]

I noticed when I turn the master reverb up on my kb300 I get a small distorted sound from the horn. Turn it off and its perfectly fine. I didn't take the assembly for the coil out when I serviced it so I don't know if anything is messed up inside. I did order a replacement to be on the safe side as these amps are 30 years old. I plan on taking it out when the replacement arrives.
But im wondering has anyone experienced this specific issue?

[dak]

I once got myself a KB300 and the opamp driving reverb happened to be shot and making quite a racket (somewhat curious since most reverb tank drivers I've seen are using the opamp output directly, and the KB300 circuit has buffering transistors that should make life easier for the opamp). Replacing it with an LM4562 made a definite improvement over using the original 4558 type in this place.

You can check whether the opamp is at fault by putting in some input opamp from a channel you don't need for testing, like using 2U3 in place of the possibly defective U5 (assuming a 1983 model).

[kk4msa]

Excellent information. I'll tell you one thing, those capacitors aren't getting any cheaper, had a couple of 5,000 UF with 55 working volts. I replaced those with 4700 UF 50 volts. Is there a place where I can find the owner's manual and some schematics for The Late '80s Model kb300? There are some differences I'm noticing between the ones I have and my amplifier.