Beta Lead bad treble distortion

[Kel Varnsen]

Hey guys! Long time reader, first time poster.
Got a little Beta Lead issue... when I play with a mild overdrive setting, there's this harsh clipping in the treble range when I play around the 10th-12th fret on the low E string. Ever heard this? Kind of like a crappy distortion on top of the rest of the (good) sound. I've replaced the filter caps. (I didn't expect that to cure this problem, but just for the record, it did nothing)
Do Betas just sound like this, or is there something wrong with mine? It sounds the same on both the A and B channel. I'm just using a Telecaster's neck pickup...nothing hot. I can mitigate it by turning the treble down, but then the amp just gets too dark.
What do you think? Should I replace the rest of the electrolytics or anything? Bad input or power amp IC?

[Addam]

Is it possible that it's the guitar?  Maybe try a different one with the amp.  If it isn't at least you've ruled it out. 

[Kel Varnsen]

Is it possible that it's the guitar?  Maybe try a different one with the amp.  If it isn't at least you've ruled it out. 

Tried it with a guitar with slightly hotter pickups, and the problem is worse. It almost sounds like a torn speaker, but i've tried many speakers with it so that's not actually the problem.
At this point, I'll probably just need to get ambitious and start replacing stuff around the input/output sections of the preamp (since the problem is exactly the same on both channels)

[Kel Varnsen]

Might be on to something!
The service manual says the voltage in to the drive IC should be +8v, and the outputs should have around +4v. However, mine has between +9 and +10v in to the drive IC, and between 5 and 6v on the outputs. Assuming I'm measuring correctly, this is probably not good, right?
I attached a picture of the circuit where I measured. Doesn't look too complicated, but regrettably I'm not too knowledgeable with this stuff.
I plan on replacing the three big (original) 680uf capacitors (preamp filters?). Might that help?

[loudthud]

The voltage at that point will vary depending on the IC installed. Try all the different brands and date codes you can find. Either a 74C04 or 4069 will work in that socket. You can even stack ICs on top of each other or parallel up the unused gates with the used sections. If you really want to experiment, change the 470 ohm. As you increase the resistance, the voltage will go down, the gain of the chips goes up and the high end goes down.

[Kel Varnsen]

Wow, thanks for the info!  I'll play with that resistor a bit tonight and maybe order some ICs. Maybe it will resolve my issue. Definitely gives me some things to experiment with.
But do you mean it's probably okay that it has 5-6 volts at the outputs even though the service manual says it should be 4? I suppose they could've used a different IC in mine than in the service manual.