Orange Crush Pro 120 thin and raspy

[Randall]

CR120 took a power surge at a club the first night of a two week tour, has sounded bad ever since. Dirty channel sounds thin and raspy, with not as much gain as it should have. Clean channel I THINK is OK, he doesn't use it, and I have read it doesn't sound all that great anyhow. Symptoms are replicated out thru the FX jack.

I cannot find a schematic, so I am flying blind here. It has six TL072 in the preamp. I see signal on all of them except U2 pins 1 - 3. But when the dirty channel is switched on I get a very peaked and ugly looking wave on the leading edge of the sine on U3 pins 1, 2 and 3 as the gain control is turned up, with not much difference in amplitude between them. This distortion can be followed down the line to the ICs downstream. The other amp in U3 looks fine. I replaced U3, but no change. All ICs have V+/- with none on any other pins.

Tough to troubleshoot this with no schematic. All I could find is some home brew drawings of the preamp, but I don't trust them. I already found a couple of things that don't resemble this amp.

Anyone have any sage advice or a source for a preamp drawing?

[Chuck H]

I couldn't find anything "comprehensive" either (Where's vintagekiki!?! Damn it)

It's more trouble without a schematic, but you could attempt tracing the signal through the amp. We're all familiar with what a reasonable distorted wave form looks like. And hopefully even how it looks approaching that in the earliest stages. If you start at the input and work your way through something is bound to look wrong somewhere and you'll have at least achieved some isolation.

[g1]

Here's a schematic someone worked up in another thread. I can't vouch for accuracy but it might give you a better idea as to the path of the separate channels. (edit: I guess this may be the drawing you mentioned that you don't trust)
As it was a surge and they are the first active devices linked to the outside world, the K30A fets may be good suspects.

https://music-electronics-forum.com/...100#post745100

[Randall]

Those home brew drawings may have the gist of the circuit at heart, but they really don't line up with the designations on the board much at all, if any. The problem is downstream of the FETs. I see abnormalities around a couple of the TL072s and associated pots, but I really don't know which half of which IC corresponds to which op amp on those drawings. The board is labeled U1 - 6, and the drawing uses each op amp separately as U1 - 12. I have changed out U3 and 4 on the board as my best guess, but to no avail.

I'm throwing in the towel. It's not worth more time to futz around with no schematic. I have better things to do. It is what it is.