Fender LTB PA Head Distorted Output

[PepeSilvia]

Hey guys, looking for some advice on how to troubleshoot this amplifier. The output is distorted, not terribly, but noticeably. I've checked all the inputs and the aux return, it was constant across all of them, and the main output (preamp out) is coming out clean.

So it's got to be in the poweramp. My question is, how do I narrow it down from there? I've attached the schematic it, and you can see it's just a series of different output stages. I guess I could just start replacing transistors, but that's not very cost effective. Any advice?

Fender LTB PA - Schematic.pdf

[Enzo]

Series of different output stages?

I see one output stage - one pair of transistors, Q3, Q4.

Just replacing a lot of parts is a bad idea, what if it isn;t a bad part at all?

First, apply a sine wave to the power amp and scope the output. What does the distortion look like? Early clipping? C;ipping on one side only? Missing part of the wave? etc.

And do it with no speaker load first. Then with a load. Does the waveform change when the load is connected?

The system runs on +/-33VDC. Are both supplies present? Are they within a couple volts of 33v? And more important, aside from polarity, are both pretty close to each other?

[PepeSilvia]

You're right, I wasn't sure how else to refer to the transistors in front of the output pair.

I don't have a signal generator, I'll try it out with a bass, hopefully that will tell me enough.

I measured 38.8V with my DMM at both ends.

[PepeSilvia]

So this has sat in the closet for a few months while I've been working on other things, but here's the scope shot of the output waveform. It looks like just plain old clipping to me, but I'm not too experienced at diagnosing based on scope shots. This is with no load. Any ideas?

[Enzo]

Well, see what happens WITH a load.

The system runs on +/-33vDC I said earlier. If I read your scope shot correctly, your waveform is clipping at that level - 33v. In other words, normal.

SO see if it behave different into a load.

[Jazz P Bass]

I guess we will never know the outcome of this repair,
Here is a repost of the schematic.

[PepeSilvia]

Sorry, I wish I could remember what I did, but I fixed it!

[Jazz P Bass]

"The system runs on +/-33vDC I said earlier. If I read your scope shot correctly, your waveform is clipping at that level - 33v. In other words, normal."

Turned down the input level?