Is it "ghosting"?

[drewl]

Got a Peavey 400BH that had some funky distortion on it (so I was told) and when looking at the output on a scope the top half of the sinewave has like a smaller clipped wave or oscillation or something running through it.
Is that what ghost noting looks like?
I haven't played it yet, I was just looking at it at work.
There does seem to be a bit of ripple on the supplies, so the caps have to be changed.

[Enzo]

SOunds like a parasitic, but check power supply first. The main HV +/-52v rails will have a little ripple, it is a simple one stage filter so there is a limit to how pure it can be. Not a lot, and the same amount on each. If one is more than the other, that's a clue.

There are two sets of +/-15V rails. the zener ones off the HV are for the power amp itself, and should be pretty darn clean. The ones from the three leg Vregs are for the preamp only. SO check them if you are using the preamp at all for this testing.

I usually run the 400BH - or any of their stand alone amps - by itself. Pull the 5-hole Molex off the corner. You can clip-lead a signal to the pins there for test.

Check the solder on all connetions on the output transistor board, especially the two three-wire connectors back to the main board.

Top center of the page are two 2k 5w resistors and C19. If a resistor is open the transistors won't be biased right, but if the cap is dried up, you might not know, and it is there for stability.

Check the large open inductor in the center of the board. The wires at the ends can break - usually right at teh solder point/ Tug on it to see if it comes loose.

REmove the 478 chip in U2. It is the limiter and not required for operation. This will see if it is involved. Once you are done, I highly recommend it is installed, but during test I often omit it.

[drewl]

Thanks for the tips E, I'm running only the power amp, same problem.
I swapped out the TL074, and removed the 70487478, same thing.
This one is sort of a cross between a 400B/G and 400BH so I have to interpolate both schems.
The 2k bias resistors are good, as is the smoothing coil. I replaced the 50uf caps and a couple other little electrolytics and I'm going to see how it looks now.
It has a strange 'noise" on the signal back at the TL074, and the compressor plug, don't know if it's being fed back or if it's originating from there.
There was also a strange ac signal on Q10, so I changed it also.
Now I have to wait until lunch to try it out.

[drewl]

And it was the filter caps.....
always monitor your power supply!
The +52v would start drifting down, replaced the big cans and it's okay.

[Enzo]

The oficial replacement for those no longer available 5000uf/55v caps is 4700/63v. Warch the lead spacing, I sometimes wind up drilling a new hole to match the lead spacing on the new part.