JCM800 decoupling/grounding/oscillation/normal phenomenon

[nsubulysses]

Wondering what would cause this waveform to distort in the manner shown. Only does it when you get into clipping, and the farther into clipping the worse it gets, until the scope cannot even trigger it anymore.

Clipping a .001 cap over one of the plate resistors for V1 or V2 it goes away. Also clipping the scope probe ground to the chassis makes it stabilize. is this a grounding or decoupling scenario? It ONLY does it on the high gain input. On lower gain input you can go into heavy clipping and the signal is stable.

I believe these are the correct shames
pre -- http://www.drtube.com/schematics/marshall/2203prem.gif
power/psu -- http://www.drtube.com/schematics/marshall/2203pwrm.gif




Been referencing these threads. It does not squeal though, this is only visible through a scope
Marshall JMP making squealing noise
http://music-electronics-forum.com/t38879/

[Enzo]

Of course, clip the scope ground to the chassis, or at least to the circuit common, if you have that 100 ohm, cap and two diodes ground thing. If you rely on the ground through the power cord fo the amp and the scope, it might work a lot of the time, but as soon as anything gunusual happens, please use "official" connections.

Does it sound funny at all? It looks like a parasitic, but if grounding the probe to the chassis stops it, I'd say it is an artifact of your test procedures.

[nsubulysses]

Whoops I made a simple error and my curious nature got the best of me. It does not sound funny at all.

It's becoming more and more weirdly distorted because even with master volume on 10 I have to turn up the preamp volume a bit to get to full output (5 or 6 range). When I leave the preamp volume low (like below 2), and turn up the volume on my signal gen it stays clean all the way up, even into heavy clipping. My test signal was too weak and it was distorting from cranking the pre volume, just like we all like it to, if you're a rocker

[MarkusBass]

I hope that you know that with RIGOL DS1054 you don't have to make photos of the display. You can easily save the screen as a picture. To do this, you have to plug a USB-flash disk into the oscilloscope and press the Print button. The image will be save to the flash disk.
Here is an example:



You see what is the difference between this picture and a photo you posted.

Mark