Marshall JCM800 2203 with oscillation on output

[rf7]

I have a 2203 on the bench that has ultrasonic oscillations riding on the output. I have traced it with a scope to the output of tube V2a. Swapping tubes does not help. The output of the amp looks very jumpy and noisy into a load resistor. The rails look ok. Any ideas?

[xtian]

Because no one has replied, I'll throw some ideas out. Tried different tubes in V2?

There is a 470p peaking cap across the 470K grid stop between the volume pot and V2a's grid. Try removing that temporarily. Any change?

Tap all the caps in the tone stack with a chopstick and with the volume up. Any of them gone microphonic?

[rf7]

Because no one has replied, I'll throw some ideas out. Tried different tubes in V2?

yes, made no difference.

There is a 470p peaking cap across the 470K grid stop between the volume pot and V2a's grid. Try removing that temporarily. Any change?

No difference.

Tap all the caps in the tone stack with a chopstick and with the volume up. Any of them gone microphonic?

None of them really stick out as being microphonic.

This is a similar problem: 2203 Oscillation (and solution).

I added caps across the plate resistor as in that thread above and it did not fix the issue.

[Chuck H]

If the amp has undergone any work recently there's the possibility of a lead dress issue or even a phase error in the OT wiring. I've seen a couple of amps that were strangely almost stable with an OT phase error.

Check grounds. If the decoupling caps are very old or if the B+ rail has been altered that could also be a problem. Make sure the grid leads are raised high and all other leads are pushed low. Keep the purple "presence" lead swung away from the preamp as well as the NFB lead from the impedance switch.

Since even tapping off high end hasn't made any change (odd) I need to ask about the specifics of the problem. Is this just something you see on the scope or is there some audible problem? What are you seeing on the scope? Can you provide a pic? If this is an audible problem are there any adjustments, like higher treble or gain settings that exacerbate the problem?

[rf7]

If the amp has undergone any work recently there's the possibility of a lead dress issue or even a phase error in the OT wiring. I've seen a couple of amps that were strangely almost stable with an OT phase error.

The amp is completely stock & unmolested.

Check grounds. If the decoupling caps are very old or if the B+ rail has been altered that could also be a problem. Make sure the grid leads are raised high and all other leads are pushed low. Keep the purple "presence" lead swung away from the preamp as well as the NFB lead from the impedance switch.

I checked & re-soldered all of the grounds. It did not help. Moving wires does not affect the oscillation really.

Since even tapping off high end hasn't made any change (odd) I need to ask about the specifics of the problem. Is this just something you see on the scope or is there some audible problem? What are you seeing on the scope? Can you provide a pic? If this is an audible problem are there any adjustments, like higher treble or gain settings that exacerbate the problem?

Tapping off the high end slightly reduced the issue, but I ended up getting some 120Hz stuff on the amp's output. The amp sounds kind of harsh and bright to me. The oscillation occurs periodically at 120Hz on the output.

Here's some (blurry) scope pictures with my cheap phone:

This is the oscillation on a waveform.

This is the output with my signal generator output set to zero. With the input shorted to ground, there is no oscillation.

It's weird that it is periodic like that. Noisy diodes? Caps?

[Chuck H]

If it's occurring at 120Hz as well as ultrasonic, and the amp is bone stock, I would definitely consider replacing all the electrolytic capacitors in the amp. Some guys scoff at replacing anything other than grossly bad caps in vintage amps. But what else should one do? Not replace the bias supply caps? The other power supply filters of the same make and age?

You could jumper another cap across the main filter and see if that improves the issue. If it does I'd suggest a full cap job.

[The Dude]

Can't tell from the pictures, but it would be helpful to know scope settings for the pictures or have stated peak to peak voltages. Also, for clarification, are those scope shots of the speaker output? From what we know thus far, I'm inclined to agree with Chuck that there is a filter cap problem.

[Chuck H]

Right. I looked at the scope pics and was utterly confused so I omitted them from the equation.

One thing we still don't know is how the amp sounds.?. I'll guess that it still puts out something a little less than it's rated power with no apparent squealing oscillation. Which is the sort of oscillation the top end bleeders are supposed to cure. Rather, I'll bet it creates a ring modulator type of tone on several notes. A sort of out of tune note that follows the fundamental. More apparent when clipping and especially around Bb and B. If this is the case it's long overdue for a cap job.

[rf7]

I figured it out. It's embarrassing. I noticed that this problem started to show up on other pieces of gear today. It turns out my signal generator has gone bad and was creating the oscillation. The amp is ok. I hate when your test gear goes bad. Thanks for the help everyone.

[The Dude]

Shi+ happens and test gear, like any other electronic thing, does fail. These are the things we don't forget and you'll be all the wiser for figuring it out. Glad you got'r!