Marshall MOSFET 3210 Motorboating

[JustMike]

 I just replaced the Mosfets with Exicons and it powers up and works, but at any volume and with all knobs all the way down, there is a motorboating sound. Any Ideas?

[PRR]

> Exicons

? ? ? ?

Ah.... http://www.photonage.com.au/exicon-to3-power-mosfet-2sk134-ecf10n16-n-channel-p-8988.html

So it's nominally the right part-number. Though transistors, and especially MOSFETs, vary a LOT one to another, and Marshall may have worked to the parts they had 23 years ago.

Why did you replace the originals? Blown? What else may have suffered in the blowup?

At 23 years, many of the electrolytic caps may be at or past their peak freshness. If a supply sneakback needs 10uFd to kill motoboating, and Marshall used a 22uFd for a little safety, and it has decayed to 11uFd, it's gonna be unstable.

The caps in the resistor-string deriving +/-49V 34V 15V would be prime suspects, and don't look real generous to me. R46 C40 looks like 13Hz which is not far-far below the audio bandpass.

http://drtube.com/schematics/marshall/3210-iss7.gif

[JustMike]

 I replaced the two C40 & C41 .47u caps and there is not much difference. I assume I should replace C24 & C27 next...but I also noticed another problem. When I plug in and play, the motorboating seems to kind of turn into a tremelo sound on the guitar signal as it fades. The motorboating sound also seems to go away after I play for a minute.
 There also appears to be some channel bleed. When I play thru the clean channel, I can still hear what sounds like the OD channel "Ghosting" faintly in the background. Anybody know what this could be caused by?

[sluckey]

I replaced the two C40 & C41 .47u caps and there is not much difference.

Those caps are 47µF NOT .47µF.

I suggest you first replace those big 2200µF caps first. That's C48 and C49. Then replace C40 and C41. Finally, replace C38 and C39. If these caps are on a PCB that's hard to access, I'd just change all six at the same time. It's only $2 or $3 extra.

[JustMike]

 47u not .47u...typo.

[JustMike]

OK, I've replaced "most" of the electrolytics (still waiting on 3  22u's)and the motorboating has changed somewhat. When I play thru it, the motorboating goes away and comes back when I stop. The clean volume (VR4) seems to not work properly-like as I turn it up there is a non linear volume increase and lower settings are louder than higher settings. There are several little orange drop electrolytics in the area (are these tantalums and are they suspect?).
Thanks again.

[JustMike]

I replaced the tantalums and no difference. Does it matter if the reverb chamber is disconnected? I would think not. I'm going to start checking all of the resistor values. Feel free to chime in anytime.

[John]

Before you drive yourself too crazy, build a listening amp (do a search for it, I can never find it). That may help you find where the motorboating starts. You only need a few, cheap parts to build the listening amp.