Need help diagnosing a Marshall guys...John Philips?

[JohnnyL]

I have a Marshall 4210 which has always operated fine. I began to play and started experiencing some microphonic noise at V1. I moved the tube around and it seated fine and all was ok-that socket seemed a little loose anyway...The next time I used it-it began to make a 'motorboating' type noise. I began to first reseat and then replace tubes one at a time. When I got to V5 and installed a different tube there was a thud type of sound when I powered back on, the amp volume dropped and the 'motorboating' sound was not as intense but still there....I turned the amp off and called it a night.

I checked it today and the motorboating noise was present and the amp volume was still low. I exchanged the power tubes in their sockets and the amp is now quiet-no motorboating but still running at a volume loss.

Additional information: The PI socket was replaced and will be the first place I will look once I open the amp. I should be looking for a bad connection or broken solder joints?...The power tubes have some age on them-I'm not sure how many hours but they test fine in my simple Sencore.

What all should I look for and any advice is appreciated? Unless it is a real simple fix, I will take it to a tech. I just want to be as informed as possible.

Thanks in advance!

John

[Old Tele man]

...someone else might have to help you because Mr. Philips is currently on a self-imposed sabatical.

[JohnnyL]

Thanks Teleman...I have been offline for a while myself but this is the first place to come when I need this kind of help!

Thanks,

John

[Tommy_G]

I had motorboating on a preamp after I had did some modifications. I resoldered some parts, and the motorboating disappeared. I was mainly working with cathode bypass caps. I suspect I had a cold solder joint, perhaps you have the same, or a bad capacitor somewhere.

[RedMan]

Sounds like a leaky coupler to me.

[JohnnyL]

Redman: Can you expand on a leaky coupler? Thanks. John

[RedMan]

The coupling caps are there to block DC between stages. If one or more of them is leaking DC thru it, it could cause the symptoms you're describing. They're the ones that usually come off the pins 1+6 of the preamp tubes. If you lift the side that doesn't go to one of those pins you shouldn't read any DC there.

[JohnnyL]

Any other comments?....

Thanks.

John