Marshall 2210 Reverb hum/buzz....

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Deric
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Marshall 2210 Reverb hum/buzz....

Post by Deric »

Working on a 2210. Customer said reverb was intermittent then stopped completely then amp shut down.

Opened it up, no obvious issues. Tested tubes - all ok. Fired it up with a light bulb limiter - all good. Checked bias and all was good. Amp sounded fine. Checked reverb cables (which appear to have been replaced). One had a loose ground on the tank end and one had loose signal wire. Fixed those and reverb fired up but is very noisy.

Have a nasty 60hz hum/buzz when the reverb is turned up. Problem is there even with the MV at zero. Swapped tubes - no change. Grounded grid of reverb return - no change. If I ground either end of R42 or switch the reverb off - buzz goes away. I checked for DC on R42 and there is none... Could C29 be an issue?

I haven't tried it but I suppose I could pull TR2 and see if it changes...

Any ideas or suggestions?

Thanks!

http://drtube.com/schematics/marshall/2210prem.gif
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martin manning
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Re: Marshall 2210 Reverb hum/buzz....

Post by martin manning »

The fact that grounding the input end of R42 kills the hum removes suspicion from T42. C29 is not likely to be a problem, and apparently is isn't leaky. Did you swap V4 for a known good tube? What about the filter C47? Have you tried substituting that out?
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Deric
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Re: Marshall 2210 Reverb hum/buzz....

Post by Deric »

I did swap V4 with no change (not that it matters but in this amp V3A and V4B are swapped so V4 does all the reverb duty).

Will try another tube - just because I can...

Did not swap C47 but did clip a new cap across it with no change. Will swap it out and see what happens.

Thanks
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martin manning
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Re: Marshall 2210 Reverb hum/buzz....

Post by martin manning »

The filter is a long shot since it would affect the loop return stage too. Based on the testing you described the source is the reverb recover stage.
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Deric
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Re: Marshall 2210 Reverb hum/buzz....

Post by Deric »

Replaced C47 with no change. Swapped several known good tubes - no change. Disconnected the reverb transformer - no change. Chopsticking lead dress makes no difference.

I guess I'll start swapping C29, R38 and R39. If that don't work I'll pull TR2 just because I'm running out of ideas.
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Deric
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Re: Marshall 2210 Reverb hum/buzz....

Post by Deric »

Another observation...

All other controls set to zero. With reverb at zero all is quiet when switching between normal and boost channel. When reverb is turned up switching to the boost channel increases noise quite a bit...
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Deric
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Re: Marshall 2210 Reverb hum/buzz....

Post by Deric »

Pulled TR2 - hum/buzz gone completely on clean channel. Still there when switched to boost channel but no nearly as bad.

So....ground loop with the switching supply? The grounding in this thing is weird. There are 2 ground wires coming off the PCB and soldered to the first three pots, then pots bolted to the chassis. Disconnected one of the wire (it is still connected to chassis ground somewhere) and buzz is gone in Boost channel too. Guess I'll put TR2 back in and see what happens....
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martin manning
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Re: Marshall 2210 Reverb hum/buzz....

Post by martin manning »

Presumably it worked wired as it was... How's the switching supply look on a scope?
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Deric
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Re: Marshall 2210 Reverb hum/buzz....

Post by Deric »

Presumably it worked wired as it was...
My thought too...

+ side of 3300uf cap looks smooth on the scope. Will put TR2 back in and see what it looks like.

Thanks again for your input.
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martin manning
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Re: Marshall 2210 Reverb hum/buzz....

Post by martin manning »

Can you use your scope to find out where the hum appears in the signal path?
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Deric
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Re: Marshall 2210 Reverb hum/buzz....

Post by Deric »

Got a chance to put TR2 back in. Have a little less than 10mV ripple on the switching supply but that jumps to about 60mV when the boost channel is engaged.

The hum appears to be just in the reverb return but will double check with the scope when I get a chance.
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