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Author Topic: marshall 5212 channel bleed fix?  (Read 2023 times)


  • Chipper
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marshall 5212 channel bleed fix?
« on: February 18, 2020, 05:01:03 PM »
hi folks,
restoring two of these that i picked up. replaced all the electros and tants, and they are now nice and solid and dependable... so far ;)
but i get a bit of channel bleed happening from the dirt "channel" into the "clean" channel when i have the gain pegged on it, even with a proper footswitch hooked up. the clean channel sounds great only if i turn the dirt gain to zero.
been messing with pedals and amps for years, but never really dealt with this kind of situation for the switching before, and not really sure how/where to look, or if there may be a modification i need to make possibly... maybe add a snubber to ground somewhere or something?
here's the schematic


also, another question. it seems to me marshall dropped the ball on the design in one crucial way...
its not really two "channels" like in, say, a fender amp or something, looks more like its just switching gain stages? or am i as confused as i think? ;)

the problem is that the clean volume is affected by the master volume. the gain channel has gain and volume, both of which sound best cranked. it appears they figured  you'd crank the dirt gain, lower the dirt volume and then set the master for both channels.. but it sounds better with the gain and volume cranked, and the master lowered.
BUT then the "clean channel" gets overdriven and sounds bad... like a silverface 70's master volume that doesn't belong on any fender.

is there a q&d mod that could be made to take the output of the clean channel volume right from the wiper and dump it just post the master volume instead? that way, the master could be used to really dial in the distortion, and the clean, well, ya just turn it up, it doesn't really NEED a master volume imho.

thinking breaking the node at r10 and r9 and ic1a pin 2, and instead connecting the freed side of r10 to the node at r27 and the fx send jack?

would love advice opinions and guidance if anyone can spare a few minutes to take a look


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Re: marshall 5212 channel bleed fix?
« Reply #1 on: February 19, 2020, 01:23:24 AM »
As to your suggestion; No because VR1 is Not A voltage divider (aka plian old volume control) it's a gain control for IC1b.

Rather than all the work why not just split the input into 2 separate input sockets, (if you have room on the panel).
now just use an A/B box to select channels. That will cancel bleed over.
But you would need to kill the switching setup

As for the imbalance of the 2 channels I doubt there is a simple solution.
Best I can think of is to lift the output of R10 (clean out) and mirror IC1a section and mix back into send out at output of R27. but again a lot of work,, which may or may not work.

You are correct in thinking these circuits like most amp circuits now are NOT 2 separate channels they are either channel switching or multi channel rigs and I'm yet to meet the owner of one of these multi channel rigs who is happy with all channels,, lol.
Most end up using one section (often the clean) and resort to pedals for all the other sounds. Also I've found single channel rigs are less likely to have problems, Or like the older rigs 2 separate channels with maybe common reverb. The amount of times I've had to trouble shoot circuits only to find the audio path actually works fine,,but it's all the crappy switching stuff that fails.
« Last Edit: February 19, 2020, 01:25:22 AM by phatt »


  • Chipper
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Re: marshall 5212 channel bleed fix?
« Reply #2 on: February 19, 2020, 01:05:23 PM »
thanks, phatt!
awww, shoot. just hoping it would work out there.. seems i wanna tie the clean channel in after the "master volume" of the dirt channel to get it independent, but ..... oh well, guess that won't work.
i DO have room to add another jack... the panel is punched for another input, tho covered with the gold overlay sheet. but that seems like a lot of work.
frankly, the whole reason i use these amps is cuz i love the dirt side, i never use the clean cuzza the dirt bleed thru... my venerable randall rg120 had that bleed issue as well, so i may have to rebuild the switching section.
but for all intents, i am fine with just the dirt side and reverb. i DID find a thing somewhere about modding these circuits to fix the bleed problem, but it seems to have vanished from the net recently.
i may try your suggestions tho... would love to separate the "channels" to more independance.
i'll try contacting marshall and see if there was a tech support ticket for this dealt with... if the guvnor were still with us, be easier.... i remember him actually picking up the phone one time when i called there decades ago... but now, they're tighter than a billionaire with the last yuppie bisquit to steal in their hands. lol.

again, thanks... will letcha know how i make out. waiting on parts to arrive to recap it, and one of my roadies busted off a pot the other nite loading it up. ouch.