Any Amp Techs Available to Lend Me A Hand With My 1985 Marshall 3203?

[kernelxsanders]

As my instrumental interests has been shifting from guitar towards the drum kit, I decided to plug up my guitar rig and do some jamming through my 1985 Marshall 3203. I almost forgot how damn amazing it sounds, and how loud it is! I barely even had the volume on 1, and it could be heard from outside my home! Needless to say, I love this amp!

Now a problem has arisen. For whatever reason, the amp has mood swings. I can be cranking the amp and jamming the hell out, when all of a sudden, ALL of the power goes away! Like how i mentioned about how loud it is on volume setting 1, i have to crank it around 9 o'clock to be able to hear it well just standing by the amp, but will randomly cut back on. It seems as though it cuts back on once i reach a certain threshold of volume, so when i slowly turn the master up, all of a sudden, i'll be around 9 o'clock and BAM I'm deaf for a few minutes from the amp SCREAMING almost directly to my ear, scaring the shit out of me. If you have ever turned on an amp unaware of how high the volume setting is, which is anybody on this forum who plays guitar, you know my pain. Imagine that happening every other minute or so RANDOMLY. Believe me, it gets very old very quickly.

Also, the Reverb doesn't seem to work, when i turn it up, it only seems to add that "reverb-colored tone" but with no reverberating tail whatsoever. I'm not quite as concerned about that though, because i tend to favor external verbs most of the time, but still, i would like to at least hear how this reverb sounds!

Anywho, it is because of these problems that i cannot use this amp for recording, and this... it saddens me, very much so. What I am looking for is somebody who knows a thing or two about amplifier technology, which by the looks of it, it seems that is a lot of you guys. There is a well respected local team who is great with that kind of stuff who i could pay to fix it for me, but i figured i would go the better route, asking for help by you guys so i can actually LEARN about it myself instead of having mommy fix my mistakes for me. Especially seeing as how i strive to be an all around, general musical super... I gotta get there somehow, right? and tbh, i have learned way more from this forum than i have anywhere else, including the years of guitar class i took in high school. Well, relatively anyways, because i was learning gutiar performance instead of general music. Hmmm, maybe i should have taken the Sound Production class to help me out even more...

 

[streetwise]

New tubes & rebias. Almost always solves the problem with a faulty tube amp. I don't know about the reverb though.

The amp doesn't have adjustable bias. You have to replace R2 (here http://www.drtube.com/schematics/marshall/3203pwr.gif) with a trim pot + resistor. Try something like 39k + 25k trim. Or have an amp tech do it. Or you could install new tubes and measure idle plate dissipation and see if you're satisfied with it without adjustment.

The problem could also be a cold solder joint, faulty volume pot or faulty C6-cap in the weird bias circuit. I've never seen a capacitive voltage divider to produce the bias voltage in a Marshall amp...

 

[kernelxsanders]

Oh, I apologize for my lack of information. This is a 1985 Marshall 3203 MOSFET Lead 100 amp, which is a solid state (i know, shocking). I should have been more specific. Thanks for the info though, even if it is like a second language to me right now Would is still be applicable to my amp or exclusively for the 3203 TUBE version?

 

[streetwise]

Oh, I apologize for my lack of information. This is a 1985 Marshall 3203 MOSFET Lead 100 amp, which is a solid state (i know, shocking). I should have been more specific. Thanks for the info though, even if it is like a second language to me right now Would is still be applicable to my amp or exclusively for the 3203 TUBE version?

Sure it's 3203, not 3210? I couldn't find a schematic for 3203 mosfet.

I actually forgot the most common malfunction in my last post. Effects loop jacks often get oxidized if they're not used. If you don't have anything in the loop, try a patch cord from send to return. If that solves the problem, replace the jacks. I recommended having an amp tech do it if you don't have a decent soldering iron and soldering skills, as the copper trace on the PCB is easy to melt with too much heat. The jacks are most likely PCB-mount and the whole PCB has to be removed (remove jack nuts, knobs and powersocket screws.)

If it's not the loop or possibly some other jack or a faulty pot I mentioned in my other post, then I don't think there's an easy fix.

 

[kernelxsanders]

You could be right. As far as i am sure of, it is the Mosfet lead 100, when i typed it in google, there was a review article calling it the "3203 mosfet lead 100" and come to think of it... i remember watching youtube videos before i got the amp and it was labelled as the "3210", so i think you are right.

Anywho, I just tested your idea about the fx loop and tha fixed the problem with the volume! Now all that's left is the faulty reverb issue.

 

[streetwise]

You could be right. As far as i am sure of, it is the Mosfet lead 100, when i typed it in google, there was a review article calling it the "3203 mosfet lead 100" and come to think of it... i remember watching youtube videos before i got the amp and it was labelled as the "3210", so i think you are right.

Anywho, I just tested your idea about the fx loop and tha fixed the problem with the volume! Now all that's left is the faulty reverb issue.

Great! No idea about the reverb though. Check the cable and connectors at least?

 

[Sloan]

Turn amp on and wiggle preamp tubes to see if problem changes/occurs.

Clean tube sockets/pins with non-residue contact cleaner
check to make sure tube sockets are not loose


also, use contact cleaner on all jacks, paying special attention to preamp out/in or FX loop jacks.


if it's not dirty/loose tube sockets, then swap the preamp tubes around to see if you can track down which one is the culprit.


REMEMBER
12ax7's are DUAL TRIODES, which means they are actually two gain stages per tube, so one side can work great while the other doesn't work as good or at all. Schematics will show you which side does what.

 

[Firaxis]

Contact cleaner recently fixed a volume issue with my amp. Well, I hope so anyway, haven't tested it properly yet!

 

[Shinozoku]

Yeah, I was gonna say check what would be the effects loop. I'm experiencing an identical problem with my Randall after modifying it with new caps in that area. Annoying as hell.

 

[kernelxsanders]

Well i suppose you guys haven't read up this thread haha but Streetwise has helped me with the volume issue. and this amp is not a tube amp, it is solid state.

The only thing i need assistance with at this point is the reverb.

 

[~BURNY~]

Well i suppose you guys haven't read up this thread haha but Streetwise has helped me with the volume issue. and this amp is not a tube amp, it is solid state.

The only thing i need assistance with at this point is the reverb.

It probably doesn't help but from what I remember, the reverb is just awful.

It was one of my first amp but that was almost twenty years ago.

 

[Sloan]

lol, i obviously scanned that first post. shit, sorry about that. BUT, in addition, dirty pots are also a big culprit of volume drop etc...


not sure about the 'verb, I'm at work and just blast through posts, not a lot of time to really 'dig in'.

 

[kernelxsanders]

well in the case that the reverb isnt too great, i'm still not too worried about it. but like i said, it would be nice just to have it for whatever reason, so if anybody has any ideas, don't hesitate to share.

also, i found my foot switch for the amp. it doesnt work at all and sometimes heavily decreases output when i plug it in. im not the most familiar with foot switches and which ones are compatible with particular amps, but this kinda looks to be an MG series switch because the guy i got it from said he had a few MG heads so he coulda just thrown that in with the amp, maybe that could be why it doesnt work properly?