Marshall 4212 1989 Clean Channel Problem

[browniesteve]

dirty channel working. when i switch to clean. the master volume works really loud but none of the clean controls work at all. and the reverb has stopped working. this amp is turning into a nightmare.

 

[Chris-in-LA]

Are you posting to complain or do you have specific questions about how to fix your amp? Your questions might get more responses if you post in the Workbench section.

 

[ampmadscientist]

dirty channel working. when i switch to clean. the master volume works really loud but none of the clean controls work at all. and the reverb has stopped working. this amp is turning into a nightmare.

There is nothing unusual about a 29 year old amp that requires maintenance.
This is normal.
All the owners of older amps (in this forum) already know it. There is no such thing as a tube amp that works forever without maintaining it.

That is pretty much why this forum exists - to help maintain. We have helped each other keep going.

Getting back to the channel switching: I am looking at the schematic.
Dr Tube has Marshall Schematics to help us maintain also. (he's a righteous dude)
4212 is the same as: 2205 head.
The schematic is here:
https://drtube.com/schematics/marshall/4210.gif

There is 4 basic things that can go bad.

1.Try another foot switch, it might not be the amp at all. It might be a bad foot switch.

"...the reverb has stopped working..."

That is a clue that the jack or the foot switch may have gone bad.

2. The foot switch jack is corroded - the jack has switch contacts inside and when those contacts oxidize, the switching can stop working.
(doesn't matter if you never use the foot switch)
3. A bad BC184 transistor.
4. A bad diode in the switch circuit.

So the first thing to try is:
A. Plug / unplug a cable in/out of the foot switch jack several times. This could make the jack contacts work again at least temporarily. (now re-test)
Or spray cleaner into the jack, then plug / unplug a cable in/out several times.

B. Plug another RTS foot switch into the jack. This is a 3 conductor (stereo) plug with 2 switches on the foot pedal. (a standard foot switch) See if the foot switch makes it work again.
The foot switch bypasses the contacts inside the jack.
If it does work now - the jack contacts are corroded. (clean or replace the jack) OR the old foot switch is bad.

C. Bad transistor / bad diode: it's not the end of the world. The parts are ridiculously cheap.

Oh and
welcome to the Marshall Amp Forum.

 

[browniesteve]

There is nothing unusual about a 29 year old amp that requires maintenance.
This is normal.
All the owners of older amps (in this forum) already know it. There is no such thing as a tube amp that works forever without maintaining it.

That is pretty much why this forum exists - to help maintain. We have helped each other keep going.

Getting back to the channel switching: I am looking at the schematic.
Dr Tube has Marshall Schematics to help us maintain also. (he's a righteous dude)
4212 is the same as: 2205 head.
The schematic is here:
https://drtube.com/schematics/marshall/4210.gif

There is 4 basic things that can go bad.

1.Try another foot switch, it might not be the amp at all. It might be a bad foot switch.

"...the reverb has stopped working..."

That is a clue that the jack or the foot switch may have gone bad.

2. The foot switch jack is corroded - the jack has switch contacts inside and when those contacts oxidize, the switching can stop working.
(doesn't matter if you never use the foot switch)
3. A bad BC184 transistor.
4. A bad diode in the switch circuit.

So the first thing to try is:
A. Plug / unplug a cable in/out of the foot switch jack several times. This could make the jack contacts work again at least temporarily. (now re-test)
Or spray cleaner into the jack, then plug / unplug a cable in/out several times.

B. Plug another RTS foot switch into the jack. This is a 3 conductor (stereo) plug with 2 switches on the foot pedal. (a standard foot switch) See if the foot switch makes it work again.
The foot switch bypasses the contacts inside the jack.
If it does work now - the jack contacts are corroded. (clean or replace the jack) OR the old foot switch is bad.

C. Bad transistor / bad diode: it's not the end of the world. The parts are ridiculously cheap.

Oh and
welcome to the Marshall Amp Forum.

appreciate your answers. will check re check footswitch but think diode /and or transistor are more likely.

 

[browniesteve]

Are you posting to complain or do you have specific questions about how to fix your amp? Your questions might get more responses if you post in the Workbench section.

All in frustration. Solve one problem and something else goes wrong. a tech checked it out last week and couldn't find a problem, but it still sounded terrible. now things are going wrong. maybe just a coincidence

 

[Shae201]

If you or a tech can read schematics, its not hard to trace a problem like this down. As Mad Scientist was saying about the jack, the simplest solution is many times what fixes the problem, also clean your fx loop jacks while your at it with electronics contact cleaner from local auto parts store.

 

[Gunner64]

And don't forget the the pot to board connections. In my experience with my 2205 these joints crack, and while mine still switched, the controls for one channel did effect the other.

 

[ampmadscientist]

All in frustration. Solve one problem and something else goes wrong. a tech checked it out last week and couldn't find a problem, but it still sounded terrible. now things are going wrong. maybe just a coincidence

You need another tech.
A qualified tech would have already solved it.
Where are you located?

 

[geddy]

There is more than 1 circuit architecture dependent upon year. This effects the routing through the valves and the channel switching. What year do you have?