Marshall 3203 problem

[jgcable]

My Marshall 3203 has really been acting up lately. I haven't done any mods to it yet.
The volume fades in and out randomly. I tried cleaning the loop jacks. Sometimes it works perfectly and sometimes it fades in and out. It does this whether or not something is plugged into the loop and in both channels.
Is there a specific component that could be causing this?
I replaced the EL34 tubes and it still does it.
Lastly, is there a place anywhere near Milford, CT that works on Marshall amps if I can't figure it out?

[DonP]

Re: Marshall 3203 problem

How about putting the old tubes back in and see if it still does that?

How old is the amp? Has it had the caps replaced recently?

Might be a resistor overheating, but I really don't think so.

[jgcable]

Re: Marshall 3203 problem

It fades in and out regardless of the tubes. I tried several sets. It almost sounds like a dirty jack. Maybe I should check the speaker output jacks. The amp is very clean inside.

[DonP]

Re: Marshall 3203 problem

Is the fade in and out fast or slow?

[jgcable]

Re: Marshall 3203 problem

[ QUOTE ]
Is the fade in and out fast or slow?

[/ QUOTE ]

Its a slow fade. It doesn't happen all the time and the fade varies from a very minor drop off to occassionally the volume dropped almost 50%. It also gets fuzzy when it fades. Almost like I turned the amp on standby while I was playing out of it.

[DonP]

Re: Marshall 3203 problem

I stick to my original diagnosis. Either filter caps going bad or resistor overheating.

[jgcable]

Re: Marshall 3203 problem

I'll rip her apart tonight and check the caps. Does a cap vent if it goes bad? I suppose I could just replace all of them. I work for an electronics company and I have access to just about everything.

[DonP]

Re: Marshall 3203 problem

They could swell and ooze stuff out. Cap life is 10-15 years, less if the amp doesn't get used (a closet queen), longer if the caps get excercise every once in a while.

[jgcable]

Re: Marshall 3203 problem

She's 17 years old so I better take a good look inside.

[DonP]

Re: Marshall 3203 problem

[ QUOTE ]
She's 17 years old so I better take a good look inside.

[/ QUOTE ]

Is the amp in minty shape? Often a bad sign that it hasn't been used frequently enough.

How bad is the hum (any?). With an all tube amp that's an easy sign, but your hybrid will only hum through the power tubes, and they don't amplify hum like a pre-amp tube does.

[jgcable]

Re: Marshall 3203 problem

[ QUOTE ]
[ QUOTE ]
She's 17 years old so I better take a good look inside.

[/ QUOTE ]

Is the amp in minty shape? Often a bad sign that it hasn't been used frequently enough.

How bad is the hum (any?). With an all tube amp that's an easy sign, but your hybrid will only hum through the power tubes, and they don't amplify hum like a pre-amp tube does.

[/ QUOTE ]

There is no hum at all. The amp works and sounds perfect except for the volume drop off. This isn't a minty amp. I am sure it was used alot. What I meant by the internals are clean is that it has never been messed with inside. Its completely stock right down to the Drakes.

[DonP]

Re: Marshall 3203 problem

Before ripping out all of the caps, I think maybe you should either take it to a tech, or do the tech work yourself.

If I were checking out the amp, I'd look at the bias voltage on the power amp tubes to see if it's doing something funny like swinging up and down.

Issues like these are a pain to figure out.

Did you change the phase inverter tube as well? The single 12AX7 in there?

I'm running out of ideas.

[Soloist1]

Re: Marshall 3203 problem

Did you try a new phase inverter tube? Those need to have matched triode sides for the power amp section.

[jgcable]

Re: Marshall 3203 problem

I didn't try a new phase inverter tube. I have 12AX7's but this one has a different #. Its like 12AX7T or something like that. It looks original.
When I checked the bias I did notice it fluctuating a little. The voltage started around 22ma and eventually rose to 33.1 and pretty much stayed there on both tubes.
Where can I get a phase inverter tube?

[DonP]

Re: Marshall 3203 problem

[ QUOTE ]
I didn't try a new phase inverter tube. I have 12AX7's but this one has a different #. Its like 12AX7T or something like that. It looks original.
When I checked the bias I did notice it fluctuating a little. The voltage started around 22ma and eventually rose to 33.1 and pretty much stayed there on both tubes.
Where can I get a phase inverter tube?

[/ QUOTE ]

Any 12AX7 will do, if it is a 12AX7. Fender used 12AT7's quite a bit. They are interchangable, but the 12AX7 has a lot more gain than a 12AT7. The 12AT7 gives a more powerful (amperage wise), cleaner drive signal.