1982 JCM800 2205 is driving me nuts

[Jason Patrick]

Who has experience with working on these? This is an early one without the chip and I’m not to familiar with them and I’m fixing up a friends that sounded like crap cause the tubes were dead. I Cleaned the pots, installed new tubes, cleaned jacks, the normal procedures. Now it roars BUT, the volumes interactions are weird. The boost volume really affects the normal channel even with the boost off. I hear that this is an issue with these and is small potatoes for what I’m really trying to figure out. At certain settings of the volumes and gain, I’m getting a very starved fuzz pedal sound with like a ring modulator gated envelope type sound. When I play the guitar when it’s doing it, you really have to dig in to get a note to pop out. I noticed there are spots depending on how you tweek the gain and normal Chan volume where the volume drops out like there is high frequency oscillation going on. Lead dress is spot on, I did the chopstick poke poke, all components are in spec. I’m unfamiliar with amps using transistors tho, so I’m wondering could it be one of them? I can get great tone and crunch out of it but it just bothers me that I feel something is not right regardless. Oh and voltages seem to be golden. So, is this how these are with the diode clipping? I feel that’s where the problem may be. I usually fix old fender amps so this is a new one for me!
Thanks all!

 

[TassieViking]

You have me confused, diode clipping in a 2205 ???
There are only 2 transistors in the amp, one turns the channel LED on / off, the other one mutes the reverb signal.
There is one IC that is used to change channels and turn reverb on / off with the foot switch.
The main signal should be all tube as far as I know.

 

[Jason Patrick]

You have me confused, diode clipping in a 2205 ???
There are only 2 transistors in the amp, one turns the channel LED on / off, the other one mutes the reverb signal.
There is one IC that is used to change channels and turn reverb on / off with the foot switch.
The main signal should be all tube as far as I know.

this is an early one. No IC. Def more the two transistors in this. And yeah as far as I’ve read, diode clipping. Couple different schematics floating around. Think they changed this circuit three times

 

[Gunner64]

I had a 2205 that behaved similar. It had at least one broken connection where the pot lugs are attached to the pcb.

The way you have to wrangle the pcb out puts stress on the pot lugs, not to mention the pcb itself is partially suspended by the pots. I reflowed all the lug to pcb connections and the problem was solved.

I suspected the gain pot, but did them all.

Not saying this is your issue, but it was mine, and is something to consider.

 

[LyseFar]

I restored a 89 2210 (the 100 w version) some time ago. It was very noisy and troublesome. I re-did all grounding, full cap job, internal electrolytics also.
What I also found that these has a weak impedance selector that actually had internal bad connection due to oxidation and dirt. After a cleanup the amp and the full service it sounds killer.
The interaction between the channels (channel bleeding) is by design to mimic the 4-holers way to blend the channels.
Remember these split channel amps were new territory for Marshall and they did some weird choices.
I still have the amp and I like it. Easy to dial in and fun to play.
Here is my service “log”.

2210 noise hunting story - that ends good

Yes. Last year I made a trade for a 1989 Marshall 2210. I had a 4211 in the 80s I was very happy about and has always been regretting selling it. So. The 2210 I got had a very high noise-floor … to say the least.. not at all the way I remembered the 4211…loud humming and white noise when...

www.marshallforum.com

 

[Jason Patrick]

I had a 2205 that behaved similar. It had at least one broken connection where the pot lugs are attached to the pcb.

The way you have to wrangle the pcb out puts stress on the pot lugs, not to mention the pcb itself is partially suspended by the pots. I reflowed all the lug to pcb connections and the problem was solved.

I suspected the gain pot, but did them all.

Not saying this is your issue, but it was mine, and is something to consider.

good to know, I read about that so I took the board out and all looked good under close inspection but I’ll go back and reflow.