Is my '82 Marshall JCM800 modded?

[Tatzmann]

The dark circles and spaghetti sauce rings on the back of the pots where soldering is ... Could that be a weak grounding point?

Jus curious myself

The spaghetti sauce is rosin solderflux and doesn't damage anything, it actually protects solder from environmenta.

 

[NickkiC]

Hey guys,


I clipped the bright cap on the Pre-Amp Volume gain pot. It sounds terrific!
The amp sounds much more balanced now, probably the JMP 2204 I was referring to had its bright cap clipped too. Strange how nasty the bright cap affected my tone.
But I got a hum going on now, when I turn the Volume pretty loud, which wasn't there before. Do have an idea? Should I clip the bright cap on both sides and remove it completely?

My first thought on the “hum”, is the filter caps need replacing. Get them out of there. I would change the bias caps as well, but that’s me. I blew a cap dated ‘84 in a Laney, lesson learned.

 

[syscokid]

But I got a hum going on now, when I turn the Volume pretty loud, which wasn't there before.

Is the sound of the hum based on a 60 Hz frequency? 120 Hz frequency? Or a different type of sound? Can you record and post a sound clip of the hum?

Attach one end of a jumper wire to where the grounding buss wire is grounded to the it’s chassis connection. Use the other end to probe the buss wire and the pots for proper continuation of this area’s ground circuit.

The amp’s 40 year old filter e-caps might be fine, or they might be on their way out. Either way, they should be tested for leakage, and if they are still in spec.

Should I clip the bright cap on both sides and remove it completely?

I would. IMO, that cap is destroyed and serves no good purpose. Desolder the cap instead of snipping it. Clean up both connections of where the cap was involved.

 

[william vogel]

It’s a 10k. Those are Piher resistors. Look at the 33k at the tonestack, you’ll see it’s the same color.

 

[Spanngitter]

That’s a fairly clean ˋ82 vertical input, the one‘s which where equipped with black filter caps as a transition from Daly to LCR. Those also have a fairly low B+ (~380V) und it should sound quite brown.
For sure the Powercaps are shot on this one, so a full ReCap should be planned.
Also check all the grounds, for the pots, caps and all other GND connections, usually the black oxide washers are corroded and you have several Ohm resistance causing GND Loops, hum and funky Voltages.
I would also modify the treble peaker circuit if you cannot run it full tilt, change the Mid Pot to a 50k, reposition the NFB to the 4 Ohm Tap and add a 330uF Bypass to V2a (under the board, so it’s not immediately visible) and you are there, back in ˋ78….

 

[Taxman1966]

That’s a fairly clean ˋ82 vertical input, the one‘s which where equipped with black filter caps as a transition from Daly to LCR. Those also have a fairly low B+ (~380V) und it should sound quite brown.
For sure the Powercaps are shot on this one, so a full ReCap should be planned.
Also check all the grounds, for the pots, caps and all other GND connections, usually the black oxide washers are corroded and you have several Ohm resistance causing GND Loops, hum and funky Voltages.
I would also modify the treble peaker circuit if you cannot run it full tilt, change the Mid Pot to a 50k, reposition the NFB to the 4 Ohm Tap and add a 330uF Bypass to V2a (under the board, so it’s not immediately visible) and you are there, back in ˋ78….

Hey Spanngitter, thank you for taking the time!

I think the time has come to bring the JCM800 to a skillful tech to do the ReCap. I will show him your suggestions.
Since I'm not too familiar with the technical stuff, I guess the changes mentioned in your last senctence are about getting the JMC in the JMP ballpark?
Is there any particular brand of caps (I hear LCR is popular?) that I could mention to him?
I'm located in Germany if that matters in terms of availability.

Danke und ganz liebe Grüße aus Berlin in die Oberpfalz

 

[Matthews Guitars]

That first resistor on the board, the 10K, that's the cold clipper on a 2203/2204 and it should be that value.

LCR stopped making caps in '99 so that's out, and they weren't very good anyway. Today you'd use ARS or F&T caps.

F&T is German.

 

[Taxman1966]

That first resistor on the board, the 10K, that's the cold clipper on a 2203/2204 and it should be that value.

Hey, unfortunately I don't get the context here sorry, what are you referring to exactly?
So it is right or wrong in my amp right now? Like I said I'm not that familiar with the technical side.

LCR stopped making caps in '99 so that's out, and they weren't very good anyway. Today you'd use ARS or F&T caps.

F&T is German.

Yes, I think I meant ARS not LCR, I heard that they are popular, also hear that F&T can be more on the scratchy side tonewise, which is really not what I want.

 

[Matthews Guitars]

Filter caps don't alter tone. How can they? They're in the power supply. All that does is supply power to the circuit.

 

[NickkiC]

ARS are the same size as the Daly/LCR. F&T are shorter, same diameter so they fit in the clamps properly. I use both, depending which is available.
Again, Get them out of there. New bias caps with the service too.