Marshall JTM 60 problem with gain channel with FX loop

[J M Fahey]

Somebody should print all 3 or 4 partial schematics on, say, letter or A4 sized paper, whatever it takes, tape them together and hand draw missing connecting lines to make a real functional schematic, and I bet service Techs at "official" Marshall service centers do exactly that; OR for them it's not a big problem, since they have the chassis open, "in the flesh" , connectors plugged showing all interconnections, and "schematics" are only a help to understand the circuit and check parts values.
Maybe this way they discourage "Internet Techs"?
If so, they are doing a good job.

Compare this mess to a Fender or Peavey schematic , which also include excellent layouts , even showing see through boards so you can see components and tracks, or the best in the Industry, the unjustly despised Crate, who add a ton of test points with AC and DC voltages and scope waveforms.

Marshall has been living on past achievements for a long time now.

[torcamaniac]

Yeah Peaveys schematics are top in comparison.
I guess the problem with these JTM's is that they have 3 PCB's.
Anyway tried removing thr bypass cap from R123 and I didn't like it.
Sounded kinda thiner and sterile so I put it back.
I guess I'll just leave it this way and use stompboxes in fron of clean channel for leads and use the gain channel for riffing...
The nice thing on these amps (guess this was unwanted originaly) is when you put a patch cable in the parallel fx loop and max it's mix pot you get really fat sound and a bit of volume too. It acts as something like a subtle 2nd master volume.

[torcamaniac]

Anyway like I said in my first post, I tried all the pedals to front on clean channel but left the delay in FX Loop. Funny because clean all is fine. Then I turned one of my distortion stomps in front of the amp, and the volume of dry distorted signal was much lower.
Then I took the delay out of the loop and put it to front to the end of chain and all was good!!! So I guess the FX Loop itself is a problem and not the gain channel and the mods as I thought before. It seems that the loop can't handle anything distorted no matter is it a distortion in front or the gain channel itself.
I guess I'll leave everything in front.

[torcamaniac]

So I just ordered Morley ELC effect loop corrector box to try if it works. If not I'll just send it back

[torcamaniac]

Tried Morleys ELC on all settings. I could get the the FX loop pedals to work on the same levels on both amp chanels but....
It lowered and kinda thinned the whole amp's volume and sound a lot!!! I'd say it decreased the loudness around 50%.
Well, I sent it back. Won't use the loop at all.
The amp sounds great with all the pedals in front.
Maybe I'll get back to the whole problem when I get better and more expirienced in electronics. Right now I'm studying in school for electronics for devices and systems for 8 months. Still a year to go.
Unfortunately, tubes are not a part of it but I'm trying to learn as much as I can myself and from the net.

[torcamaniac]

Decided to measure the voltage on the second 100K voltage divider to see how hot it actually is. Than I can experiment with lowering the voltage output.
While I am at it was thinking about changing the bridge rectifier. The original is BR62 with 6A. Should I use one with larger amperage? And what would be a good way to mount the new rectifier to chassis? Because I've read that it dissipates the heat better.

Thanks

[torcamaniac]

Here's schematic with all 3 PCB's in one file:


And a look inside and imagine how messy it is to work on such a design, even when one takes everything out.

[torcamaniac]

There is one more stage after the loop. V103 but if I understand it correctly the V103 is the tone stack tube.
I think it shouldn't provide distortion anymore?
The overdrive channel uses second half of the first tube (V101B) and both halves of second tube V102.