Uneven plate voltages? - now a Marshall troubleshooting thread

[Yoda]

Here are the tube sockets:





I still need some help here. I just replaced the cap cans and the bias supply electrolytic caps as well as the bias resistor which was incorrect from the factory.
EDIT: Disregard previous comments. I was able to get the amp to turn on now without the current limiter in place. Bias is turned down all the way until tomorrow when I have tine to adjust it but for now the bass seems to be back as well as some volume. I’ll report back with any other problems.

I still need to take care of replacing the rest of the electrolytic caps on the preamp board. I am going through this amp and giving it its first servicing in 30 years.

The schematic for it is the early 2100, not the SL-X. The only schematics I’ve found are very blurry but they are for circuit boards JMP50C and JMP51C.

[Yoda]

This is the schematic for my amp, the JCM 900 2100 (non SL-X):

https://el34world.com/charts/Schematics ... x_100w.pdf

[xtian]

R12 on preamp board has a black spot. Burned, or just dirty?

[Yoda]

It’s a little bit of black silicone from the nearby caps.

[Yoda]

I also found the new F&T filter caps to be quite underwhelming in size to the original LCR caps they replaced. The F&Ts look like Legos.

[Reeltarded]

so at least some result!

Modern lytics are small and efficient. Be thankful they didn't get bigger in 30 years.

[Yoda]

I got the amp turned on, both power and standby switches turned on, without the current limiter. Then it blew the fuse after warming up for a minute. Still scratching my head on this one.

[xtian]

The OT might be shorted and drawing excessive current.

[pdf64]

The OT might be shorted and drawing excessive current.

I can't think how any type of short could cause these symptoms, which winding do you think is shorted to what?

[Reeltarded]

It actually can be the PT as well.. still dont get how a piece of wire shows two voltages at opposite ends though..

[pdf64]

It actually can be the PT as well.. still dont get how a piece of wire shows two voltages at opposite ends though..

Probing power valve anodes, especially true pentodes, can invoke oscillation, thereby making a nonsense of the meter reading.
If it really needs doing, best remove the LTP valve first.

[xtian]

The OT might be shorted and drawing excessive current.

I can't think how any type of short could cause these symptoms, which winding do you think is shorted to what?

A short in the primary that measures fine with multimeter but fails under high voltage. Wild guess, but I want in on the lotto.

[pdf64]

A short in the primary that measures fine with multimeter but fails under high voltage. Wild guess, but I want in on the lotto.

Hey, that’s my special ’bete noir’, go find your own
But seriously, as it happened whilst idling, that seems a really long shot

[Yoda]

I need to pick up some new fuses before I revisit the issue of unequal voltages on the plates, that happened before I replaced some parts.

If there’s a short in one of the transformers is it still possible for the amp to make sound? It was making sound if hooked up to the current limiter.

[Yoda]

New fuse is in. I powered up the amp with no current limiter and it turned on again. This time I measured 400-402v on all four power tube plates.

Now I have my two bias probes hooked up, I have the bias trimpot set in the middle of its range. Upon flipping the standby switch I watch the bias climb to around 22-24mA on one meter and roughly stabilize while the other meter continued to climb slowly. I shut the amp off when it got to 50mA on only one side before it blew a fuse again. Something obviously still not right here. I switched the probes to the other output tubes (still one meter per push-pull pair) and the fuse blew when I flipped the power switch without the current limiter.