Cheap Marshall JCM900 4102 "AS IS" - blow fuses, bad power tube and pots - low output

[DrGonz78]

Wow!! Read many threads here where the OP posted about getting the flu and hadn't gotten to work on the amp lately. But civil unrest against the government and a chaotic situation like that takes the cake. Hope all is going better where you are and that you can get back to amp work soon.

[spunko]

Good news Marshall half-worked.
Since clean ch sounds really really loud, we don't need to test the power amp. If the power amp works properly on clean ch fault is in dirty ch.
For this repair do not need scope.

https://drtube.com/schematics/marshall/cd0189-iss7.pdf

When switching channels does dirty ch have less output in both cases with F / S and with panel push boost sw, and does the LED light up.

To start
- Check VR2, VR8 and VR10 (1 MOhm) does is it correct and not interrupt.
- When switching channels, check that changes voltage at point C of switching circuit (TR1/ MPSA13)

- If VR2, VR8 and VR10 are correct, check the changes voltage when switching channels at
diode D5 (1N914) and IC6 (M5201 pin 1)
diode D2 (1N914) and IC1 (M5201 pin 1)
diode D1 (1N914) and IC4 (M5201 pin 1)

Hello, and sorry for the late reply, you already know

Thank you very much for all the help.
I checked again all the pots as suggested, and the volume pot for the ch B wasn’t reading well on my meter, I know it’s audio taper, but it went up and down in the reading when it is supposed to just rise when turning up.

I changed the pot and “glory”, both channels sound really loud.

I’m happy with its sound so far (1 hour playing, neighbors are complaining)

Also all the correct fuses has been installed.

[vintagekiki]

I am glad for your fixing Marshall and what you are happy how the amp works.
Welcome to the club.