Marshall JCM 900

[Clem Pendleberry]

I replaced the faulty power transformer and It worked fine,and biased well. I thought I would replace the filter cap while I was in there. I was very careful with the wiring and I brought it up on the variac to about 100 V. Iwhen it started smoking. Maybe I should have left the old filters in there But they were about 33 years old .Can anyone offer me any advice?

[Chuck H]

It looks to me like something is shorted just past R30 in the B+ rail. Possibly the filter capacitor. I'd call it out but it's not designated on the schematic.


https://schematicheaven.net/marshall..._100w_4100.pdf

[Jon Snell]

We need to know what that component is; R? before we can advise but the most common cause is incorrect fitment of components. Using carbon film instead of Metal Oxide to avoid a bonfire as you have. (Component leads not cut to the correct length and pushed through from the top of the board so far, they touch the chassis).
I have seen many where techs try to cut corners and don't want to replace components from the correct side of the board.
Not saying this is what happened here.

What do you mean by "Filters"? Capacitors perhaps or series resistors ... what is their component number?

I see many in my workshop;
How not to wire an IEC mains connector!
Caused by a dry joint. I fixed it so you wouldn't know it has bits of PCB track missing.

[Jon Snell]

It appears that you are asking the Marshall forum for advice as well as here. Good advice was given but you didn't follow it, under the handle of Hapa.
Get a tech to fix it.

[Chuck H]

That incinerated resistor looks to be R30. Ergo I called out R30 in post 2. In this image I found it looks to be 10k but in the schematic it's indicated as 22k. Could be a revision or whatever. Hard to say. I suspect a miswire in the last repair and agree a qualified tech should handle it from this point before the amp is destroyed.

[The Dude]

The sage advice of our "Great One" would have gone a long way here. "If it's working, stop fixing it". Internet lore strikes again.

[Jon Snell]

The easiest way to destroy R30 is wire the 50 - 50 uF capacitor incorrectly.

[Chuck H]

The easiest way to destroy R30 is wire the 50 - 50 uF capacitor incorrectly.

This is the reason for my reticence and suggestion of a qualified tech to handle this matter now. Because "That looks right. Let's flip the switch and see how it goes." is very different from knowing the circuits and what you're doing. I can get behind trying. That's how we learn to a degree. But when expensive amps start catching fire it's time to pull back.