Marshall JCM 900 4100 blowing fuses

[danfrank]

I was wrong!

Good for you for finding what was wrong with it and my apologies to everyone else for me being an asshole and trolling…

 

[Hapa]

I’m not sure this is the only problem but I feel I am moving forward. This may be a silly question but could use the variac to power it up or should I construct a light bulb limiter? Also does anyone know the value/ rating of the two large diodes that appear to be bad? (D6 and D7) They are considerably larger than the four that I assume make up the full wave rectifier on this same board?

 

[Purgasound]

I’m not sure this is the only problem but I feel I am moving forward. This may be a silly question but could use the variac to power it up or should I construct a light bulb limiter? Also does anyone know the value/ rating of the two large diodes that appear to be bad? (D6 and D7) They are considerably larger than the four that I assume make up the full wave rectifier on this same board?

Uh oh, wait a sec... When you first mentioned the diodes I just assumed rectifier diodes. D6 and D7 aren't part of the rectifier circuit. They are 1N5408 diodes are part of the grounding circuit. The circuit ground is kept separate from chassis ground on the 900 series in an attempt to eliminate him and EMI interference from shared grounding with other equipment.

The diodes would only burn up if something else shorted in the circuit. That's a pretty serious fault condition though. The mains fuse would always blow before that but we've established the wrong fuse was installed. Those diodes should short as opposed to open when failing so unfortunately we still don't know what the fault is.

How did you verify they were bad or not? On second thought, it's not entirely necessary to have those in play for the amplifier to function. They can be bypassed straight to chassis ground. What about the 22R or 100R 4W resistor beside it? Is that open? Again, this stuff only indicates something would have been sending enough current to ground to burn up those parts.

...and again, we need to be more specific when using part numbers to refer to specific components. I am going to assume you mean the D6 and D7 on the rear board as they are large 3A diodes but there are also diodes labeled D6 and D7 on the front PCB that are completely different components.