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.

 

[Hapa]

To clarify the large fuses were found in the V1-2 and V3-4 fuse holders. When I got the amp it had a blown 4A slo fuse installed,and no mention of what they “tried” previously. I think we’ve all seen blackface Fenders with 20A mains fuses that worked fine….until they didn’t.

Yes everything I have referred to up to now is on the rear board. These diodes measure 22ohms in each direction individually and also when measured together,I suspect this measurement is due to the adjacent 22 ohm resistor.

R 22 is well within spec at 81.9k and I cannot find R 100, R36 is the highest number I can find on this board. All 4w resistors are within spec. I should note everything is tested in circuit and none have been isolated.

 

[PelliX]

This may be a silly question but could use the variac to power it up or should I construct a light bulb limiter?

It's not a silly question; a variac limits voltage, the light bulb limiter limits current - they have two different purposes. I'd recommend both if you're dealing with a (potentially) stuffed PT. LBL even more than the variac, but both would be ideal.

Yes everything I have referred to up to now is on the rear board. These diodes measure 22ohms in each direction individually and also when measured together,I suspect this measurement is due to the adjacent 22 ohm resistor.

Yes.

R 22 is well within spec at 81.9k and I cannot find R 100, R36 is the highest number I can find on this board. All 4w resistors are within spec. I should note everything is tested in circuit and none have been isolated.

Maybe I missed this earlier in the thread, but disconnect the secondaries from the transformer. If it blows fuses then, the PT is buggered. If not, it's somewhere downstream.

Your PT is most likely blown because the bias “X” cap blew out and shorted, which basically shorts the HV winding to ground.

Without glancing at the schematic I find that dubious; this should merely take out the HT fuse (assuming it had a halfways sane rating...).

Or better stated, what is supposed to be the “x” cap was replaced sometime in the past with a non “x” cap and that shorted…

That doesn't make any sense; the failure mode for an X cap is to short, whereas a Y cap typically fails open. Drop me the schematic that you base this on?