Marshall 100 blows mains fuse when Ht is engaged

[Tubetec]

So yeah I called over to a buddy today ,
I had a look at a jcm 800 series that was blowing a mains fuse when the standby switch was thrown , I had it down to either transformer , diode bridge or filter cap , a quick cold test with a bog basic meter told me one of the diodes in the HT rect was shorted , I procured an In4007 from a defunct Smps , snipped out the offending component and made a solder bridge without having to hoik the pcb from the chassis , job done back to high voltage rock n roll .

 

[Tubetec]

no need to stress all the wired connections to the pcb by lifting it , nib out the offending component and tin the pre existing leads .

 

[Tubetec]

Go the hard way and move the pcb if you want to be a perfectionist , easy way is better and less prone to fails.

 

[fazer]

Agreed

 

[john12ax7]

Interesting, maybe they were using under rated parts?

JCM800 is a classic.

 

[Tubetec]

IN4007 is the default , its just about within its ratings but they fail short the odd time , for no apparent reason .

some amps put two diodes in series on each leg of the bridge , its a bit more 'failsafe' in case of breakdown .

 

[john12ax7]

some amps put two diodes in series on each leg of the bridge , its a bit more 'failsafe' in case of breakdown .

Does the 800 not do that? Usually a single 1N4007 is not enough with the higher voltage transformers.

 

[Tubetec]

The circuit diagram of the 50w jcm 800 amp shows a full wave rectifier (two diodes) , the 100w shows a bridge config with centre tapped secondary . As far as I remember the JMP 50 watt uses four diodes in fullwave connection .

https://schematicheaven.net/marshallamps/plexi_superlead_reissue_50w_1987X.pdf

 

[CJ]

jmp2000 added .001 caps across 1n4007 diodes
fender twin silverface master uses .002,

yeah, lifting those marshall pc boards is a major pain in the posterior, i would rather work on a Bentley while it was on fire on some cargo ship than lift another one of those boards.