JTM60 Combo rectified diodes fried

[Psabin8951]

I have just been given a JTM60 combo.
Its HT rectified diodes D105,106,107,108 are all blown (literally!).
Obviously I can replace these but what would have caused this?

[Enzo]

Possibly nothing.  One diode shorts from random component failure, and causes the others to fail.  Otherwise, a shorted power tube is the first source of damage in a repair until shown otherwise.  Beyond that and all farther down the list would be anything to short the B+ to ground, like bad filter, bad flyback diode, shorted OT, and so on.

[trobbins]

Was the mains or HT secondary fuse blown?
Are the power transformer windings ok?

[Psabin8951]

Just got back to this.
No fuses blown.
I've replaced the diodes and its definitely not just them.
Does anyone have a pic of what voltages lurk where on the mains transformer as it's not maked. I then can rule that out hopefully.
Thanks in advance

[phatt]

Do you have the schematic? That will help you decipher which is which.
The pcb will likely have the reference numbers printed on the pcb.
Maybe post a pic as that will help us.
I'm guessing you have about 8 secondary wires, 2 for HT, 3 for Filaments and another 3 for the opamps.
HT will be ~ 300 ish VAC,  Filaments will be around 3v5-0-3v5 AC and the opamps will be ~ around 12-0-12 VAC.

not sure about the bias,, might be tapped off the HT?
Others here will know more.

You will need to build your self a limiter before you turn it on other wise you might just blow up more parts.
See here for details on how to make one;
https://www.ssguitar.com/index.php?board=2.0
3rd post down.

Phil.

[Enzo]

Transformer unlikely to damage diodes.  SOmething downstream drawing excess current through the diodes burns them up.

Have you removed the old power tubes?  What exactly is the amp doing wrong at this point.

[Psabin8951]

Phatt/Enzo,

Limiter already being used and bulb glowing brightly until tubes removed.

[Enzo]

bulb glowing brightly until tubes removed.

Failed power tubes are the most likely failure, and that sure supports that theory.

[Psabin8951]

Limiter bulb glows even on standby so surely that's not tubes is it?

[Enzo]

Unless I misunderstood:

Tubes installed bulb glows brightly.

Tubes removed bulb no longer bright?

Limiter already being used and bulb glowing brightly until tubes removed.

That at least points at the tubes.

[Psabin8951]

Sorry, having double checked it glows bright with or without tubes and on standby

[Enzo]

OK then, find the two high voltage wires from the power transformer, should be on posts W104 and W105.  Pull them off the posts so they are in free air.  Now does the bulb glow bright when powered on?  If so, the transformer is suspect.

Is F102 open?

There should be two wires to the standby switch, are they on posts W103 and W106?

[Psabin8951]

With wires from W104 and W105 the limiter bulb still glows brightly and yes standby switch does go to W103 and W106. F102 is ok.
So sounds like I need a new transformer right?
If so can anyone suggest best place to obtain them in the UK?

[Enzo]

Starting to look that way.

UK folks would know better, but doesn't Marshall deal with private citizens over there?  At least they could refer you to a retail supplier of their parts if not.

[Psabin8951]

Hi all,
Ok so I got a new transformer from Marshall all fitted ok.
I now get a less bright limiter bulb and no smoking diodes, channel switching works but no sound at all. No sound plugging in to fix return either.
Where do I look next?