JMP blowing main fuse with odd symptoms.

[EyeHeartVintage]

I have a 50w jmp that is blowing fuses. Can anyone lead me in the right direction to solve this problem?

Here are the symptoms-

1. When plugging in amp to wall outlet, with power and standby switched off, the filter caps start to hum and vibrate.

2. Turning power on no change, filaments light up as normal.

3. Switching standby on fuse blows and hum stated in point 1 stops.

I thought at first a bad tube, but after pulling all the tubes it's still doing the same thing so I'm a little confused. No smoke or smell from the PT so I am thinking/hoping a winding didn't open up. Any thoughts?

[NickC]

How old is it? Has it ever been recapped?

Electrolytic power supply caps must be replaced every decade or two.

[EyeHeartVintage]

i think its a '71. all electrolytic caps read well over 10m on my meter which should mean they are operating correctly.

[Reeltarded]

No way that is happening unless the mains switch has burned closed. Unplug and drain then check power cable to both sides of switch in both on and off condition. Yes. It's closed, right?

Last time I had that happen to a working amp it flashed and started screaming just in time for the fuse to pop. There was a fire too!

Over 10m? You have to check them for voltage drop, but I fear they are gone with the switch honestly.

[Firestorm]

I have a 50w jmp that is blowing fuses. Can anyone lead me in the right direction to solve this problem?

Here are the symptoms-

1. When plugging in amp to wall outlet, with power and standby switched off, the filter caps start to hum and vibrate.

With the power switch off, AC should not make it to the transformer, hence no DC should make it to the filter caps (and, IIRC, most Marshalls have the filters on the cold side of the Standby switch anyway).

If it's making sounds the minute it's plugged in, I'd be looking for a short, maybe the switch itself. It likely needs a recap, too, but there's something else wrong.

[EyeHeartVintage]

It was a rectifier issue and the power switch was bad.

Thanks for the input guys.