Marshall JMP 100W blows fuses.

[Luthin You]

I am a tech but this is blowing my mind , this marshall was blowing a mains fuse randomly about once a week , I pulled the tubes - no change , pulled the stby fuse - still blows , replaced all filter caps - still blows , replaced diodes - still blows , but now its blowing instantly as soon as the stby goes on and the only thing connected is the new diodes ( on a tag strip incase it was the board) surely the transformer can't be at fault ? HELP !!

[Enzo]

Got a model number?

Make and USE a light bulb limiter, stop blowing fuses.

Photo of your tag strip.

[Luthin You]

Thanks Enzo , its from 1975 and model is 8086 , cct board says ST1, its a 100W super lead 2 . Im trying to find a suitable bulb for limiter . I can't work out how to post a pic ( over 60) but each ac line from standby has an anode going to + , and a cathode going to ground , centre tap not connected right now .

[Enzo]

Your description sounds backwards from here. Anode is the triangle side of a diode, cathode is the line end.

[g1]

Not sure about the 8086, but this might be it? https://drtube.com/schematics/marshall/1959mk2u.gif

[Luthin You]

Yes that's the schematic , its a Mk 2 SLA . Hard to describe diodes , but they're right . Turns out one of the new filter caps was a dead short . Ill try to get it going today and let you know . Thanks very much .

[Alf]

I was gonna say : maybe a diode but I saw you replaced them already.

[Amp Kat]

You have to work from a no load situation and varify the tranformer will operate without a load and all the voltages are correct and the primary isn't shorted. Then start connecting components 1 by 1 starting with the bridge rectifier and moving downstream to preamp tubes to power tubes. Imo sounds like a wiring proble or component in the wrong place. Just an FYI but I only use F&T HV caps and have never found one shorted or bad since I've been using them.

[vintagekiki]

Is the main voltage selector set to the correct voltage.
Is the main fuse of the appropriate value and slow blow type.

[pdf64]

Do not connect the HT winding’s CT to chassis 0V, as bad things will happen. It sits at half the HT VDC, thereby balancing the voltage across the two reservoir caps.

[Luthin You]

After installing new JJ filter caps , one of them was a dead short , This took out a diode in the bridge and also , as it was in series with its mate , caused it to boil and blow . This happened while using a current limiter . I replaced the diode and caps and its back in service now , thanks everyone .