JMP-1 Plate Voltages

[Wilder Amplification]

I've got a Marshall JMP-1 MIDI preamp in which I haven't worked on in quite a few years. Does anyone have any idea what the DC HT voltage should be in these and/or the AC voltage on the HT winding? Out of all the supplies on the schematic that's the one supply that it doesn't show the voltage on.

[Wilder Amplification]

Nevermind. Power transformer is shot. 55VAC on the HT winding and the sucker gets really hot even when under a no load test when removed from the PCB (the dual primaries were paralleled during the no load test).

So I removed the bottom cover and broke out the side of the plastic housing...sucker got fried damn good. The only things I can think of that would cause it to fail like that would be either the first filter cap drawing high leakage current (since none of the decoupler resistors exhibited any signs of getting hot) or a rec diode exhibiting an early breakdown. Unfortunately they don't fuse the HT suppy so there's nothing to protect the PT should something of this nature occur. The filter caps/rec diodes test fine on the meter but the meter only tests them at a couple volts so it's more than likely not enough voltage across the components to show up on the meter.

For less than $5 I can shotgun the whole HT supply circuitry minus the PT so that's more than likely what I'll end up doing to ensure that whatever took out the PT has been addressed. Question is if this transformer is even still available or not, which I'm currently looking into, since Marshall discontinued the JMP-1 a couple of years ago. If not I'll have to see if I can find a company to custom wind me one at a reasonable price.

[kbhe]

JMP-1 Plate voltages

Hi,Jon! The jmp-1 plate voltages are 160 volts.Antique Radio Supply has low power plate transformers on page 36.(2009 catalog) Good Luck.

[Enzo]

The power transformer is Marshall part T306, listing for $62.75. Doesn;t say discontinued next to it, so it ought to be available. That doesn;t mean they have them in NY, if it has to come over from England, better pack a lunch.

I'd apply maybe 300VDC to the B+ circuit just to see what reacts. it might not be a bad component. it could also be an arc, a solder bead, a loose nut, screw or plug tip rolling around inside.


I think you are locked into an original replacement or a rewind, since it not only has to have the B+ winding, but also the +/-15 winding, and the heater/5VDC winding. Not really much room in there for multiple transformers.

[Alex R]

I've seen three of those in the last two years. All had bad PTs. Marshall were supplying them the last time I asked. They solder in easy enough.