Marshall Lead 100 Mosfet

[goldtop5]

I have Marshall Lead 100 mosfet model 3210 that's blowing fuses. I checked every component that may be the possible cause, but can't seem to pin point what and where the problem could be. I noticed when I loosen the mounting screws on one of the mosfets (it doesn't matter which one), the short goes away, but the problem comes back when I tighten them down. The mosfets are well isolated. I used new insulation washers. What's your suggestion?

[Enzo]

Do not connect a speaker load. have you been loading the amp? If so, does it still blow when the load is reemoved?

Instead of blowing ANY fuses, look up light bulb limiter, make one, and use it.

Amps that put out DC will blow fuses with a load but will sit there with DC on the output as long as you like when unloaded.

SO if we are talking loaded, anything anywhere in the power amp that shoves the output over to DC will make fuses blow. One tiny open resistor can do it even.

If it blows fuses unloaded, then either both positive and negative sides are turning on at the same time, such as with an open bias string, or we have shorted outputs.


Then again you may have a shorted rectifier in the power supply. SHorted filter caps are much rarer, but happen.


And most rare of all, is the shorted power transformer.

[goldtop5]

I've been powering the amp up without a load. Yes, it still pulls current. I've been using a light bulb limiter to keep from wasting a bunch of fuses. The outputs have been replaced. I pulled them to check if there were any shorts but they tested fine. The big filter caps were replaced too. If both positive and negative sides are turning on at the same time, how would you troubleshoot? Can you explain the open bias string?

[Jazz P Bass]

Voltage checks are in order to determine why the two sides are turning on.
You can't guess at this stuff.

[olddawg]

Did you check continuity to the heat sink with an ohm meter? Don't trust visual inspection for the insulators. Measure with your ohm meter from the heat sink to every leg any output device with them properly installed before you turn the amp on and unplugged. There should be no connection. When you are un-mounting them you are disconnecting something. Btw, I usually buy these amps for next to nothing to reuse the cabinets. Everyone ask me about my old Plexi when it is an 18 watt clone in a Lead 100 MOSFET box. You might be able to buy a working chassis for less than another set of outputs cost you if you look around.

[loudthud]

Did you replace the output Fets? Any chance you got the N-Channel and P-Channel parts reversed?

[g1]

The outputs have been replaced.

These outputs are pretty much extinct. Any that are not very expensive NOS are possibly fakes. Where did you get them?
As Loudthud asked, any chance they are reversed?