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Marshall VS 8100 blowing fuses

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9/9/2005 9:41 AM
Aris	Marshall VS 8100 blowing fuses I disconnected the PT secondaries and the fuse doesn't blow...otherwise, the fuse instantly blows in a bright flash of blue light. Any ideas on where/how to start looking for the problem? Are there known problem areas on these amps? Thanks for your help. Aris

9/9/2005 11:10 AM
Aris	Oh, yeah...the output transistors are ok...no shorts between legs.

9/9/2005 12:39 PM

you can check the diode rectifier and filter capacitors, but I suspect that they're fine.

I recommend doing all testing at this phase with a lightbulb in series with the mains supply.

Does it also act up with the speaker disconnected. If the amp is going DC and the output is slammed up against a supply rail, it'll prolly blow the fuse. Don't ever have a speaker connected to an amp that you're fixing, unless you are reasonably sure it's working.

You can try powering up the amplifier with the power supply working, but the output transistors removed. You might want to check the bias transistor (and the bias pot) and the driver transistors. You can measure the voltage between the bases of the output transistors. There should not be much voltage between them. maybe two-three diode drops at most. If there are several volts on there (or if one of the base lines is kissing the supply and the other's somewhere completely different) or more, this is what caused the output devices to short the power supply and pop the fuse. check the bias supply and drivers/predrivers. If you are reasonably sure that everything in the amp is working fine, you can solder some resistors (~100 ohms seem to work) from base to emitter of where the outputs would go, and then see if the amplifier can pass a signal (scoping the output), the resistors let the drivers drive the output somewhat, but it can't output any real current. If this works, and the bias voltage between the two bases checks out, you cna probabally carefully try it again with the outputs.

9/9/2005 8:49 PM

In my experience with those it is almost always a shortd output device. When it is not, it is then more likely a rectifier in the power supply than anything.

Beyond that, shorted filters are possible, but rare.

And don't forget the little "invisible" things like small caps across the rectifiers and small caps across the transformer windings. SInce those are on the pc board, they would be out of there with the secondary wires removed.

With the tranny disconnected, measure resistance between the posts where the wires went. And from each post to ground. A short should show up that way.

9/13/2005 2:06 PM
Aris	it was a shorted diode bridge rectifier....replaced it and all is now well thanks, a

10/27/2005 8:04 AM

Enzo - Another Marshall blowing fuses....

...and I'm wondering if the PT is bad. MG100 blows fuses until I disconnect the red secondary wires from the board. Output transistor is good, as is rectifier bridge and other power supply diodes. I'm unclear about your previous post...when disconnected, there is continuity between the 2 red secondary wires, and between each red secondary wire and ground. Is this normal in a healthy PT, or is this one bad?

10/27/2005 9:33 PM
KB	With the secondarie wires disconnected measure the voltage across the post and you should get a voltage close to the rail voltage usually from 40 to 60 volts AC. I honestly wouldn't think the tranny is bad but it is possible I guess., KB

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