Marshall MF350 question.

[khingpynn]

This is a continuation of a prior corispondence with Doug... all are welcome to comment.

I'm working on my Marshall MF350 Mode Four.

The problem I'm having is it blows the main fuse when plugged in even with the power switch in the off position.

As suggested Doug I isolated the four power chips... still have the fuse blowing.

I looked at the rectifier diode pac TS15PO3G and I see it has four terminals.

I would like to test it with my multimeter... calling the terminals one through four starting and the left most terminal being one... how do I do this.

[52 Bill]

The problem I'm having is it blows the main fuse when plugged in even with the power switch in the off position.

The first thing to check is the power cord, fuse holder, power switch, etc. All of the primary circuit components, anything from the plug to the power switch.

While there may be problem with your output chips and or your rectifier, if it blows fuses without turning it on there are other problems to deal with first.

As for checking a bridge rectifier, use the diode setting on your meter. Attach the red lead to terminal 1. Touch the Black lead to terminals 2 and 4 and note the reading. Now reverse the leads and note the readings. In one direction you should get low readings and in the other direction you should get high or open readings. Do the same test with terminals 3- 2 & 4.

[khingpynn]

Excellent... thanks very much.

[Enzo]

Oh, I missed the part about blowing fuses without even being turned on.

Does the detachable power cord blow fuses all by itself - with no amp attached? Or are we talking the main fuse in the amp?

After that, is the wiring to the power switch correct? No loose wires touching anything?

The mains circuit here is pretty basic. There is a IEC power cconnector which includes a mains filter. Wires to the fuse holder then on to the power switch.

Note where each one goes, and pull the wires off the power switch. Leave the switch in the off position. Now use your ohm meter to test the terminals of the switch to see if any are shorted together.

Also, you can then tape the bare ends of the wires so they touch nothing and power the amp with switch disconecteed. If fuses no longer blow, it is switch related. If fuses still blow, then there is wiring trouble or something touching somthing.

[khingpynn]

Everything is fine i can run just the one side of the tranny that runs the fans... if i disconnect the 30 volt secondaries of the tranny then I blow the main fuse.
When i disconnect the power chips ( all four ) I still blow the main fuse.

The main fuse is a T8... what amperage is this fuse... I'm used to the ones that actually mark te volts and amps on the terminal end of the fuse.

[Enzo]

Then check the rectifiers and main filter caps for shorted.

[khingpynn]

OK will do and thank you very much!