Marshall JMP Super Bass 100 Head Needs Repair and Clean-up

This Marshall head suddenly began making strange, high-pitched noises at low volume. Jovan asked, while we were fixing it, could we restore this fine old Marshall head back to the 1972 version? Absolutely!

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This is real Made in England Marshall goodness.

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Of course, the Bass head makes a great guitar rig, as well.

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Name, rank, and serial number, please.

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Once the electronics come out of the head, we always need to remove the knobs in order to properly clean and re-lubricate each control. These knobs are held in place with a set screw; this really indicates an English-built unit from the 1970s.

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Someone else has been here before, and got in a hurry when they were reassembling everything.

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The other end of the pinched wire goes here, so we’ll replace it and improve the workmanship while we’re at it.

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Here is the source of the high-pitched audio. Without the lead wire, the effective capacitance of this part is in the tenths of nanofarads.

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A little research shows that the original part was a silver mica capacitor. Here, we’re auditioning a few parts to see just how much difference the value makes in how this input circuit sounds.

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Anything with my fingerprints cannot go back out the door with workmanship like this.

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Most of the filter capacitors are of two sections, paralleled with a piece of wire as shown.

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All of this comes off in order to replace these parts. Do you see the signs of smoke?

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For bass amp performance, the cathode bypass capacitors are a little larger than the normal values found in guitar amps. Four of these are going into this unit.

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Remember the signs of smoke that we saw earlier? Soot has contaminated the chassis. Usually, this isn’t an issue, but in this case, the soot is mildly conductive, adding phantom resistors between the tube pins.

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Each of the sockets were removed for deep cleaning. One of the sockets was replaced because of a crack I found during cleaning.

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Sockets are back in; filter capacitors are changed out and wired as before.

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To make everything align properly, two washers are used behind the input jacks on the front panel. Can’t leave them out!

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The unit is ON and running fine!

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The last guy used sheet rock screws to hold the unit together. Either the screws are too small, or the holes are too big.

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So I decided to keep the screws, and decrease the size of the holes by gluing birch sticks, from cotton cleaning swabs, into the hole. This bottle of Titebond hide glue is five or six years old, and I need to be using it up. It will work just fine to keep the sticks in the holes.

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The sticks were installed, with glue, all around.

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Here, the finished electrical chassis is sliding back into the cabinet. Next step: a four hour test!

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Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626