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!

.

This is real Made in England Marshall goodness.

.

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

.

Name, rank, and serial number, please.

.

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.

.

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

.

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

.

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.

.

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.

.

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

.

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

.

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

.

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.

.

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.

.

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

.

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

.

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

.

The unit is ON and running fine!

.

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

.

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.

.

The sticks were installed, with glue, all around.

.

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

.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Rickenbacker Bass from Reverb.com Looks Great But Doesn’t Play Fair

Craig found this incredible Ric bass on Reverb, and thought he could flip it. When he unboxed it, he discovered several electrical problems that needed to be fixed to protect HIS reputation as a seller. Could The Unbrokenstring track down the electrical issues?

.

Can anyone tell me what the model number is?

.

I never get to see the tuners up close on cool instruments like this.

.

The bound and sealed fret board is just outstanding on this instrument.

.

Rickenbacker calls the neck pickup “Bass” and the bridge pickup “Treble.”

.

This model comes with a built-in thumb rest. Not.

.

The bridge and saddles are just massive.

.

Each pickup is wired to separate outputs e.g. The Rick-O-Sound system. We will fix the issue of the jacks extending past the nuts like this. Anyone who has played out knows that the extended jacks makes it much more difficult to plug the cable from the amp into the instrument when you go on-stage. And it’s dark. And you are nervous. And the jacks are on the other side of the instrument where you can’t see them. This is a pet peeve of mine.

.

One pickup is completely dead, and one of these controls will not rotate at all, and these controls are all out of order. What’s up with that?

.

The pots have been changed from the original 330k units, but these are the Correct tone caps.

.

The pick guard and bass pickup are from your favorite parts vendor, located right here in Houston, Texas.

.

Inside the control cavity, we see some signs of human presence. The 4001 is the model number. The rest of it…?

.

Well, this explains a lot. The wires are literally tied in a knot, and shrink-tubed together.

.

Here is more Boy Scout knot-tying merit badge material.

.

The connection to the switch is wrapped, not soldered, and it also appears to be glued with a clear adhesive.

.

I detached one end of each of the tone caps to check them for value and leakage at stated working voltage. These are in perfect shape.

.

Here is more of that clear glue. It’s not Super Glue, but some sort of crystal clear hard plastic adhesive, I guess.

.

Here is more glue. Now, it just hit me. The reason that the pot doesn’t turn is that some of the glue leaked inside the control and locked it in place. Oh boy.

.

Here is another connection made with glue. If it were silver rather than clear, I’d say that someone used Liquid Solder. Which has actually happened, but that’s a long story involving a Heathkit home stereo from the 1970s that is best saved for another day.

.

The output jacks are mercifully unmolested. These were cleaned and checked OK.

.

The blue material is the automotive heat shrink tubing that can be shrunk with a cigarette lighter. The presence of soot on the blue tubing confirms this.

.

Our Boy Scout was here too, wiring up the bass pickup to the rest of the electronics.

.

The treble pickup (next to the bridge) had a broken wire. The small wires were broken from the stress from moving the larger wires.

.

While we’re here, let’s see what happened to the bridge ground. No, I’m not taking the strings off, just leaving them slack. Nobody changes bass strings.

.

This takes the cake. Duct tape. Or is it Duck Tape?

.

A dab of non-conductive hot glue secured the ground wire and the duct tape removed. We’re about done with our exploratory surgery, so the bridge can be reinstalled.

.

500k pots are ‘pretty close’ to the original 330k pots, which are only available on the used market for Big Money. All the wiring is soldered correctly. The location of the controls is marked as shown. You can see, from the earlier picture, that this layout is different.

.

A marker was used to clearly identify which jack goes with which pickup. The output jacks are ready to be installed. The blue heat shrink in the foreground is some of mine, properly shrank with hot air, not a cigarette lighter.

.

Note that the output jacks are flush with the nuts. This makes it a lot easier to plug in when you walk on stage.

.

The wiring is done and the bass is set up. It’s actually a lot of fun to play!

.

Here is the correct knob layout, in case you don’t download the Users Manual.

.

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Jim Dunlop Uni-Vibe Pedal from eBay Is Non-Functional

Joe wanted a new pedal board, so he used this occasion to get something he’s always wanted, a Joe Dunlop Uni-Vibe pedal. But the elation of the winning bid was replaced with the realization that this unit was defective. Could the Unbrokenstring Crew bring it back to life?

.

The owner said that the LEDs were intermittent and no audio came through the pedal in any mode, even bypass.

.

The Correct power supply was delivered with the unit, and it works fine. Note the +18v output.

.

Jiggling the cables when plugged into the connectors seemed to have an effect, so we will begin by looking inside for mechanical issues.

.

These knobs are guitar knobs with set screws… all metal construction here!

.

The wrap-around case is very sturdy!

.

Like most used pedals, adhesive from Velcro tape leaves a sticky mess. This is offensive.

.

A little elbow grease and alcohol on a rag removes the sticky mess. That’s much better!

.

Three circuit boards make up the internals of the pedal. The first board to come off has the optical ‘stuff’ in the silver box, used on many electronic tremolo designs. I think that’s how the rotating Leslie effect is realized, as well. We’ll check this out later.

.

Connections to the next board are done with gold plated pins, mating with sockets containing gold plated pins. This avoids any problems with dissimilar metals.

.

Interestingly, the parts inside the silver box were hand-soldered after the rest of the board was (probably) wave soldered. This makes sense, as the parts inside the silver box are probably not compatible with the heat of the wave solder process.

.

The middle board has all the remaining electronics, such as power distribution, audio routing, and interconnection.

.

The back side of the middle board has the wiring for the external control potentiometers.

.

In true Leo Fender style, this metal bar serves as a washer for the controls, grounding everything together.

.

While we’re here, we will clean this up. A sanding stick is just the thing to knock the rust off.

.

This washer is steel, so a little lubrication is in order to keep it nice for the coming years.

.

There. Shiny on both sides!

.

Now that everything can be accessed, the power jack is cracked. Here is a new one.

.

The holes in the circuit board are cut as slots, not drilled as round holes.

.

This solder joint is cracked. This may have something to do with the lost audio, although this circuit is the ‘sleeve’ of the plug that attaches from the outside world. All of the rest of the solder joints on the connectors inspected under the stereo microscope and were cleaned up.

.

The solder attachment points for those rear panel jacks are under this piece of insulating plastic, which separates the jacks from the metal bodies of the controls. This piece of plastic needs to cover all of the attachment points under the controls.

.

Here is another view of the insulating plastic and the long washer holding the controls in alignment.

.

A quick check shows another source of intermittent audio: the Pedal input jack. The electronics thinks that an external pedal is installed when it isn’t. These quarter-inch jacks have a built-in switch that is sometimes used to steer signals based on whether a plug is inserted. This contact pair was dirty. To clean it, a plug is inserted to open the contacts, a piece of clean paper is soaked with cleaner, the plug is removed, and the paper drawn through the contact pair.

.

This stuff is really expensive these days, so I’m trolling for a corporate sponsor deal.

.

The silkscreen printing on the case is VERY fragile. Most of the existing printing has come off.

.

My Brother P-Touch label maker, loaded with clear tape, is just the thing to make some nice, new labels.

.

As the pedal goes back together, contact cleaner is used extensively to clean up any junk and to give some assurance that the contact won’t open up again in the future.

.

I just had to take a look at the insides of the optics on this pedal. Here is the silver box.

.

The silver box reflects light from the incandescent lamp in the center and illuminates the four light-sensitive resistors spaced around it.

.

We are checking the internal clocks, used for ‘Rate’ which are created by the middle board.

.

Here is the optical system in action, minus the silver reflector box.

.

Now that we’ve had our fun, the box will go back in its place.

.

The hold-down wire is soldered on both ends to the circuit board, and the remaining slack is taken out by crimping the wire as shown. We don’t want this to move, as mechanical vibrations will be transferred to the audio chain.

.

The first circuit board is done.

.

Reassembly includes getting all the spacing washers between the jacks and the cabinet back in the right place. I see these left out when techs get careless or in a hurry.

.

These jacks are secured by these plastic nuts.

.

The whole jack is insulated from the rear panel to break ground loops (remember the long metal washer that we cleaned up above? They are connected to the chassis. These jacks are not…) and so they are made from insulating material. No wrenches were used to tighten these fasteners, but just finger-tight with a socket.

.

Screws and knobs are back on!

.

No RED LED on the right indicates that BYPASS is asserted.

.

The RED LED shows that the EFFECT is active and the GREEN LED indicates that the ROTATING SPEAKER or CHORUS effect is asserted.

.

The RED LED on the left indicates that the PITCH MODULATION or VIBRATO effect is asserted and no RED LED on the right tells us that BYPASS is asserted

.

In this mode, the RED LED on the left indicates that the PITCH MODULATION or VIBRATO effect is asserted and the RED LED on the right tells us that the EFFECT is active.

.

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626