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

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?

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Can anyone tell me what the model number is?

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

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

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

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

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The bridge and saddles are just massive.

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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.

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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?

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

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

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

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

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

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

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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.

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

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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.

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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.

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

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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.

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

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

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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.

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

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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.

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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.

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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.

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

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

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

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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?

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

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

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

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

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The wrap-around case is very sturdy!

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

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

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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.

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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.

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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.

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

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

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

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

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

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There. Shiny on both sides!

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

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

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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.

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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.

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

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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.

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

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

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

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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.

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

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

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

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

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

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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.

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The first circuit board is done.

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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.

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These jacks are secured by these plastic nuts.

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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.

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Screws and knobs are back on!

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

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

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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

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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.

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Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Acoustic B600 Head Acts As If It Is Always Muted

This unit lights up but does not make a sound. Matt said that other techs couldn’t figure out why there was no audio coming out of this almost new bass head. As the last resort, could the Unbrokenstring Crew succeed where no tech has succeeded before?

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The plastic face plate, bezel, and knobs were coated with a synthetic rubber material that turns into a sticky mess with time. Here, I’m using a little elbow grease to see what works to remove this particular flavor of sticky rubber stuff on the bezel, which surrounds the front panel.

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This rubber coating, seen on this mouse, is popular on all sorts of consumer and pro-sumer gear, and the Internet has many articles and YouTube videos regarding how to clean this stuff up. Once the rubber coating begins to degrade, some sort of sticky chemical leeches out over all exposed surfaces. I suspect that the chemical has contaminated the MUTE push button, causing it to be, in an electrical sense, ON all the time, thus muting the unit. Other techs were looking for an electrical problem, not a chemical or material science problem.

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I took lots of pictures while disassembling the unit. There are no schematics available.

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The pictures would be invaluable when reassembling the unit.

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Here is another cable, hidden on the side.

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The central section of the unit is this chassis, which is a power supply and Class D power amplifier. The chassis serves as a heat sink for all the active electronics. A cooling fan on the top circulates air through the unit.

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With the chassis out of the way, the screws securing the front panel PC board in the chassis can be accessed. Of course, the knobs have to come off. Don’t they always?

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The front panel has all the controls, and some blue LED lamps. The MUTE button is labelled in the silk screen (white lettering next to each component.)

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The push buttons covers themselves don’t need to be removed to allow the unit to be disassembled, but they need to be removed to clean the switch. And they, too, are covered with the sticky rubber stuff.

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We are looking at the world’s first spa for Acoustic Amp push button caps. After trying several different chemicals and soaps, straight isopropyl alcohol seems to work the most quickly.

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Now that the switches and controls are cleaned, we can start the process of re-assembling the unit. The machine screw with the red fiber washer fits into a slot in the edge of the circuit board.

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Everything is reassembled in the reverse order of disassembly. The jacks and buttons on the rear panel are cleaned and lubricated. None of these were contaminated with the sticky rubber stuff.

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Power is applied, and all functions are checked out. Unbelievably, the soldering iron remained cold for the entire repair action. Sometimes you don’t need to be an electrician to make things work again!

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Thanks for reading all the way to the end!

David Latchaw EE

281-636-8626

Fishman Loudbox Mini has Defective Tolex from the Factory

This Fishman Loudbox Mini is about a year old. The owner loaned it out, and when she got it back, the covering was in tatters. Could The Unbrokenstring Crew do something?

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Fishman will give you a Return Authorization to return these little amps back to the factory, at your expense, and another unit would be returned to you. With supply chain issues, no stock at the factory, and not really wanting to be without the amp until a replacement became available, the owner decided to have it recovered locally.

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What was really irritating was, whenever you picked up the unit, the space under your fingernails would fill with this rubbery ‘stuff’ that was shedding from the original covering. This is particularly annoying for finger-pickers with long fingernails.

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To make matters worse, the owner would have to vacuum up little bits of covering two or three times a week in her apartment, and her car and the stage at church was always a mess after she used her amp.

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The first thing we did was remove the feet. The old covering stuck to everything!

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The handle bolts hold down this wooden stiffener on the top of the unit. Off it goes!

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This little hex bit is used to remove the bolts that hold the electronics in the chassis.

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Six bolts hold the chassis in the body of the cabinet. The unusual style of this cabinet will make the job of recovering the unit a little out of the ordinary compared to the usual Tolex job. Along the edges, both sides as well as the edge will need to be covered, with closely-matching seams.

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I’ll take a few pictures to show where the wire harness connects. These are polarized, but it is possible to swap a couple around and cause who-knows-what havoc.

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Power to the board comes from a power transformer inside the cabinet. The secondary voltages come through this cable.

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This is an audio connection from the front panel. We don’t have to remove this, but I snapped a picture just in case it came loose later.

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The front grille is held in place by three screws, almost hidden inside the electronics compartment.

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The grille tilts away from the cabinet, but is still held captive at the bottom somewhere.

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This little tab fits in the slot cut into the cabinet. We need to keep track of this slot lest it be covered with Tolex and forgotten.

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The inside corners, next to the control panel, are covered with these little plastic corner covers. Naturally, the sticky covering is all over these corners. A neat little feature of these corners is, some of the seams of the original covering come together under these corners, and are hidden from sight and protected from damage.

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These corners need to be cleaned up. Heat is not an option, as the plastic will probably deform. Most chemicals will likely attack the plastic.

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A little old-fashioned elbow grease is applied to break the adhesion between the old covering and the plastic. It rolls off slowly, with this electricians’ screwdriver.

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Once the posts are cleaned, this can be set aside, ready for reassembly later.

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While we are cleaning up hardware, every item that touches the old covering requires cleaning. Here we are cleaning under the heads of the machine screws that hold the electronics in the cabinet.

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Little washers under the heads of the screws that held the power transformer are particularly sticky. Here, we are using a sharpened edge of a popsicle stick (excuse me, they are called tongue depressors nowadays) to scrape the goo from the washers.

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Even the edges of the hardware that holds the handle strap to the unit is messy.

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We now turn our attention to the cabinet itself. We will remove the loudspeakers and keep them in a safe place while we work on the cabinet itself.

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As with most Tolex removals, a heat gun is just the thing for speeding the process along. This is a paint stripper, run on the lowest setting.

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Once an edge is heated, the old covering can be pulled away from the cabinet material. Here we go!

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The cabinet is sealed particle board, giving us a good surface to support the new Tolex.

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The old covering is shedding each time we handle the cabinet. What a mess!

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Now, we’re starting to get somewhere! Note that the aluminum tape shield around the electronics has been peeled back, so that the old covering could be completely removed.

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To re-Tolex the small panel under the handle, we are arranging to have all the edges meet at the screw holes, so that the pressure of the handle screws can secure all the edges. No over-lapping of Tolex is allowed anywhere, because of the tight tolerances between the existing cabinet and the pieces that fit into it (the electronics, the grille, and this panel.)

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One side of the cabinet is done. Here, we are opening up the holes before we forget where they were.

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Once the sides are covered, a simple rectangular strip covers the top, back, and bottom. But rather than get contact adhesive on the new Tolex, we will mask off everything, starting with these inside corners.

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I don’t think that it will be a deal-killer if adhesive is accidentally smeared on the speaker baffle, but my obsessive/compulsive disorder forces me to do it anyway.

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The blue tape indicates the start and ends of the last piece of Tolex to be applied. It wraps around the bottom, back, and top of the cabinet.

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The masking tape must conform to the Tolex already installed to prevent any wicking of the adhesive, so this brush pushes the masking tape into the contour of the new Tolex.

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This corner, at the bottom, is the fussy-est edge because of the small geometries involved. The contact adhesive on both the Tolex and the cabinet must be completely sticky and dry to hold this thin edge down. Here, more masking helps control the spread of adhesive on the bottom of the cabinet.

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I took a clear picture of the bottom before the Tolex was applied, because I will need to find those holes again, for the feet as well as the power transformer.

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And this is why we need to mask off the edges. The disposable brush works well, but it’s a little hard to control near the edges. The white stuff is the contact adhesive.

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When the adhesive is dry, it turns clear. A little heat will help speed the process along where I put a little thicker layer of adhesive on the cabinet.

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Here is the top and the right side of the covered cabinet.

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Here is a view of the back and bottom of the recovered cabinet. This is starting to look nice!

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The power transformer is bolted to the bottom. Remember those little washers that we were cleaning up earlier? They go here.

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The clean feet are returned to where they belong. Note the pink eraser; It’s just the thing for cleaning up spots on the new Tolex where adhesive leaked under the masking tape. It rolls right off, without a trace.

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The tweeter is mounted back where it belongs. The wiring is polarized so that we don’t need to worry about + and – connections on either loudspeaker.

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The other loudspeaker is reconnected and installed now.

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Remember the slot that holds the tab on the grille? This is it.

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This is a better view of the tab/slot system.

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The tab goes into the slot, and the grille fits tightly against the new Tolex.

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You have seen this before, from another angle. Three screws hold the grille in place.

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A grounding lead on the electronics is attached to the aluminum shield inside the cabinet, as shown here.

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These big screws hold the electronics in place. Recognize that little hex bit I showed you earlier? Here we are again!

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Two screws hold the handle and the rectangular spacer in place on the top of the cabinet. This project is all done!

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Ampeg BA115HP Combo Rescued from Pawn Shop

This pawn shop find doesn’t work, parts are rattling around inside, and a knob is missing. Watch The Unbrokenstring Crew get this into the practice room!

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St. Louis Music moved their production to China, which IMHO was the beginning of the end of this fine brand. Que sera sera.

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I had always wondered if stuff from a pawn shop with obscured serial numbers was a ‘little warm to the touch,’ if you know what I mean. But who would steal something this big?

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The electronics are out of the cabinet for inspection. A replacement Gain knob is on order from eBay, because someone cranked it up and ripped off the knobs. The ‘Style’ switch, which was broken internally, was put on order as well.

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This amp sports three band EQ and a Master Volume

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You can play your CD through the Line Input RCA jacks on the top, and mute the amp to play through headphones. In the muted mode, a built-in chromatic tuner comes on. Neat!

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The internals of the tuner was one item rattling around inside the unit.

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The translucent lens over the LED array diffuses the individual LEDs that illuminate when each note is played. This was very dirty for some reason.

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The tuner electronics and LEDs are one assembly, which makes me think that this was an option for this model.

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This power cord needs attention. All I need to do is unclamp it, shorten it, and re-clamp.

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The Chinese did not go cheap on the power transformer. This is the power supply section, with the filter capacitors seen on the left.

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The power amp is all bipolar transistors. The large white objects are wire-wound resistors. Some of these resistors are open-circuit, but the transistors are all intact. This is interesting…

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Removing the preamp board, we note that these spacers are required behind the input jacks. The gain control, which needed a knob, is fine.

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Speaking of spacers, this toothed washer is required behind the Style switch (the white and green item in the center of the picture) to properly set it’s geometry. This is the new switch. The original switch was defective.

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The ‘sleeve’ contact on this input jack was open-circuit, which caused hum with nothing plugged into the amp. Do you see anything?

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The contact on the right is not closing when nothing is plugged into the jack.

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The easiest way to fix this is to just replace the jack, although these can be carefully disassembled and bent back in place. These are inexpensive enough to just replace.

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The open resistors were identified and removed. Note the round holes in the circuit board.

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These round holes are for air circulation. I believe that there is a relationship between the plugged holes and the defective resistors.

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Some of the holes were plugged. The material plugging the holes is adhesive that helps to mechanically support the resistors when the amp is moved. Someone at the factory was sloppy with the hot glue gun!

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The new replacement resistors are modern high-temperature resistors, spaced above the circuit board for additional heat dissipation. All of the holes in the circuit board are clear now.

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This part of the circuit board is all cleaned up and ready to reassemble.

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The raw corners around the mute button can be stained to match the black anodize aluminum. A permanent Sharpie pen works well for this task. A little blending with a finger, before the marker fluid dries, makes the repair invisible.

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The front view of the finished unit is pretty unremarkable, since the controls are on the top and back. The control in the lower center is a pad for the tweeter. This unit is now 100% and ready to get back to the practice room!

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Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

The Left-handed Squire Stratocaster Gets Upgrades

Lindsey wanted new Texas Special pickups installed in her beautiful Squire Strat. While we’re at it, we’ll upgrade the tuning machines to Fender Locking tuners. Note that this is a left-handed instrument.

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Name, rank. and serial number, please! As we disassemble this instrument, we will see that this Squire is just a little bit nicer than the instruments coming from China.

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This is a good picture of ‘break angle,’ where the strings are stretched over the nut.

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I just love the red pick guard with the red burst finish! The strings are off.

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Lindsey purchased a new set of Fender Locking tuning machines. The original tuners are coming off.

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This is a picture of a pile of retired Squire tuning machines.

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Oops. These new Fender Locking tuners use a little different mounting pattern than the Squire tuners they replace.

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Little dimples next the the dowel pockets show where the new dowel pockets need to be bored. So there IS a difference between the Squire tuners and the ‘similar’ but full-sized Fender Locking tuners.

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The new tuners are lined up with a steel rule, to get them absolutely straight.

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After plugging the original holes with ash and boring the new dowel holes, the locking tuners are in place. No one can tell now that any gun-smithing was performed. They look pretty spiff!

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True to its brand, this neck plate says “Squire.”

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To remove the pick guard, the neck must be removed. Why? Because the neck has an ‘overhanging’ fret board.

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What is a fret board overhang? This.

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The Unbrokenstring Crew will augment the shielding around the electronics by adding copper foil to the body routes and the bottom of the pick guard.

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The ‘ground’ wire to the conductive shielding paint is removed so that the copper foil can be installed underneath it.

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This triggers my Obsessive/Compulsive Disorder (OCD.) Did someone add a center spring and just push the black ground wire out of the way?

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This film tone capacitor is just fine for this application.

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The output jack is liberated from the ‘football’ plate. The jack and cable will be removed from the body entirely.

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This neck pocket is VERY clean and true, and requires no remedial work to remove finish or glue. This is NOT a plywood guitar! It appears to me that a black Magic Marker pen was used to color the corners near the neck to cover the raw wood.

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The CNC machine cut one direction around the cavities of this body, causing tear outs in the wood grain. Then, they were just painted-over. This needs to be cleaned up and smoothed out to give the copper tape a proper surface to which to adhere properly.

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Here we see some router tear-out because of the conflict between the rotation of the router and the grain of the wood. The router bit edges tear and lift the grain up and away from the body. When routing by hand, the correct procedure is the change the direction from which the router approaches this part of the wood. But you can’t tell a CNC machine to stop and work backwards. Well, you can, but only if the programmer is conscientious, rather than cost-conscious.

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The white material is left-over polishing compound, used to buff the finish.

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With considerable application of elbow grease, the body routes are clean and smooth. The black paint is electrically conductive and helps a bit to shield the electronics. But we can do better.

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For some reason, I always start the copper foiling process in the output jack route. Note the tabs that will later connect to the foil on the underside of the pick guard.

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The body cavity routes are foiled at last!

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Turning our attention to the electronics, we remove the nylon tie wrap which bundles all the wiring under the pick guard.

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The bridge and neck pickups from Squire are exactly the same, and have white wire insulation for the signal. The center pickup wire has yellow insulation.

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The Squire pickups are removed and bagged. Perhaps someone can use some new Squire Strat pickups in their FrankenStrat Build.

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This aluminum foil sheet will be replaced with copper tape. I have no idea what the “H” stands for. Is it an “I”? Or The One?

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We can still see the original protective sheet that covered the pick guard under the knobs.

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This aluminum sheet is easily removed with a fingernail.

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To save my fingernail, a tooth brush handle is pressed into service as a scraper. This foil is mostly paper, with an inconsequential layer of aluminum over it.

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The remaining tape adhesive is dissolved. This chemical wash treatment will leave a surface on this pick guard to which the copper tape will securely adhere.

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The pick guard gets the copper foil treatment. The holes in the pick guard are cleared of copper, and the strips of copper are tack-soldered together to form one continuous shield.

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To remove the pickups from the pick guard, the wires that were soldered to the switch are clipped. Then the remaining wire ends are de-soldered. However, this switch was broken (probably at the factory) and the terminal was broken off. The wire was soldered directly to the printed circuit board upon which the switch is built.

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Enough of the circuit board trace remains so that a solder connection can be made.

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The new Texas Special pickups are the correct size. However, the holes in the Squire Strat pick guard are sized for the more diminutive Squire Strat pickups. So, a little gun-smithing is in order.

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The pickups are installed and the pickup leads are dressed where they belong. We are ready for final wiring.

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Before we solder the pickup leads in place, let’s just see how these Texas Special pickups ohm out. The display on the ohmmeter is a little hard to read, but the neck pickup measures 7.02k ohms.

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The resistance of the middle pickup is 7.22k ohms.

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This is the Texas Special bridge pickup resistance.

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The jack is oriented in such a way that the contacts and the wiring cannot touch the copper foil shielding. If it does, the instrument can go mute.

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The wiring is fished back through the drilled holes in the body.

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Finally, my OCD itch is scratched. The claw ground wire is underneath ALL of the springs. This guitar is to be set up as a ‘hard tail’ e.g. the bridge is not floating but is pulled into a stationary position by spring tension.

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Here’s one last look at the electronics before the guitar goes back together.

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This finish and pick guard are just spectacular!

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With the strings off, it’s the perfect time to condition the fret board.

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Restring, setup, bring the pickup heights back to where they belong, and this beautiful guitar is ready to play!

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Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Fender Blues Junior Puts On A Light Show

The Unbrokenstring Crew is amazed at the tough life that this tweed Fender Blues Junior has endured. Yes, it doesn’t work at all. Can we bring this poor thing back to life?

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Brian salvaged this amp from the curb in front of a house in North Carolina while volunteering in the cleanup following Hurricane Florence in 2018.

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Water damage is clearly evident on the tweed fabric, with stains and mold inside and out. The glue holding the fabric on the amp has failed, particularly on the bottom half of the cabinet.

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Starting from the bottom up, we use hide glue to stick everything back down. The lacquer coating on the tweed fabric has saved it from completely disintegrating.

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We are employing hide glue because it is not water based; we don’t want to make the wood cabinet swell any more than it already has. The hide glue can be easily cleaned up afterward, even after it dries, with warm water and a rag.

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Here, we’re removing the chassis. Fortunately, the rust is not too bad on this chassis.

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Someone has been here before, and they probably didn’t have a Fender employee badge.

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Too much heat and rework has destroyed the plated-thru holes in the circuit board. We can repair this.

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The connections (called ‘nets’ in circuit board parlance) are restored with small bits of stranded copper wire, tinned and soldered in place.

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The heart of any tube amplifier is the output transformer. It bridges the gap between high voltage power, tubes, and the loudspeaker. This HiPot (high potential tester) is measuring a complete failure of the insulation between the primary plate circuit windings of the output transformer and the secondary loudspeaker windings. Surprisingly, the loudspeaker is fine!

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Hidden on the back side of the chassis, the output transformer has lived. And Died. Alone. In The Dark.

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Fortunately, The Unbrokenstring Crew has a supply of original parts for boutique Fender amplifiers and clones, from Texas Amplification stock. This nice example of original Fender iron fits perfectly on this chassis.

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Testing the 6BQ5 / EL84 tubes, on the other hand, produces a light show. The purple glow is ionized gas inside the tube, and the blue lights hitting the paper behind the tube are beams of uncontrolled electrons.

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The red filaments are the only colors that should be there. After these pictures were taken, I had to replace the socket adapter on my TV-7U tester because it melted internally. The rest of the tester is fine and was re-calibrated – with a new socket adapter.

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After the light show from testing the tubes, each section of the amplifier is tested separately, in order to discover any other collateral damage from either the water or the failed output transformer. This amp will be Good To Go once the glue dries!

.Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

MusicMan RD50 Combo Amp Repair and Inspection

AJ played this wonderful MusicMan combo amp, until it quit suddenly. He was aware of the Big Names in the music business that repaired these, but was there anyone local? D’oh!

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As you might expect, this combo has one 12 inch loudspeaker and has a fifty watt Class AB push-pull pair of 6L6 tubes. One vacuum tube serves as a preamp, and the rest of the amp is built with solid state techniques.

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This looks a lot like a Fender amp, doesn’t it? Leo Fender had sold the Fender company to CBS, but wanted to continue making instruments and amplifiers despite a non-compete agreement that he was required to sign as part of the deal with CBS.

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So the MusicMan amps were born. The Mid-shift switch indicates a slightly different tone stack design.

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IIRC, production was shifting away from the Fullerton factory to Anaheim, with offices in La Brea California.

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The rocker switch for the ground select function and a three wire power cord is evidence that the older design of Fender amps was changing to meet modern regulatory requirements.

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Of course, model numbers were entirely new, and serial numbers had little resemblance to the old way of doing things.

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I have no idea what the paper label to the right is for. Any ideas, anyone?

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The chassis is sound, if not a little cosmetically ‘challenged.’ We can blame the humid Gulf Coast environment.

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Another change from the Old Fender was this pilot light, which consists of a neon bulb and a limiting resistor. The package is held in with a push nut.

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Yet another thing we don’t see here is the iconic brass sheet upon which most of Fender’s controls and jacks are traditionally mounted.

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The wire harness dress is excellent as is the workmanship. The black switch is the Tone Shift switch seen earlier on the front panel.

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These front panel controls work smoothly and are noise-free.

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The Bright/Normal switch is found next to the input jack.

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The dual-section 12AX7 lives here. This amp has been re-capped, including cathode bypass capacitors and all electrolytics. I’m not touching any of this!

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The rear panel jacks are Switchcraft, the best you can get.

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This ‘death’ cap is original. The red paint on the solder joints is an interesting way to indicate that they passed QC. This makes it easier to see where past rework/repairs have been done.

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The fuse holder held a too-high value 32v automotive fuse. The correct 250vac 3A part is installed.

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Most of the preamp duties are done with operational amplifiers. Those connectors in a square configuration are for the reverb tank and pedals.

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This unit is very well built and maintained.

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Can you see the problem? The pair of transistors on the pink heat sink form a phase inverter that drives the output tubes. In the 1970s, televisions, ham radio gear, and other consumer electronics were commonly built using ‘hybrid’ techniques e.g. solid state parts with power tubes. Leo Fender knew his TV stuff, and applied that technique in his new line of amps.

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These high powered resistors are part of the phase inverter circuit. They must be matched closely for good performance. Obviously, these are no longer matched.

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The devices on the heat sink are 75 watt 15 ampere 80 volt power transistors. They should be closely matched for best sonic performance. Also, transistors will drive the next (tube) stage with a bigger voltage swing than two sections of a vacuum tube, because they are inherently lower impedance.

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The main board is coming out of the chassis so that we can solder and desolder parts.

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In the late 1070s, circuit board design was performed on computers. Thus, graphical images could be added to the artwork. Also, this circuit board is electrochemically plated tin, which is a fresh new technology not previously seen in Fender products.

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On a lark, we will measure the value of the remaining 6.8 ohm resistor.

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It reads a little high. No problem. The resistors will be replaced with a matched pair.

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The old transistors are coming out for testing.

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The transistor curve tracer shows that this part is good.

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The other part is shorted internally.

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The heat sink is removed and the old resistors are desoldered. Here, we’re cleaning up the circuit board where the resistors go.

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These two parts were sourced from new stock and selected because their value matches better than 0.1%.

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These transistors were sourced from new stock and were matched on the curve tracer. See the new resistors above and to the left?

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The solid state phase splitter drives the tube stage as it should.

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The tubes are in and it’s time to fire it up!

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This amp comes with the official MusicMan pedal, controlling reverb and distortion.

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When a function is selected, the LED comes on. This is nice on a dark stage!

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This unit is ready to go again!

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Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Sears Silvertone 1484 Amp Head Refurbishment

Andy found a ‘beater’ Silvertone amp on eBay and was curious if it could be restored. At first look, it was pretty rough. But could the Unbrokenstring Crew work some magic to undo the damage caused by UPS and previous amp techs?

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Danelectro built these amps for mail order distribution. In the 1960s, Amazon was a river in Brazil and the Internet was a military thing. But the Sears Catalog brought you nearly anything you wanted.

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The Sears catalog number was the URL of merchandise. With that number, the world was your oyster.

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The split splined shaft on the reverb was broken, and the original knob was long gone. The control does work electrically, but the reverb function was not functioning. Or something like that.

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The main volume knob was missing, because the split shaft was compressed and didn’t have enough remaining ‘bite’ to retain the knob. This could be repaired.

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One of the preamp tubes is a Chinese 12AX7. That tube tested bad.

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However, the rest of the tubes were in fine shape and were kept in service. By the looks of the power transformer, I believe that this amp was dropped on its end, because the transformer is leaning to the right.

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One of the preamp tubes is original, a Silvertone 6FQ7, made in U.S.A.

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The chassis is hand-wired. We are starting at the preamp section. Those brown capacitors are mica capacitors. Wherever two of them are next to each other, they are taped to each other using black electrical tape.

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The pilot light is good. The red and blue wires to the right are all shielded signal cables. Power wiring is on the left.

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The transformer on the left is an interstage coupling transformer. Masonite, a pressed fiber board material, is used extensively in this unit. Here a big chunk runs right down the center of the amp chassis, and many terminal strips are riveted to the Masonite for mounting components.

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The red cylinders are the filter capacitors, and the small silver cylinders are the power rectifiers. Amazingly, the filter capacitors required almost no reforming. Normally, capacitors this old are replaced out-of-hand, but the owner preferred to keep it as original as possible.

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Each and every capacitor was screened for leakage at working voltage and capacitance value. Do you see the scorch mark?

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At this end, we can barely see the output transformer, which has been replaced with a service spare part. Apparently the output transformer failed at some point, leaving some scorched areas behind.

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These blue wires come from the non-functional reverb tank.

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These wires come from the other end of the reverb tank. So we can guess that the reverb tank failed, and some Jake Leg tech just cut it out of the circuit rather than fix it.

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So, we will fix it. Note the duct tape holding the whole arrangement together.

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We find more Masonite under the tape. Each end of the spring in the reverb tank is stretched between the end clips.

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A second point of contact intersects the spring about an inch away from the end. Note that this contact is bent. This is another indication that the amp was dropped on its end.

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In the middle of the spring, this wire guys the spring at the center of the tank.

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So we bend the bent contact so that it pokes into the spring where it belongs. A very high voltage signal, over one hundred volts, is applied to the contacts at one end. The insulating cardboard on each contact keeps the current from flowing through the spring. However, the high electric field induces a mechanical motion into the spring, which is carried through the spring and wiggles the two contacts at the other end. The wiggling contacts act as a variable capacitor. The change in capacitance causes a varying voltage to be produced, which is amplified and sent to the amp. A moving ribbon microphone or condenser microphone works the same way.

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Simple enough, huh? Speaking of simple, we have Upgraded this reverb tank from duct tape to wire ties. Which is kind of a big deal, if you ask me.

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The reverb tank lives here. But the chassis is filthy. Now is the time to clean it all up.

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A rag is soaked with a combination of solder flux remover and furniture polish, which is tough enough to cut through six decades of crud.

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The reverb tank is suspended over the chassis with this bracket. It just bolts on.

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A replacement control has arrived. These stamped sheet metal nuts are used to keep the controls in place. The controls are all recessed behind a trim panel. The recessed trim panel makes it a challenge to find knobs that will work on this amp, as we will find out later.

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Here, the replacement control has been lubricated and will be wired into its new home.

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The reverb control has an ON/OFF switch that literally disconnects the reverb tank from the rest of the amp. The two terminals on the back of this control are the ON/OFF switch terminals.

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That sheet metal nut will not take much torque, so it is being tightened by hand.

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Next, we will replace this two wire line cord with a three wire cord, so no one will be electrocuted if the power transformer insulation fails.

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Two of the line cord wires go to this convenience outlet. The outlet will remain in place, but will be removed from the circuit.

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The power switch wiring is being modified to switch only the ‘hot’ wire. The neutral and ground wires will not be switched, per UL requirements. Also, the ‘death’ cap will be removed from the circuit, so that, when it fails shorted, raw 120VAC will not be connected to the chassis. You will find that important if you are holding the guitar at that moment…

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Let’s see what we can do about wiring in the reverb tank. These blue twisted wires are snaked through the chassis and will be attached to the terminal strip in this picture. Here is another good look at a pair of mica capacitors taped together.

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The little terminal strip in the middle of the picture has the remnants of the original blue reverb tank wires left from where the previous tech disconnected the reverb tank. So now we know right where to reattach them.

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Another reason to love eBay is that, from time to time, the Correct knobs can be located. These were surprisingly affordable. Some of the original knobs were repaired with Super Glue and reinstalled.

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The unit is back together and ready for final test!

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Finally, the Correct knobs really improve the cosmetics of this fine old amp head. The owner wanted to leave the film of rust on the front of the unit, for that Vintage Mojo look. This head sounds fantastic, but the piezo electric reverb tank sounds like something from a Star Trek special effects soundtrack, which, might be just the sound you want!

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Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626