Fender | Unbroken String

The Korean Squire ‘Blackout Strat’ Build

January 13, 2019 by latchaw

Matt’s unicorn was to again own a Blackout Strat like the one from his youth. Could the Unbrokenstring Crew make an equal-or-better unicorn?
The body is a Korean Fender Strat with genuine wear. The new neck is in superb shape. This will be the basis for a cool project, particularly since modern parts are available to upgrade this guitar.

The neck comes off and will be updated separately. Say ‘Goodbye’ to the Squire neck plate.

The dot inlays in this neck are really spectacular.

The pictures do not do justice to the spectacular mother of pearl in this fret board.

First, we’ll clean up a couple of handling skuffs that occurred while this project was coming together.

Tinted polyurethane, various grades of fine sandpaper, and a buff polish gets us back to where we belong.

A set of vintage tuners will be fitted to this head. Here, we establishing the center line of the tuners.

Screws for the tuners will go where the scratches cross.

The pin vise is pressed into service to bore the screw holes.

This drill bit will be used as a gauge to verify the diameter of the tuner holes in the neck.

Sure enough, this neck was pre-drilled for 3/8ths inch bushings.

The bushings are fitted tightly into the neck. This is essential for good tone, as the bushing supports the capstan, which is one end of the support structure that establishes string tension. If these are loose, your strings won’t work very well.

Turning our attention to the body, we find that the ground wire was not properly soldered to the string claw.

The interior routes of the body were painted with a conductive paint. This just won’t do for the Unbrokenstring crew. This screw ties the conductive paint to the rest of the ground circuit. We can do better.

The output jack is liberated from the stamped ‘football’ socket plate. We will rework the wiring with heat shrink support in order to support the wires and make it more durable. And the Unbrokenstring Crew will redo the soldering job, because We Own It if it fails.

The neck pocket must be square and clean. Any debris or finish will interfere with the transfer of mechanical vibrations from the neck. For maximum sustain, we need to take this pocket down to the bare wood.

This side view shows that the corners are square and clean.

Copper foil will be used to create the cavity shielding. We’re starting with the hard stuff, the output jack route.

Bottom and sides are done.

All of the interior routes are lined with copper. The seams are tacked together with solder.

This is the bottom side of the new pick guard, also made in Korea. The aluminum foil must go.

All of the holes will be de-burred with this tool.

We will have a nice flat surface to which the copper foil will adhere without voids.

Foiling the pick guard takes just a few minutes! Again, the seams are tacked together with solder to form a plane.

The foil is trimmed away from the edges with an Exacto knife.

This build will use these pickups.

This set will be a very cool foundation for this instrument.

The controls are mounted right to the copper. True to Fender specs, we are using surgical tubing for pickup springs.

The pick guard assembly is done, complete with Orange Drop tone cap.

Here is another view. All that is missing is the output jack wires and the bridge ground wire.

Here, the body is going together. The output jack wiring and bridge ground wire is routed through the body and soldered to the pick guard assembly.

This workmanship turned out pretty. New Old Stuff (NOS) push-back wire insulation is used to complete the vibe.

Everything fits!

The neck gets a little prep before the strings are installed.

Remember the Squire neck plate? This is the replacement.

Meet the strings!

This new neck has a new nut, which needs to be slotted and filed. String spacing is established here.

Here, the slots are taken to the proper depth, calculated beforehand. The stackup of feeler gauge blades establishes the bottom of all of the string slots. When each file touches the feeler gauge blades, we’re done.

The string slot depth is where we want it. Next is to file off the top of the nut and polish it, which has been shown in other blog posts. When this is finished, the strings will protrude just above the top of the nut.

Now this is beginning to look like a guitar. This is set up as a ‘hard tail’ so no tremolo bar is needed, but one is supplied.

We lost Matt somewhere in the bowels of Guitar Center.

This message came into the Unbrokenstring Global Command Center after this guitar made it back to the rehearsal room:

“BTW been meaning to tell you, OUTSTANDING job you did with the recreation of Crow’s Fender Blackout Strat!

“I think can say, without any hyperbole whatsoever, that just plugged straight into an amp with no tweaking whatsoever, that is the BEST SOUNDING GUITAR I HAVE EVER HEARD IN MY LIFE.

“Completely outshines even the original Blackout he was trying to unbury.

“We’re still absolutely dumbstruck by clarity and full tonal range of it. Truly amazing work, sir!

“Without question, your finest creation to date.”

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Intermittent Fender Acoustasonic 150

December 23, 2018November 26, 2018 by latchaw

A fellow musician gave Charles this amp, which was nice gesture. However, the friend said that it was intermittent. Could the Unbrokenstring Crew turn this gesture into a reliable amp?

Styled as a unit from the Fender Brownface era, the exterior certainly checks all the boxes for Brownface goodness, with the Correct knobs and silk-screened front panel true to the archetype. One channel is reserved for an instrument, and the other channel is tailored to vocal performance, including a dual XLR/quarter inch jack for a microphone.

No metal shredders allowed. This unit has a tweeter, and an electronic gain structure that does not distort. Just the thing for Charles’ acoustic act. This badge still has the protective plastic in place.

The speaker cabinet is sealed. This polarized connector keeps three pairs of audio signals from the amp going to the correct loudspeaker and tweeter.

On the back side, we find the ON/OFF switch and the IEC power socket. Most of the rear panel is slotted for ventilation.

This is a solid state unit, with plenty of pep to be loud.

The internal architecture permits stereo operation, as is shown by the FX loop connections. I did not play around with the USB functionality, but it’s in the manual. We have bigger fish to fry.

Name, rank, and serial number, please.

This circuit board holds all the connectors for Line Out and effects loop functionality, as seen on the rear panel.

This assembly is an AC to DC power supply on the left, and an efficient Class D audio amplifier on the right.

Digital signal processing (DSP) is used to create the reverb and other effects. The DSP functions are on the mezzanine board on the left. The thin white cable in the center is the USB cable. The main printed circuit board handles the clean audio chain and the connections to the front panel controls. The flat cable on the right brings power from the AC to DC board and sends audio to the amplifier.

A lot of surface-mount components are found in this unit. Those little cans are electrolytic capacitors; black squares are integrated circuits. Each of those little black squares does the job of two vacuum tubes. I feel old and obsolete.

The check mark probably means that someone tested this at the factory, I guess.

So the audio processing hardware is seen at the top of the picture and the power stuff is at the bottom.

The AC wiring comes from the switch directly to the circuit board, where there are filters and a fuse.

This power stuff is actually a switching power supply, which efficiently creates the various operating voltages.

If you look closely at the gold rings on the circuit board, you will see solder that looks ‘strange.’ It does. Gold atoms mix into the molten tin/lead alloy while the solder joint is in the liquid state. The gold makes the solder brittle.

The entire circuit board is gold plated. This plating is among the flattest finishes available for bare circuit boards, perfect for surface mount technology (SMT) components but is a metallurgical compromise for thru-hole components..

As you can see, for thru-hole technology components such as these pins sticking through the board, the results of the soldering action can leave something to be desired. Do you see the holes in the solder joints?

Now that those holes are fixed, we can focus on the real source of the intermittent operation. Do you see that light blue resistor with two red stripes hiding behind the capacitor and the heat sink?

That light blue resistor was soldered here. Or to be more precise, it was soldered there at one time. The cracked solder joints became intermittent conductors. Here I have removed the resistor and cleaned away the old solder in preparation for making a new pair of solder joints, free of gold contamination.

Another issue with this amp is that someone has been playing with the loudspeakers.

This loudspeaker fits the cabinet perfectly, but electrically, it is a 40 ohm (yes, forty ohm) loudspeaker, designed for use in a public address paging system (you know, that mess that you hear at the doctor’s office playing MUZAK, mercifully interrupted by an announcement for someone to call a telephone extension? Yeah, that.)

The Correct part is available.

Who would have thought that you could actually replace a bad loudspeaker with a new one of the correct type?

Do you like those TV shows where they have a build-up to the ‘Big Reveal’? I don’t either.

Fortunately, we have the correct part and are ready to install it.

Once we replace the grille, you will never know the difference.

See, I told you that you couldn’t tell the difference. This unit plays beautiful music and the functionality is solid.

Support this musician, winner of a Texas Music Magazine 2018 Album of the Year: http://www.charlesbryantmusic.com/

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Fender Rumble 350 Bass Combo Repair and Modification

December 20, 2017 by latchaw

I can’t help but think that this is not a real Fender, but “Fender Bass Amplification” is the nameplate on this unit, not the cursive Fender logo seen for decades. This unit is very versatile, works better than most bass combos, but just screams “Designed and Manufactured In China.”

A lot of ‘motor noise’ came from one of these loudspeakers in this Fender Rumble 350 bass combo amp. The owner said, while we were at it, could we wire this unit so that it could be used as an external cab, driven by another unit? The UnbrokenString Crew said, ‘Sure! Why not?’

If you listen closely, you can hear the warped voice coil rubbing against the magnet gap inside the bad loudspeaker.

Access to the inside of the cabinet is accomplished by removing the loudspeakers. We are using an electric screwdriver to drill a pilot hole for a switching Neutrik connector that will allow this unit to be used as an external cabinet.

This Forstner bit is just the right size to clear the body of the connector.

Using the pilot hole, we can cleanly cut through the Tolex and into the wood cabinet. The scratches in the Tolex were not part of this project.

Yes, this looks like a hole to me.

We will replace both Chinese loudspeakers with a matching pair of 200 watt ea. bass guitar loudspeakers. The original loudspeakers were rated for 75 watts each, which is strange considering that they were tied to a 350 watt amplifier. The bass loudspeakers have a different hole pattern, so we are drilling new locations for the Tee nuts.

These Tee nuts have little barbs that help keep them in place.

I am using this clamp to squeeze the Tee nuts into the drilled holes in the baffle.

Now we can begin wiring this unit up. The black and white pair connect the two loudspeakers in parallel.

The red and black wires come from the power amp in this unit. This Neutrik connector will disconnect the power amp when an external amp is connected to the cabinet at this port. The soldering is done outside the unit. BTW this is a Neutrik NL4MD-V-S. The mating connector, an NL4FX, was supplied to the customer for his own wiring.

That looks pretty nice, in spite of the marks in the Tolex.

The new loudspeakers are in place and this unit is ready to test!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Fender Rhodes Electric Piano Amp Refurbishment

June 19, 2017 by latchaw

A Southeast Texas area church had a wonderful Rhodes Piano that needed some attention. Whenever the unit was powered on, a loud hum was all that came through the speakers. Time for the UnbrokenString Crew to go to work!

The Fender Rhodes Piano consists of a keyboard section, containing keys, action, and tuned rods that work in the same manner as tuning forks. The rods vibrate when struck, and the motion is sensed by a coil not unlike a guitar pickup. From there, the signal is sent to the other section, a powered stereo loudspeaker assembly seen here.

The speaker cabinet is two-faced e.g. loudspeakers fire from both the player’s side and the audience’s side. A pair of loudspeakers are assigned to each output of a stereo amplifier. The pairs of loudspeakers are across from each other in the cabinet, one firing forward and the other one back. This enhances the swirly, phased sound of the instrument.

These loudspeakers are Fender branded CTS units. The metal box in the background contains power and input circuitry.

CTS built these loudspeakers in June of 1975.

This part number indicates that these are 32 ohm AlNiCo loudspeakers. This is a standard-issue Rhodes Piano unit.

This voice coil is totally cooked. The motor drags badly in the magnet.

This voice coil is open-circuit but moves smoothly in the magnet. Is a repair possible?

There is the broken voice coil wire. This wire is really cooked, so we will elect to replace the loudspeaker with a pair of modern 8 ohm units wired in series, to yield the proper 16 ohm load to the amplifier.

We have removed the panel at the end of the cabinet. The power transformer is visible to the right. Each channel has a separate input here. Also, a special cable from the Rhodes keyboard attaches here.

The power cord for this unit is no different than an extension cord.

Instead of a regular extension cord, we will use a SmartPower unit to power-up the unit and protect it from surges. Think ‘mini-Furman unit.’ I also sell these, BTW.

These transistors read as short circuit. I think we now know everything we need to know to make an intelligent quotation.

Name, rank, and serial number please.

One output of the power supply assembly is 25vdc for the keyboard section.

The keyboard voltage is set by a potentiometer accessible through this hole.

The power supply filter cap is in great shape for its age!

Likewise, these guys look great and test good.

Everything here is as it should be.

Steven removed one of the damaged loudspeakers.

Over the years, the gasket glued itself to the cabinet.

A little extra cleanup won’t hurt a thing.

The circuit board for the power amplifier is a hand-drawn affair, typical for the 1970s.

The board designer was nice enough to add is some text that would help the amp tech find his/her way around.

Some power resistors were burned up. All of the components to the right of the transformer were replaced. The transformer is for inter-stage coupling, not power.

Some power transistors were hand-selected for duty in this amplifier.

The repaired amplifiers are re-installed in the bottom of the cabinet.

We are ready for final test!

Amplifier design has certainly changed over the years. This is a unique design that has withstood the test of time very well! The customer was VERY pleased with the finished job. Weather Report Cover Band, anyone?

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Fender Hot Rod Deluxe Car Wreck

February 21, 2017 by latchaw

This seasoned road warrior was happily touring and now was called up for recording duty. However, at the final live set, the unit became very warm and the audio dropped way off. Could the Unbrokenstring Crew work some magic to return this guy to the studio?

This unit has been in the shop once in the last year for a defective plate load resistor in the phase inverter.

Apparently Fender received a batch of defective plate load resistors. The vendor corrected the problem, but a few of the amplifiers built with that particular batch of resistors have been in the field for years.

The circuit boards need to come loose to access everything. It turns out, the heat is coming from several sources.

The schematic is on the monitor. Unbrokenstring is trying out the Paperless Office Experience.

This is one of the open-circuited plate resistors on the phase splitter tube.

Here is an NOS carbon composition resistor from stock. It looks right at home in this amp.

The real issue today is that the capacitors have begun to vent. We need to address this with a recap.

These units are all on the same circuit and are all the same age. So they are all to be replaced.

A previous non-Unbrokenstring repair around the low voltage regulator circuits fixed some burned circuit boards.

These large resistors limit the Zener current. They are too close to the printed circuit board.

The capacitors will be replaced with units that have a higher temperature rating.

Here, the Zener diodes and power resistors are spaced above the circuit board for better ventilation.

Some amp techs touch each cap with a screwdriver to discharge them. The positive lead carries about 400vdc and represents a safety hazard. Those who ‘know’ will use a discharge wand on the tube socket or choke terminals to discharge this high voltage. For the screw driver’s sake, we’re installing Teflon tubing on the exposed high voltage points.

We are changing out the plate load resistors. All of the preamp tubes use a 100k resistor in the plate circuit. This resistor is also part of the audio path e.g. the amplified signal appears on the plate end of this resistor, so we will benefit from modern 21st century components for duty in this part of the circuit. Again, they are spaced above the circuit board to keep the body temperature (and noise) low.

More plate load resistor goodness…

The plastic input jacks are replaced with American-made Amphenol units which are bolted directly to the front panel.

The circuit boards are placed back into position and the unit is placed under test. After an hour and a half of a four hour burn-in, the amplitude dropped off again.

Bias current climbed a few milliamps and could not be put back to normal. Here is a temperature reading at the base of one of the output tubes.

This is the temperature of the other tube base. Obviously, the circuit is unbalanced now because the other tube is a lot hotter. We will check the tubes again, but something else is happening here.

Here is the temperature of one of the preamp tubes.

Surprisingly, everything returns to normal when the amp cools. The chassis is on the bench now to take a closer look. Something looks wrong with the output transformer. Running the unit on the bench while applying a heat gun to the output transformer duplicated the problem!

A new output transformer is still available from Fender! Let’s substitute the new one and restart the test.

The red and brown leads are the ends of the output transformer primary winding that are driven by the output tubes.

The red wire is the center tap, which is fed high voltage. The black wires connect the power supply choke into the circuit.

The output windings of the output transformer go here. Pay attention to the striped, unstriped, and black wires.

All of the wires to the output transformer go through this TieWrap. We’ll snip this to get it out of the way for now.

Here are the output transformer leads. Now, we unscrew the transformer from the chassis.

I compared the two transformers. The bases are lined up, but the top is misaligned. What the…?

Yes, the feet and bottom of the frame is aligned.

But the top of the lamination stack is definitely displaced on the top.

Here, the laminations are parallel, but the straight edge shows that the old transformer is definitely parallelogrammed.

A phone call to the owner reveals that this amp was in a serious car wreck about five years ago.

That wreck explains why the bottom of this amp is filled with broken safety glass. The force of the car wreck deformed the output transformer. I’m guessing that as the deformed transformer heated up, the internal insulation broke down somewhere because the windings were stretched by the deformed transformer.

Let’s bolt down the new transformer. Here, the leads are fished through the grommet in the chassis.

A replacement TieWrap is installed in place of the old one. The small transformer is the power supply choke.

More TieWraps are applied to dress the leads in a manner similar to the original.

I’m pretty confident that the unit will work fine now! Let’s put everything together, including all those knobs!

Here are the new jacks.

The new output transformer deserves a new set of tubes.

These preamp tubes were ordered at the same time.

A one ohm resistor in series with the output plate circuit allows us to directly read the idle current of the output tubes. 60mV corresponds to 60mA of bias current, exactly what Fender specifies.

Another temperature check is performed after everything warms up.

This little thermometer came from Harbor Freight. It works surprisingly well for a $20 infrared thermometer.

We are restarting the four hour test from the start.

If you look closely, you can see a blue beam of electrons flying into the tube envelope through a small aperture in the plate. These bottles are happy!

This road warrior has a new cover!

Listen to this amp in action at http://billybourbonmusic.com/ and support this artist!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Fender Twin Custom Foot Switch Repair

January 24, 2017January 9, 2017 by latchaw

Matt complained that the reverb function of his amp was erratic when he used the factory foot switch. Could the Unbrokenstring Crew look it over and find the rat?

Let’s take a tour of the unit. This one is in excellent shape. Meet the ‘normal’ channel,’ on the left side.

Effects channel is in the middle.

Everything in this amp works well. I wonder what the issue is?

Name, rank, and serial number, please!

One thing I really like about this unit is the ON and STANDBY switches sport dust boots. Good practice to keep these switches trouble-free for years to come!

The foot pedal goes here. This is a stereo jack, to support two functions. Is this the problem?

Matt supplied the foot switch. This looks as if it has never been out of the studio.

Inside the unit, we see cast frame Eminence units. Very nice!

The tube diagram is as it should be. I understand that the schematic is the same as the original Blackface Fender, only updated with modern components and largely built upon a printed circuit board. Nice stuff.

No reverb here! There is an intermittent within the pedal.

The vibrato section works fine, so we need to investigate the foot switch on the right.

Let’s verify that everything is OK here. Very clean inside!

Amazingly, the reverb foot switch does not actuate every time it’s pressed. A new unit is pulled from stock.

Here is the new reverb switch. Now, the reverb is functional, but there is another source of intermittent operation.

Aha! At the plug end, the wire insulation has pulled back, allowing the inner conductors to touch the case and each other. This is a mess!

There are actually three conductors in the factory cable; two are used for switch functionality and the third is the braid, a ‘common’ conductor for both circuits which doubles as a ground shield as well.

Here, I’m carefully pulling one conductor out of the center of the braid while leaving the braid intact.

Here are the three ‘wires’ that we need.

I pulled the braid until it was a solid conductor. This piece of clear tubing will insulate it from the other wires in case the insulation on the wires pulls away again.

This is the finished termination. I had to use a big iron on the solder joint to the outer shell, and the insulation is a little worse for wear. I’ll do better next time. A tie wrap was added on the exiting cable to help the strain relief do its job.

Everything is back together and works per spec.

Another satisfied customer!

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Peavey MaxBass 158 Combo Amp Repair

October 27, 2016 by latchaw

Replacing broken input jacks and switches are the bread-and-butter of the amp repair business. This Chinese-built practice amp needed a new input jack. The owner was a college student and didn’t have the money or space to upgrade to something bigger. Could the Unbrokenstring Crew make this unit play again?

The cabinet is a simple, sealed-back unit. The electronics chassis is accessible, but the speaker wire is threaded thru a hole best accessed from the hole where the loudspeaker goes. So, here we are taking off the grille.

As soon as the electronics chassis was slid out the front, the printed circuit board assembly came loose.

Most of the mechanical support for the circuit board is provided by the input jack, which is plastic. As a rule, I replace input jacks with all-steel Amphenol jacks. However, that won’t work here. This forces me to replace the input jack with a similar plastic unit in order to reassemble the amp back the way it was.

The old input jack is gone. Good riddance!

A new, identical jack is sourced from a commercial vendor. Now, we have a chance of a more durable assembly because the new jack is Made In USA.

When sourcing an alternative part, the electrical function must be the same. In this case, input jacks are single-pole switched jacks; the tip circuit is grounded until a plug is inserted into the jack. This keeps the amp quiet whenever nothing is attached to the input.

Likewise, the footprint of the alternative part must mate up correctly with the rest of the amp. Here, we see that the new connector pins match the circuit board exactly!

A little solder to seal the deal!

This part of the job is ready to go.

Cleanup of the unit is easier when everything is apart.

Likewise, any electrical problems can be fixed while the unit is apart. We’re checking this guy out to verify that everything works.

We have achieved success. Time to button it up.

With the chassis in place, the leads to the loudspeaker can be pulled back into the speaker box and secured. My hands were a little full, so I didn’t take any pictures while the loudspeaker was out. Sorry.

Our job here is finished.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Fender Power Chorus Combo Amp Is Intermittent

October 21, 2016 by latchaw

The lights are on but no one is home regardless of what knob we twist. Can the Unbrokenstring Crew sort this out?

More Made In U.S.A. goodness!

Why am I not surprised?

The chassis comes out the back of the cabinet. Nothing is really amiss at first glance.

Touching these components causes the audio to work properly. The problem lies somewhere in this vicinity!

All those pretty red Fender knobs come off. Fortunately, they are all the identical.

And, all those nuts come off before the circuit board can be removed from the front panel.

These screws fasten standoffs that support the rear edge of the circuit board.

It appears, at first glance, that the heat from the power resistor melted the solder at this joint. This is directly underneath the large rectangular power resistor seen in an earlier picture.

On closer inspection, we can see that the trace leading away from the component lead is cracked. I speculate that the resistor expanded at a different rate than the circuit board and cracked the copper trace. Once the copper was cracked, it became a ‘hot spot’ and accelerated the failure of this connection.

A piece of 24AWG copper bridges around the cracked trace. It will take a long time for this to crack!

At least I have some automation to put all these fasteners back where they belong.

While the unit is apart, we can take an opportunity to clean up the rest of the unit.

This unit appears to be unchanged from the first picture, except now it plays.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Fender AcoustaSonic Pro Combo Amp Refurb

October 11, 2016September 23, 2016 by latchaw

My customer found another wonderful old amp in the pawn shop. Could we refurbish this unit to its former glory?

The unit worked, but the grille cloth was dry-rotted. We also need to do something about the broken push buttons. The stereo preamp assembly lives on the left side of the faceplate, and an effects processor is found on the right side. This amplifier is an early attempt at what is called an ‘acoustic amplifier’ these days, suitable for voice and acoustic instruments.

Made in U.S.A!

Does anyone know about the ‘PATENT PENDING’ sticker that was added to the rear panel?

The badges on the front of the unit were in excellent shape.

Tiny #2 screws hold the Fender badge in place.

We need to replace the grille cloth. The badges shall be returned to their correct positions.

The rectangular badge is pretty easy to measure.

The Fender logo, however, is a little more complicated in shape.

Fortunately, we just need to be ‘close’ and the screw holes in the baffle underneath will assure correct positioning.

Velcro loops were stapled to the corners of the grille.

These loops will need to be restored to their correct position after the grille cloth is replaced.

Here you can see the position of the Velcro hooks. As an aside, this is a sealed cabinet unit. All the wiring in the speaker cabinet is done through the loudspeaker holes.

To hold the new cloth in place, we will use similarly-sized staples.

This grille cloth pattern is vintage, and is called ‘wheat’ in the catalog.

We picked up a couple yards of the new wheat material from an Internet supplier. The color match is nearly perfect.

The new grille cloth is stretched and stapled onto the original baffle.

Sure enough, the screw holes in the baffle enabled us to properly locate the Fender logo.

Likewise, we got the rectangular badge back where it belonged. The blue handled tool is a tapered punch.

We’re done here! This will be set aside for now.

The internals were removed from the amp. This front panel is a mess, with nicotine and finger oil everywhere.

A little Gibson guitar polish cuts through the crud and cleans everything up.

The broken push button switch is problematic. I salvaged some pieces from other switches, but they weren’t quite perfect.

New switches were ordered. These are dimensionally and electrically identical, and fit the PC board perfectly.

Here are the new switches going onto the PC board. These select four preset effects when pushed.

The controls on the left were all cleaned and lubricated.

The effects processor PC assembly is installed on the right side of the faceplate.

There are lots of different hardware pieces used to attach these assemblies to the amplifier chassis. A socket finger-tightens a nut which holds the effects processor in place.

This is a view of the back of the face plate. This style of electronics is SO very 1970s!

The loudspeaker and tweeter cables pass through the bottom of the chassis.

The hole to the right is where the loudspeaker and tweeter cables pass into the bottom cabinet.

I removed the loudspeakers and tweeter to clean up the cabinet. This is a Motorola tweeter. The speaker wiring is completed from the front, as this cabinet is a sealed, un-vented cabinet.

So, we pass the cable into the bottom section of the box. When the wiring is squared away, this hole will be filled with RTV to re-seal the cabinet.

The chassis is complete. These big screws hold the chassis in the amp cabinet.

The customer wanted a foot switch. A four-button foot switch was the original Fender accessory. However, those are rare. This three-button foot switch will work well enough for the customer to select three of the four presets from the effects processor. These switches are wired in parallel with those buttons we replaced earlier.

The original wiring was intended to control a synthesizer. Out it comes!

Everything is cleaned up and desoldered. The momentary switches were just right for this application.

The bottom of the switch box will be modified for a DIN5 connector, which is compatible with the original amplifier foot switch connector on the back of the amp.

Here is everything running. This is a pretty nice unit, which represented the state of the art in acoustic amps in its day.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626