Fender

Fender Twin Custom Foot Switch Repair

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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

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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

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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

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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