Unbroken String - Guitar, Amp & Music Instruments Repair

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

Peavey XR500 Powered Mixer Repair

June 12, 2023October 4, 2016 by latchaw

Peavey powered mixers consist of a front panel with controls and a rear panel with the amplifier. The amplifier will generally have a different part number entirely, which you will need to know in order to find the correct schematic.

We have five input channels. Each channel has a treble and bass control and a gain control. A bit of that channel may be added to the effects processor input. A bit of that channel may be sent to a monitor output.

This mixer has a reverb function and a simple equalizer. The main, effects, and monitor outputs are line level.

The main output is amplified and available on the rear panel as shown.

This information pertains to the amplifier section, not necessarily to the mixer.

The front panel nomenclature is used on this tag.

The output transistors are mounted on the aluminum heat sink, coupled to the back plate of the mixer enclosure.

The power supply is a simple linear supply with three terminal regulators for powering the mixer and effects. That big brown cube-looking thing is a choke, wound on a transformer bobbin and supported entirely by the large wires of the winding itself. Not a bad idea for an inexpensive choke.

On larger powered mixers, the power transformer is mounted in the enclosure, but here, it is fastened to the rear panel.

The white diode is shorted. This is a protection diode, often seen on high voltage power supplies, to clip high voltage transients which may occur with loudspeaker failures (or if someone disconnects the loudspeakers while the unit is running, which I suspect is what happened here.)

Shorted diode is gone. Both protection diodes were replaced.

This resistor and surrounding capacitors make up what is called a Zobel network. This network is intended to neurtalize the effects of the inductance in the loudspeaker voice coil from the amplifier output terminal. When the white clamping diode shorted, this network was exposed to full high voltage, with the effects you see here.

Amazingly, the capacitors were fine even at their rated voltage, so only the resistor required replacement.

Full power testing went well. However, the power transistors themselves became very hot.

The thermal compound and spacers under each transistor were renewed and the transistors re-torqued. That black device with the brown wires coming from the top is a thermal relay that open circuits the power to the amplifier if it senses that the heat sink temperature is too high.

Everything is fine and this unit is ready to return to service!

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

Boosta-Grande Pedal Repair

October 27, 2016September 20, 2016 by latchaw

We did a quick-and-easy foot switch repair on this gain pedal. Here’s the scoop:

The operation of this pedal was intermittent. The switch didn’t click anymore. Yup. Probably just needs a foot switch.

The foot switch is on its own circuit board. This switch is a latching, 3 pole double throw unit. A ribbon cable carries all the lines between the terminals of the switch and the rest of the electronics.