Ibanez TubeScreamer Pedal Repair

Steve said that his Keeley-modded Tube Screamer pedal had quit.  Could the Unbrokenstring Crew bring it back to life?

This pedal is in great shape cosmetically!

Robert Keeley made a few circuit changes that are well-documented on the Web.  This pedal has those changes.

The battery box has its own lid.

The bottom of the pedal is shielded with the metal screen seen in the center.

A die cut sheet of black insulative material protects the circuit board from the metal shield.

Let’s get it all out of the enclosure so that we can work on it.

We’ve applied battery power and connected up to my Marshall Stack.  The original problem is still with us.

An oscilloscope probe verifies that the dual operational amplifier is defective.  Part of the Keeley mod specifies a Texas Instruments 4558 dual op amp.  This IC is socketed, so replacing it is trivial.

Here is an example of the softly clipped audio that the Tube Screamer delivers to the amp input.

Reversing the disassembly procedure is all we need to do to get this cool little pedal back together again.  We’re done.

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE

JBL SoundFactor SF12M Loudspeaker Repair

Sal is a busy DJ who had to use his backup system after one of his JBL speaker cabinets quit. Can the Unbrokenstring Crew help get him back on the air?

This loudspeaker has been a reliable performer but had lost its high-end, then quit entirely.

So we found this inductor inside the cabinet, probably part of the crossover.  This is the ‘quit entirely’ issue.

The grille and loudspeakers need to come out to get access to everything.

This horn driver is Made In The USA!

This pic documents the wiring polarity.  One yellow wire is solid and the other yellow wire has a black stripe.

All we are measuring across the horn terminals is the crossover impedance.  This is the ‘lost its high end” problem.

This speaker is also Made In USA!

This is a pic to document the loudspeaker wiring.  One of the green wires has a stripe.

This loudspeaker seems to be OK.  No dragging or other issues were noted.

The loudspeaker wiring was bundled up to make it more compact…

so that we could remove the crossover network and repair the missing inductor issue.  This is a simple soldering repair and has been covered elsewhere in the blog.

The date of manufacture of this assembly is stamped inside the unit on the bottom of the cabinet.

Some of these horn drivers are repairable by the end user.  Let’s see what’s involved with a repair.

The threaded horn adapter comes off when four screws are removed.

The voice coil can be removed from the magnet as shown.  This voice coil assembly is widely available from many Internet vendors, Amazon, and eBay.  This is a repair that you can do yourself if you are handy with hand tools.

I searched for this JBL assembly number and found the whole assembly on sale for about the same price as the voice coil.  And, it comes with a warranty!

This horn driver has a higher power rating than the original OEM part.  These are made in Brazil now.

Oh, and did I mention that it came with a warranty?

This dust cap keeps junk out of the interior diaphragm of the horn driver.

The new driver even looks cool.  Too bad it’s hidden inside the loudspeaker cabinet.

The assembly fits in the loudspeaker without modification and is compatible with the existing wiring harness.  The thread pattern is compatible with the horn lens.  It just screws on by hand without tools.  What’s not to like?


Another happy customer!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE

Luna Gypsy Spalted Acoustic Guitar Needs Fretwork

Sophia of “Pretty In Punk” purchased this guitar new, but it was almost unplayable. Could the Unbrokenstring Crew get this beautiful instrument in shape?

Nearly every fret was higher or lower than the one next to it.  To mask the problem, the action was very high.

We straightened the neck itself so that it was absolutely flat, then used the Absolutely Flat sander across the frets.

Here, you can see the large amount of material removed from one fret, but not the others.

More material removed from the high frets.

Oh, look, here is another high fret.

Here, three frets in a row were high.

Getting close to the sound board, we’re running out of high frets to sand.

After sanding, we marked the top edge of the frets so we don’t take any more material from them.

This nifty fret file works only on the sides of the fret to round them over.  This file was reviewed in an earlier blog post.

Now, we’re getting somewhere.  This fret board is flat and the frets are even.

Sophia prefers these strings.

If you look closely, some of the over-wound strings at the end almost crested the top of the saddle.  If they get too close, I have some washers that slip over the string and sit on the ball end.  The string would then be passed thru the hole in the bridge from the inside of the guitar.  However, when tuned to standard pitch, we had no trouble here.  Missed It By That Much!


I think she’s happy with the results!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE

Peavey MaxBass 158 Combo Amp Repair

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

Fender Power Chorus Combo Amp Is Intermittent

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

Peavey XR500 Powered Mixer Repair

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