Acoustic B20 Bass Combo Amp Repair

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Acoustic is an in-house brand name, sold by a national music store chain.  No service information is available, as these are built in China and are intended to be disposable, not service-able.  Could the Unbrokenstring Crew fix this one?

This combo bass amp is dead.  We will tear it down to see how it works, and perhaps fix it if we find something wrong.

Name, rank, and serial number, please!

The grille covering the loudspeaker is held in place with Velcro hook and loop fasteners.  Off it comes!

Removing the loudspeaker is a good way to gain access to the interior of a combo amp.  This pic documents where the wires go.

The loudspeaker leads are secured with hot glue.  This also keeps the cabinet sealed, acoustically.

This picture documents where the wiring goes.  The red and yellow wiring is AC from the power transformer.  The black and white wires go to the loudspeaker.

The knobs and other hardware has been removed, allowing us to remove the circuit board.

This guy gets very hot.  This is the power amplifier device for this amplifier.

This device is a TDA2050, commonly found in small, active loudspeakers and in other consumer applications.  This circuit appears to be a copy of the circuit appearing in the Application Note section of the data sheet.  Who needs a schematic?

To put the least stress on the circuit board, the bad amplifier device is unbolted from the heat sink.  Then, the leads of the device are individually cut at an angle  The device is then removed and the holes in the circuit board are cleared of solder (and of the old legs.)

The old device is gone.  We will add a little white heat sink grease when we install the new device.

Here is the new device, with some of the mounting hardware.  This heat sink compound is the same stuff used for years for power transistors and processor chips in personal computers.  A small tube like this will last a long time.

This doesn’t have to be very neat.  But coverage needs to be complete.

The legs of the new device are threaded into the holes in the printed circuit board.

The insulating mica washer is slipped behind the device, and the device is bolted in place.

A rectifier had become very hot.  This is the forward resistance reading…

And this is the reverse resistance reading.  One ea. new rectifier is now installed in place of this one.

These new capacitors take the place of two radial-lead components.  These axial leads fit well into the other footprints.

These capacitor leads and the power amplifier device leads are now soldered and trimmed.

The heat sink is bolted back down to the chassis.  These fasteners will have some thread-locker applied to keep them from coming off.  But, I cannot find the second hex nut.  I had this threaded spacer, left from another project, that has the same thread pattern, so I’ll use it here.  Hey, the additional metal will just add thermal mass to the heat sink.

This variety of thread-locker is thin enough to weep into the threads of “pre-assembled” hardware.

It’s time to reinstall all the front panel hardware, which also mechanically supports the front of the main circuit board.

The ground lead goes here.  The unit is really noisy if this is left off.  Don’t ask me how I know this.

The wires for the loudspeaker are fished through this grommet in the bottom of the chassis.

After looking over everything, I decided that the new capacitors would be more mechanically stable if they were glued to each other and to the circuit board.

While I’m thinking about it, I’ll put those knobs back on.

The loudspeaker wires are now fed through the hole in the cabinet as the electrical chassis is slid back into place.

The chassis is secured with four of these machine screws.

When we started this project, the speaker wires were sealed in place where they entered the cabinet.

So, we’ll seal them in place, and seal the cabinet, with this clear RTV.

This looks a whole lot like how we started.  This unit works very nicely, now!

Oops.  I found that missing hex nut.  It was stuck in the socket.  Perhaps I can use it on the next project!

Thanks for reading all the way through!

CONTACT – David Latchaw EE
281-636-8626

DigiTech Whammy Pedal Checkout

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This pedal wouldn’t work regardless of how it was connected. He tried the supplied power supply and a pedal power unit in his pedal board, with no luck.  Could the Unbrokenstring Crew straighten out this mess?

We put the pedal and the power unit (wall wart) on the bench and verified that nothing worked.

Here is the clue.  The customer provided a DC 9v output wall wart, a power standard which is commonly used in the world of guitar effects pedals.  That power standard is not compatible with this unit.

Not much else to see around the rest of the pedal.  The MIDI control is a nice touch.

This is the ‘correct’ wall wart.  This one says 9vac output, which is what this pedal needs.  I love eBay!

Something is rattling inside this unit.  Removing these Allen head cap screws will allow us to open the case.

This is what is rattling.

And here is where it goes.  This hardware works well, so we can set the switch aside for now.

The electrical connection between the circuit board ground and the chassis is made via this metal stand-off.  This connection was intermittent until we took the wire brush to the end of the stand-off.

The light spot on this circuit board is called ‘measling.’  This indicates that something got very hot.

This inductor goes between the power supply and the chassis ground of something plugged into the pedal.  So, it failed.  The head of the destruction caused the measling seen above.

Here is another view.  This inductor serves as a simple noise filter when nothing is plugged into the guitar input jack.

The inductor is replaced and the unit goes back together.  Working with the correct wall wart, this pedal is fun!  It does pitch bending and chorus effects surprisingly well, considering its age and the technology used.

Thanks for reading all the way through!

CONTACT – David Latchaw EE
281-636-8626

Ibanez TubeScreamer Pedal Repair

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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
281-636-8626

JBL SoundFactor SF12M Loudspeaker Repair

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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
281-636-8626

Luna Gypsy Spalted Acoustic Guitar Needs Fretwork

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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
281-636-8626