Acoustic B600 Head Acts As If It Is Always Muted

This unit lights up but does not make a sound. Matt said that other techs couldn’t figure out why there was no audio coming out of this almost new bass head. As the last resort, could the Unbrokenstring Crew succeed where no tech has succeeded before?

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The plastic face plate, bezel, and knobs were coated with a synthetic rubber material that turns into a sticky mess with time. Here, I’m using a little elbow grease to see what works to remove this particular flavor of sticky rubber stuff on the bezel, which surrounds the front panel.

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This rubber coating, seen on this mouse, is popular on all sorts of consumer and pro-sumer gear, and the Internet has many articles and YouTube videos regarding how to clean this stuff up. Once the rubber coating begins to degrade, some sort of sticky chemical leeches out over all exposed surfaces. I suspect that the chemical has contaminated the MUTE push button, causing it to be, in an electrical sense, ON all the time, thus muting the unit. Other techs were looking for an electrical problem, not a chemical or material science problem.

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I took lots of pictures while disassembling the unit. There are no schematics available.

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The pictures would be invaluable when reassembling the unit.

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Here is another cable, hidden on the side.

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The central section of the unit is this chassis, which is a power supply and Class D power amplifier. The chassis serves as a heat sink for all the active electronics. A cooling fan on the top circulates air through the unit.

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With the chassis out of the way, the screws securing the front panel PC board in the chassis can be accessed. Of course, the knobs have to come off. Don’t they always?

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The front panel has all the controls, and some blue LED lamps. The MUTE button is labelled in the silk screen (white lettering next to each component.)

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The push buttons covers themselves don’t need to be removed to allow the unit to be disassembled, but they need to be removed to clean the switch. And they, too, are covered with the sticky rubber stuff.

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We are looking at the world’s first spa for Acoustic Amp push button caps. After trying several different chemicals and soaps, straight isopropyl alcohol seems to work the most quickly.

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Now that the switches and controls are cleaned, we can start the process of re-assembling the unit. The machine screw with the red fiber washer fits into a slot in the edge of the circuit board.

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Everything is reassembled in the reverse order of disassembly. The jacks and buttons on the rear panel are cleaned and lubricated. None of these were contaminated with the sticky rubber stuff.

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Power is applied, and all functions are checked out. Unbelievably, the soldering iron remained cold for the entire repair action. Sometimes you don’t need to be an electrician to make things work again!

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Thanks for reading all the way to the end!

David Latchaw EE

281-636-8626

Gibson ES125 Tune-Up

Dr. John has collected this beautiful ES-125 (a Gibson Electric Spanish guitar with an MSRP of $125 back when it was produced) but it sounded as if it were underwater. Could the Unbrokenstring Crew toss it a life saver?

 

This instrument is in collectable condition, with all original hardware. The finish is finely-checked as you would expect a seventy-year-old musical instrument to be. A new hand-wound pickup was included in the instrument case, if the original one was defective and could not be easily fixed.

 

Years of oxidation and skin oil had made the neck sticky, particularly when the humidity is high (which is all the time in Houston.)

 

The sticky finish ends at the head stock, which implies that the finish is OK but the skin oil is the culprit.

 

Here, fine polishing compound is mixed with Dr. Duck’s Axe Wax to rub out the finish and remove the oxidation.

 

Next, we will look under the pick guard to investigate where the underwater sound is coming from.

 

This pick guard is shaped in such a way that it holds all the controls, and only a hole for the ground wire to the bridge and a slot to clear the pickup is needed in the sound board to electrify this instrument.

 

The ground wire to the strings appears to be a piece of lamp cord. The solder joint around the ground wire did not alloy to the ground wire between the pots, but slides up and down the wire.

 

This ceramic cap is the tone cap. It bleeds off high frequency to ground under the control of the tone pot.

 

This tone cap is marked 0.02uF at 50 volts.

 

On the capacitor tester, the value is correct.

 

However, the dielectric is very leaky, which would probably change things in the tone circuit for the worse. This is probably where the ‘underwater’ sound comes from!

 

Some high quality film capacitors are retrieved from stock.

 

These are the same value, 0.02uF, but are rated at 400v in case the guitarist plugs the instrument into a wall socket. At least the capacitor will survive. The player, not so much…

 

Dr. John lives about seventy miles away. As each change was made, a sound file of the instrument was emailed to him to monitor progress.

 

A free copy of ProTools First and Ableton Live came with the interface, which will amazingly run pretty well on this old rack-mount controller PC that I have on the bench.

 

John decided that the new pickup didn’t add anything to this fine old instrument, so it remains in its original condition as of seventy years ago (with a new tone cap, of course.)

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Epiphone PR-5E Acoustic Gets New Electronics

This wonderful instrument was rescued from an abandoned house.  It sounded good acoustically, so the new owner asked if the Unbrokenstring Crew could repair the electronics and get it playable again?  Let’s get to work!

This instrument is only a few years old, purchased and then cast aside.

 

An online source for specialty guitar parts had a 2018 model of the electronics.  Can we make it work?

 

The mounting scheme uses four screws in the corners.  Cleats will be added to the guitar so that the screws hold.

 

But the REAL advantage of the 2018 electronics is that the enclosure matches the curve of the body.

 

Common wood items such as yard sticks, tongue depressors, popsicle sticks, and paint stirrers are made from birch, the straight-grained ‘poor relation’ to maple.  So the paint department lady at Home Depot gave me this stirrer.

 

Cleats to tightly hold the electronics in the body are fabricated by hand.

 

Next, the cleats are sanded to fit the curve of the body on the inside of the bout.

 

These are ready to trial-fit.  The bevel on the corner, lower-left side, clears an internal brace in the guitar.

 

One shim goes here.  Note the angled pieces, which will catch the screws in the electronics.

 

To protect the finish, low tack painter’s tape is used all around.

 

These clamps will hold the cleats in place as the hide glue sets up.

 

Both cleats are installed and the hide glue is curing.

 

Additional reinforcement is added in the corners to give the screws more material to grip.  These bits will be in compression, so they don’t need to be super-strong, only tough.

 

Before we get too far, we need to do another trial fit.

 

I think we’re going to be good here!

 

I will pre-drill the holes where the screws will be installed.  This should minimize splitting of the small cleats.

 

The actual drilling will be finger-powered, using this pin vise.

 

All four holes are pre-drilled.

 

Yes, I know.  This isn’t terribly interesting.

 

The screws are installed.  Not bad!

 

The jack plate holds the battery and has both 1/4th inch phone plug and balanced XLR connections.  It was temporarily removed to give more access to the interior of the instrument.  The new electronics hook right up to the jack plate.

 

This is a new one on me.  This battery was in the guitar when it came in.  The Golden Thumbs-Up Emoji Award!

 

And, while we’re here, we’ll clean the instrument, string it, and do a setup.

 

The electronics come alive!

 

This is a cool tuner.  The LCD screen has a pointer that swings across its face.

 

Beauty is skin-deep, but what really counts is just below the surface.  This rescued guitar is ready to make music!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626