March, 2018 - Unbroken String

Peavey KB4 Keyboard Amp Disintegrating

March 27, 2018 by latchaw

This Peavey keyboard amp works intermittently, but the modular AC outlet has completely come loose from the rear panel. Can the Unbrokenstring Crew fix this?

This unit is pretty cool, with a built-in luggage roller and extendable handle as standard equipment from the factory.

The modular AC plug was covered in RTV rubber. Was this a user ‘fix’ or did it come from the factory this way?

Fortunately, all the electrical conductors were insulated. Otherwise, we would have sparks.

User controls are on the top.

The ground polarity switch is a throw-back to the days of two wire electrical cords.

A simple mixer is integrated into the unit.

Name, rank, and serial number, please.

The panel layout allows for some space for the handle. Good Job!

We removed the head from the cabinet,

Here is our intermittent. This power resistor had broken free from its solder pad.

A circuit board trace had broken. An Exacto knife clears away some of the solder mask to allow for a repair.

This crack was very small, so a good solder jumper is all that is needed here.

This unit appears to have been wet. Do you see the minerals left behind after the water evaporated?

Here is another little blob of mineralization. This may have been from solder flux residue left after the assembly was manufactured. Some fluxes turn white in the presence of water.

This screw was loose inside the grille. This screw holds the loudspeaker in place. This is not good.

Most of the screws were loose. While we have this unit on the bench, we should be sure that the loose screw is not a sign of a more sinister problem lurking with this unit.

Here is the loudspeaker in this unit. Nice!

My guess is, humidity has softened the baffle upon which the loudspeaker is mounted. These Tee nuts will be removed and new holes drilled in the baffle in different locations. The Tee nuts will be reinstalled and we should be Good-To-Go.

Some black nail polish will camouflage the new fastening hardware. You do have black nail polish, don’t you? Doesn’t everybody? Hint: This also makes good thread locker.

OK, now for the IEC power jack. This unit has mounting ears, so we won’t rely on friction or glue to keep it in place.

As an added bonus, this IEC jack has a built-in noise filter.

The hole in the chassis was enlarged to accommodate the new jack. This hand grinder is adequate for the job.

Yep! Just fits.

Now we will bore the steel panel to accommodate the mounting hardware. This will be SO much better than glue!

This doesn’t look too bad, does it?

All of the original wiring goes straight onto the new IEC jack. This is better than factory! Hot glue was apparently used at the factory to secure the switch and the old IEC jack to the rear panel. So that answers that question. Shame on you, Peavey!

Here is one last look of the internals before we reassemble the head. The mixer is at the bottom and the power amp and power supply is at the top.

This unit is literally ready to roll!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Behringer Ultrabass BX4500H Bass Head Needs a New Fan

March 19, 2018 by latchaw

Billy said that this unit worked very well until it overheated and shut down after about an hour and a half of use. He also didn’t hear the fan run. Could something this simple be all that is wrong with this bass head?

The Unbrokenstring Crew operated this unit for an hour and verified that it overheated and shut down, as it should.

Here is a tour of the rear panel. The power switch is in the back of the unit.

I really like the Neutrik Speak-On connectors for power at this level. The 1/4th inch plugs could be operated above their data sheet limits for current and voltage if used at the 450 watt level.

We’ve removed the top of the unit. Removing the finger guard lets us verify that the fan itself is completely locked up.

To get to the fan, we need to remove the heat sink assembly, which is held in place with these screws on the bottom.

This pic just allows us to keep the wiring straight for reassembly. This circuit board handles the power output duties.

The fan is bolted to the end of the heat sink assembly. The power amp is also attached to the whole stack.

We need these specs in order to specify a replacement.

This power cable is specific to the mating connector on the circuit board that supplies the 24v for the fan. We need to keep this and transfer it to the new fan.

There is a chance that we can remove the whole cable assembly from the old fan and move it to the new fan.

The new fan has three wires. The third wire is probably a tachometer output so that the speed of the fan can be known.

The wires on the new fan are soldered to the fan with lap joints.

So, we can make new lap joints when moving the old cable to the new fan. I’ve got a good feeling about this.

The new fan is bolted back on the stack and the whole arrangement is put back together.

This amp ran for four hours continuously until the neighbors called the cops complaining of a noise ordinance violation. I believe that the overheating problem is fixed!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Red Ibanez A200 Acoustic Electric Guitar Gets Fixed Right

March 13, 2018 by latchaw

Sophia called The Unbrokenstring Crew after her prized red Ibanez acoustic/electric quit during a gig. Could we fix it?

This is a surprisingly solid instrument, with a red finish that just won’t quit.

I also have a similar Chinese acoustic electric which is a surprisingly good guitar for the money.

The tuner still worked although the output from the guitar was silent. Therefore, the problem is between the electronics and the output jack. The electronics and tuner are removed to gain access.

That didn’t take long to find. Someone had just soldered the wire to the connector pin. This eventually flexed and failed.

The other end of the broken cable goes to the output jack. The balanced output jack is a nice touch.

Let’s try to take some pics inside the guitar. But we need more light. This LED flashlight will do the job.

This is inside the guitar. The wire tie holds the harnesses in place so that they don’t rattle while the instrument is played.

I didn’t need to remove the piezo pickup, but I’m taking it out of the guitar anyway so that it is not damaged. The pickup and the tuner are one piece, so we can store them all safely while other work proceeds.

Meet Ms. Output Jack.

The broken cable solders to the circuit board. The location and function of the wires is recorded in the notebook.

The unbalanced TRS output jack is wired to the circuit board as shown. This picture is for documentation purposes.

Can you see a problem? The TRS output jack should fit down inside bosses molded into the body of the output jack assembly. Whoever tried to fix this before installed the TRS jack 90 degrees out from where it belonged.

This is the end of the broken cable.

A new cable was secured from a guitar junkyard on eBay. Looks the same, doesn’t it?

The old cable was desoldered and the new one installed as shown. We are ready to solder the wires to the PCB.

Soldering complete. Note that the TRS jack, on the right, is correctly installed now.

The output jack goes where it belongs.

Before we install the electronics, let’s show a little love to the bridge with some Dr. Duck’s Axe Wax.

The new cable installs as shown. I think this pic was taken while I was still testing everything out prior to reassembly.

I forgot to ask Sophia what gauge of strings she wanted. These are probably 11s.

A phone call confirmed that these were what she wanted! This guitar is repaired better than it was when Sophia purchased it. The Unbrokenstring Crew makes the world just a little better than it was before.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Intermittent Ashdown EVO III 500 Bass Head

March 5, 2018 by latchaw

This head was soldiering away in the studio when the output signal became distorted. Can the Unbrokenstring Crew un-distort the output and keep it from happening again?

Stock photo credit: Ashdown Engineering

Name, rank and serial number, please.

The oscilloscope shows the waveform presented across an eight ohm resistive load. A sinewave is applied to the input jack. We should have a sinewave here. But we don’t. This gives us something to work on!

Oops! As soon as we touch the chassis, the output waveform changes!

Since we’re on a roll, let’s touch it again! This is what we should have seen all along. I think we know where to look.

You might be surprised to know that the big metal heat sink I was touching has a high voltage on it. So here I am safely draining the high voltage before I touch it and get shocked. Like in the previous picture.