Intermittent Fender Acoustasonic 150

A fellow musician gave Charles this amp, which was nice gesture.  However, the friend said that it was intermittent.  Could the Unbrokenstring Crew turn this gesture into a reliable amp?

Styled as a unit from the Fender Brownface era, the exterior certainly checks all the boxes for Brownface goodness, with the Correct knobs and silk-screened front panel true to the archetype.  One channel is reserved for an instrument, and the other channel is tailored to vocal performance, including a dual XLR/quarter inch jack for a microphone.

 

No metal shredders allowed.  This unit has a tweeter, and an electronic gain structure that does not distort.  Just the thing for Charles’ acoustic act.  This badge still has the protective plastic in place.

 

The speaker cabinet is sealed.  This polarized connector keeps three pairs of audio signals from the amp going to the correct loudspeaker and tweeter.

 

On the back side, we find the ON/OFF switch and the IEC power socket.  Most of the rear panel is slotted for ventilation.

 

This is a solid state unit, with plenty of pep to be loud.

 

The internal architecture permits stereo operation, as is shown by the FX loop connections.  I did not play around with the USB functionality, but it’s in the manual.  We have bigger fish to fry.

 

Name, rank, and serial number, please.

 

This circuit board holds all the connectors for Line Out and effects loop functionality, as seen on the rear panel.

 

This assembly is an AC to DC power supply on the left, and an efficient Class D audio amplifier on the right.

 

Digital signal processing (DSP) is used to create the reverb and other effects.  The DSP functions are on the mezzanine board on the left.  The thin white cable in the center is the USB cable.  The main printed circuit board handles the clean audio chain and the connections to the front panel controls.  The flat cable on the right brings power from the AC to DC board and sends audio to the amplifier.

A lot of surface-mount components are found in this unit.  Those little cans are electrolytic capacitors; black squares are integrated circuits.  Each of those little black squares does the job of two vacuum tubes.  I feel old and obsolete.

 

The check mark probably means that someone tested this at the factory, I guess.

 

So the audio processing hardware is seen at the top of the picture and the power stuff is at the bottom.

 

The AC wiring comes from the switch directly to the circuit board, where there are filters and a fuse.

 

This power stuff is actually a switching power supply, which efficiently creates the various operating voltages.

 

If you look closely at the gold rings on the circuit board, you will see solder that looks ‘strange.’  It does.  Gold atoms mix into the molten tin/lead alloy while the solder joint is in the liquid state.  The gold makes the solder brittle.

 

The entire circuit board is gold plated.  This plating is among the flattest finishes available for bare circuit boards, perfect for surface mount technology (SMT) components but is a metallurgical compromise for thru-hole components..

 

As you can see, for thru-hole technology components such as these pins sticking through the board, the results of the soldering action can leave something to be desired.  Do you see the holes in the solder joints?

 

Now that those holes are fixed, we can focus on the real source of the intermittent operation.  Do you see that light blue resistor with two red stripes hiding behind the capacitor and the heat sink?

 

That light blue resistor was soldered here.  Or to be more precise, it was soldered there at one time.  The cracked solder joints became intermittent conductors.  Here I have removed the resistor and cleaned away the old solder in preparation for making a new pair of solder joints, free of gold contamination.

 

Another issue with this amp is that someone has been playing with the loudspeakers.

 

This loudspeaker fits the cabinet perfectly, but electrically, it is a 40 ohm (yes, forty ohm) loudspeaker, designed for use in a public address paging system (you know, that mess that you hear at the doctor’s office playing MUZAK, mercifully interrupted by an announcement for someone to call a telephone extension?  Yeah, that.)

 

The Correct part is available.

 

Who would have thought that you could actually replace a bad loudspeaker with a new one of the correct type?

 

Do you like those TV shows where they have a build-up to the ‘Big Reveal’?  I don’t either.

 

Fortunately, we have the correct part and are ready to install it.

 

Once we replace the grille, you will never know the difference.

 

See, I told you that you couldn’t tell the difference.  This unit plays beautiful music and the functionality is solid.

 

Support this musician, winner of a Texas Music Magazine 2018 Album of the Year:  http://www.charlesbryantmusic.com/

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

 

Rickenbacker Rick-O-Sound Bass Repair

Craig is a Ric Man.  This beautiful blue bass in only one of his many Rics.  This one has problems with getting both pickups to work.  Could the Unbrokenstring Crew get all this sorted out?

This instrument plays beautifully and has no real discernable setup issues.  However, the wiring seems to be amiss!

 

More than a few of you will have heart palpitations seeing this head stock.

 

Two output jacks allow a mono mix of both pickups as is usually found with most multi-pickup instruments, plus a special “Ric-O-Sound” jack that presents the signals from two pickups as two separated signals, accessible with a stereo cable (TRS.)  This gives the player the ability to run two preamps, two effect loops, two separate amps, etc.

 

Immediately after looking under the pick guard, we find a loose wire.  Poor soldering here.

 

This isn’t even tinned.

 

Here is another broken wire, another ground wire.

 

We have a stack of three inside-tooth lock-washers under each output connector.  Not much spring action available from the teeth of these locking washers, so they don’t really lock.

 

A magnet is the quickest way to clear all this extra hardware out of the control route.

 

This is the stereo jack handling the “Ric-O-Sound” duties.

 

The mono jack handles the single-ended output from this instrument.

 

Soldering workmanship on the switch needs some attention as well.

 

Disconnecting each pickup allows us to do some cleanup in the wiring cavity.  The neck pickup is ohmed-out.  The neck pickup is sometimes called the ‘treble’ pickup.

 

And we do the same with the bridge pickup.  This pickup is also called the ‘bass’ pickup, which seems redundant.

 

Perhaps this soldering was done with plumber’s solder.  It is awfully dull.

 

Another look at the switch.  Not much to brag about here.

 

The ground connection to the control is redone.  Smooth and shiny is the name of the game when soldering.

 

I labelled the controls BV (Bass Volume,) TV (Treble Volume,) BT (Bass Tone,) and TT (Treble Tone.)

 

Now that the controls are identified, we can install the knobs in the Correct location.  Yes, they were in the wrong place.

 

Earlier I pointed out the stack of three inside-tooth lock washers.  Here’s why I mentioned it:

 

When stacked, the teeth have nothing to push against.  The teeth are literally hanging out in space.

 

A flat washer in the middle will provide some ‘resistance’ for all the teeth to press against, thus restoring the action of the lock washers to that of being, er, well, lock washers.

 

The jacks are Imperial measurement and this washer must have been metric.  A little gun-smithing is in order.

 

The output jacks are wired and tightened into their correct locations.

 

Testing shows no output.  It has something to do with this volume pot.  What the hey??

 

This is the hey.  This bit of conductive solder debris was underneath the volume pot, shorting the ‘stapled’ contacts seen in the previous picture to each other, and thus, shorting the output to ground.

 

Together again, and it’s playing in stereo!

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

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

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.

 

I wish the heat sink was as well-supported as the rest of the circuit boards.

 

This whole assembly is attached to the circuit board on the bottom.  Out it comes!

 

Can you see the problem?  Me neither.

 

Close examination reveals cracked solder joints.

 

The correct repair for a cracked solder joint is to remove everything and replace the joint with fresh tin/lead solder.

 

This is a good solder joint, if I do say so myself.

 

Further examination reveals more cracked solder joints.  Guess what we’re going to do to these?

 

This little yellow grabber tool is handy to install screws in tight recesses.

 

Time to crank it up!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

 

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
281-636-8626