Month: November 2018

Intermittent Fender Acoustasonic 150

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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

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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

Ampeg B2-RE Bass Amp Gets New EQ Potentiometers

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Bass players use SERIOUS hardware, and this Ampeg B2-RE is a serious, heavy, high-end bass amp.  So why in the world does Ampeg use those tiny little plastic slide pots for the equalizer controls?  The Unbrokenstring Crew digs in.

Ampeg is a name that needs no introduction nor elaboration.

 

Three of the plastic handles are broken off.

 

A tour of the rear panel shows the cooling fan, the quarter-inch speaker jacks and Neutrik jacks for output power.

 

The preamp and power amp can be separated using these jacks,  An effects loop is nice, and a balanced output for the poor sot in the control room is a nice touch.  The balanced output has a switchable attenuator.

 

A look inside shows a big green circuit board that handles the power amp duties.  This view is dominated by the toroidal power transformer on the right.  The ribbon cable on the left carries power to and signals from the preamp board.

 

And here is the preamp board.  Interesting, the slice potentiometers for the EQ are located on their own circuit board on the left side of this picture.

 

The preamp board needs to come out to get to the slide pots.  This end of the cable comes loose easily.

 

Screws hold the preamp board in place in the chassis.

 

All the knobs come off, as well as the nuts beneath them.

 

The input jack has its own nut and washer.

 

While we’re here, an inspection of the unit reveals a solder joint on the path to failing.  This is an easy fix.

 

The slide potentiometers used by Ampeg are special for a couple of reasons.  One reason is, they have this nice vinyl flap over the slot that helps keep dust out of the internals of the potentiometer.

 

The potentiometer board is held down with these screws and brackets.  This needs to come apart next.

 

The other thing that makes these slide potentiometers special is that they use three legs to attach to the circuit board.

 

Here is the exact part number that needs to be specified when purchasing replacements.

 

A similar part is available from other sources, but if you look closely, the ‘pin-out’ does not match.  The correct part is in the foreground, and the ‘new’ incompatible part is seen in the background, with the ‘1736’ marking, which is probably a date code of year = 2017 week = 36.

 

I was curious if the slider and handle could be removed from the new parts and substituted into the old pots.

 

The interior view of the slide pot.  The black plastic thingie with the fingers, on the left, is the wiper, and the black strip on the right is the resistive element.  The fingers slide along the resistive element to achieve the variable resistor function.

 

Oh, drat.  The sliders are a different thickness.  We won’t be able to use the new parts to repair the old controls.

 

So, a week later, a batch of the new controls come in.  We are going to change them all out.

 

What do you think, folks?  Should I clean up the front panel and remove all these presets?

 

The new parts are installed.

 

The workmanship appears to be pretty good, if I do say so myself!

 

Back on the air, this Ampeg is ready to really rumble through its four hour burn-in!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Boss BCB-60 Pedal Board Repair

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This pedal board quit completely during the church service.  Joe got it going again by rewiring around everything after he realized that the power supplied from this pedal board died.  Could the Unbrokenstring Crew get to the bottom of this issue?

Joe has his name on everything!  This pedal board is actually a big rugged tool case, with a self-contained power supply for all the pedals.  Velcro holds everything in place.  As this problem did not actually involve any pedals, they were removed before we got this unit to repair.

 

This is the model number.

 

This is the serial number.

 

Company name and nation of origin.  Not China!  (Yes, I am one of those people.)

 

This panel distributes audio and power in and out.  When power is applied, there is no power out.

 

This is a closeup of that panel.  HINT: You can use any 9v negative center power source, with sufficient capacity.

 

Audio jacks are on the right, and power is on the left.  This whole assembly is wired like a loop-back panel.’

 

Removing the connectors allows full access to the circuit boards.

 

Interestingly, the middle board shares power.and audio.  See the diode?

 

Examination of the solder joints reveal excellent workmanship.

 

This diode on the center board is a anti-polarity-reversal diode.  This prevents bad power from reaching the pedals.  When this diode has failed open-circuit, it prevents ANY power from reaching the pedals.

 

This diode is equal or better than the original part and is the highest-spec’d part that would fit on the board.

 

Reassembly is the reverse of disassembly.

 

Audio and power functions are now completely restored!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626