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:


Thanks for reading all the way to the end!

CONTACT – David Latchaw EE


Fender AcoustaSonic Pro Combo Amp Refurb

My customer found another wonderful old amp in the pawn shop.  Could we refurbish this unit to its former glory?

The unit worked, but the grille cloth was dry-rotted. We also need to do something about the broken push buttons.  The stereo preamp assembly lives on the left side of the faceplate, and an effects processor is found on the right side.  This amplifier is an early attempt at what is called an ‘acoustic amplifier’ these days, suitable for voice and acoustic instruments.

Made in U.S.A!

Does anyone know about the ‘PATENT PENDING’ sticker that was added to the rear panel?

The badges on the front of the unit were in excellent shape.

Tiny #2 screws hold the Fender badge in place.

We need to replace the grille cloth.  The badges shall be returned to their correct positions.

The rectangular badge is pretty easy to measure.

The Fender logo, however, is a little more complicated in shape.

Fortunately, we just need to be ‘close’ and the screw holes in the baffle underneath will assure correct positioning.

Velcro loops were stapled to the corners of the grille.

These loops will need to be restored to their correct position after the grille cloth is replaced.

Here you can see the position of the Velcro hooks.  As an aside, this is a sealed cabinet unit.  All the wiring in the speaker cabinet is done through the loudspeaker holes.

To hold the new cloth in place, we will use similarly-sized staples.

This grille cloth pattern is vintage, and is called ‘wheat’ in the catalog.

We picked up a couple yards of the new wheat material from an Internet supplier.  The color match is nearly perfect.

The new grille cloth is stretched and stapled onto the original baffle.

Sure enough, the screw holes in the baffle enabled us to properly locate the Fender logo.

Likewise, we got the rectangular badge back where it belonged.  The blue handled tool is a tapered punch.

We’re done here!  This will be set aside for now.

The internals were removed from the amp.  This front panel is a mess, with nicotine and finger oil everywhere.

A little Gibson guitar polish cuts through the crud and cleans everything up.

The broken push button switch is problematic.  I salvaged some pieces from other switches, but they weren’t quite perfect.

New switches were ordered.  These are dimensionally and electrically identical, and fit the PC board perfectly.

Here are the new switches going onto the PC board.  These select four preset effects when pushed.

The controls on the left were all cleaned and lubricated.

The effects processor PC assembly is installed on the right side of the faceplate.

There are lots of different hardware pieces used to attach these assemblies to the amplifier chassis.  A socket finger-tightens a nut which holds the effects processor in place.

This is a view of the back of the face plate.  This style of electronics is SO very 1970s!

The loudspeaker and tweeter cables pass through the bottom of the chassis.

The hole to the right is where the loudspeaker and tweeter cables pass into the bottom cabinet.

I removed the loudspeakers and tweeter to clean up the cabinet.  This is a Motorola tweeter.  The speaker wiring is completed from the front, as this cabinet is a sealed, un-vented cabinet.

So, we pass the cable into the bottom section of the box.  When the wiring is squared away, this hole will be filled with RTV to re-seal the cabinet.

The chassis is complete.  These big screws hold the chassis in the amp cabinet.

The customer wanted a foot switch.  A four-button foot switch was the original Fender accessory.  However, those are rare.  This three-button foot switch will work well enough for the customer to select three of the four presets from the effects processor.  These switches are wired in parallel with those buttons we replaced earlier.

The original wiring was intended to control a synthesizer.  Out it comes!

Everything is cleaned up and desoldered.  The momentary switches were just right for this application.

The bottom of the switch box will be modified for a DIN5 connector, which is compatible with the original amplifier foot switch connector on the back of the amp.

Here is everything running.  This is a pretty nice unit, which represented the state of the art in acoustic amps in its day.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE