Month: September 2016

Fender AcoustaSonic Pro Combo Amp Refurb

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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
281-636-8626

Boosta-Grande Pedal Repair

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We did a quick-and-easy foot switch repair on this gain pedal.  Here’s the scoop:

The operation of this pedal was intermittent.  The switch didn’t click anymore.  Yup.  Probably just needs a foot switch.

 

The foot switch is on its own circuit board.  This switch is a latching, 3 pole double throw unit.  A ribbon cable carries all the lines between the terminals of the switch and the rest of the electronics.

 

The circuit board was carefully de-soldered using braid wick and lots of flux.  We’re ready for the new switch.

 

Here is the switch…  or what’s left of it.  The pedal enclosure was holding everything together.

 

The new switch was installed so that a minimum amount of threads were exposed on the top side.

 

The switch was adjusted so that the terminals on the switch lined up with the slots in the circuit board.

 

Everything lines up where it should.  See, I said that this would be easy!

 

The chef ordered new solder fillets all around!

 

An easy fix, and this boost pedal is VERY clean.  We’re done here!

Thanks for reading!

CONTACT – David Latchaw  EE
281-636-8626

Breedlove 12 String Breaks Strings

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I saw Leo Kottke in concert on a Thursday evening in 1975.  The next day, I gave up on the guitar solely because I felt so overwhelmed by his talent and felt unworthy to ever touch a guitar again. Confronting my sick, twisted fears from that traumatic experience included resuming my music studies thirty-five years later.  Doing her part to attack my illness, my Darling Bride, who went with me on that fateful night, purchased this Korean-made Breedlove Stage Concert guitar for me.

In standard tuning, the high G string broke more often than not. What to do?  I started by re-chamfer and polishing the edge of the hole in the tuner where the string passes.  The breaks appear to occur when the G string passes across the hole in the tuner.  When the string passes across the hole, a stress concentration could occur right at the chamfer.

 

Here is a bit copied out of my Breedlove’s Owners Manual.  The stock high G string is 0.008 inch.

 

These came from Strings By Mail. Many individual strings can be purchased ala-carte from them.

 

My plan is to throw conventional wisdom to the wind and add enough string to the post so that the portion of the string leaving the post only touches the round portion of the post, not the hole nor its chamfer.  Here, I am measuring the broken string to establish where the stress concentration occurred.

 

Then I added the length of the string that was broken off, then estimated the circumference of the hourglass-shaped tuning post and multiplied times 12.  The plan is to get enough static string on the post to cover the hole, preventing the portion of the string leaving the post on a tangent from getting anywhere near the hole.  Putting more than three or four wraps of string around a tuning post is generally frowned on, because the more string wound on the post, the longer it takes for the tuning to stabilize.  We’ll see…

 

I love the bridge on my Breedlove.  I don’t have to reach inside the guitar to push the bridge pegs out nor verify that the ball ends are up against the peg and bottom of the sound board..

 

The ala-carte strings have ball ends that do not necessarily match the color codes used in the sets.  Who would notice??

 

I polished and chamfered the hole edge to the best of my ability.

 

To keep the loose string end under control while fiddling with the length of the new string, I put my classical guitar capo across the neck to corral the G string.

 

Here’s the G string.  I can grab it, pull it, handle it, measure it, trim it, pull it, and it won’t get away from me.

 

I estimated the additional string length in a previous step, and am adding it here.

 

We have all been here.  Wish me luck!

 

Here is eleven turns of the G string around the post.  Uh oh.  I am a little short of covering the hole.

 

But, tuned up to pitch, the portion of the string that tangentially leaves the tuning post is no where near the hole.

 

Here is another view, showing the smooth transition from the last wrap.  I don’t think we will see the sharp bend in the portion of the string that leaves the tuning post tangentially that would occur if the guitar were tuned and the string were bent over the edge of the hole in the tuner.

My original goal was to load up the tuning post with enough string to cover the hole entirely, thus eliminating the chance that the portion of the string leaving on a tangent toward the nut would experience a sharp bend across the tuning post hole.  But, I lucked out this time, and now the string is no where near the hole.  Thus, I could have reduced the number of turns of string on the post.  Not too bad for my first investigation.

I have not broken the high G string on my 12 string guitar since these pictures were taken.  The high G string has stayed in tune while playing and does not appear to de-tune between sessions any more so than the other strings.

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Peavey Deuce Combo Amp Refurbish

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WARNING – A very long blog post awaits you. Typical guitar amplifiers can be described in terms of functional blocks – power supply, preamplifier, power amplifier, and loudspeakers. This refurbishment effort left NONE of those blocks untouched. Sit back and enjoy!

This Peavey needed some work before it could be added to David’s tonal arsenal.

 

I removed the chassis for inspection.  This is the ‘clean’ channel.  The LEDs illuminate to show channel selection.

 

The ‘effects’ channel has a little more tonal control.

 

This unit uses an odd solid state phase shifter scheme and a mechanical reverb unit.  The standby switch is on the front, and the power switch is on the back.  The Fender switch scheme for tube amplifiers sometimes puts both switches in the back, and of course, I always select the wrong one because I have to reach around where I can’t see to work the switch.

 

Somehow, the 8 ohm jack is open-circuit.  We will need to look at this on the inside of the chassis.

 

This is the main power switch.  The ‘ON/ON’ function allows the user to capacitively couple the chassis to one side of the AC line.  This sometimes helps with hum reduction, although the three wire AC cord defeats one side of the switch.

 

The fuse is intact insofar as the red pilot light turns on when power is applied.

 

However, this 10A fuse is the wrong part.  Someone has been jacking with this amp!

 

Sorry, this needs to be fixed.

 

Note that the scratches on the tube base indicate that the tube was inserted incorrectly.  How can this be?

 

This is how.  The guide posts were broken off.

 

The reverb tank bag is screwed down to the bottom of the unit.

 

Everything here is intact, no damage, transducers are OK.  We’ll set this aside in a safe place for now.

 

We found the first bug.  Poor little spider.

 

All of the high voltage capacitors show signs of venting.  See the little bump in the black rubber, next to the 350?

 

The output jacks on the rear of the chassis have been damaged.

 

The pin contact has been bent all the way out so that the plug cannot touch it.

 

This pic is to document that these inside star washers should be placed between the jack and the chassis.

 

Both jacks shall be replaced with new ones.

 

All new hardware was used, of course.

 

The jacks are wired together and the jacks are wired directly to the output transformer.

 

The amplifier circuit board has seen some significant pyrolysis.  Maybe this happened when the tubes were installed the wrong direction, because of the missing guide posts?

 

To remove the amplifier circuit board, the tube socket pins must be unsoldered and the wires removed.

 

With a little attention to detail, the circuit board comes off the tube sockets.

 

This is the underside of the amplifier circuit board.  I wonder if someone poked the wires a little too far, so that the ends touched the chassis?

 

This printed circuit board will be rebuild, and all the components will be moved to the solder side of the board.

 

Lots of rosin flux was used to clean off all the solder.  Lots of alcohol is used to clean off all the rosin flux.

 

The rag gives some place for the mess to go.

 

With the circuit board cleaned up, we can see if it can be repaired.  It’s too old to replace; Peavey has no stock.

 

The carbon tracks are removed.  The remaining copper is trimmed back to where it is still attached to the laminate core.

 

Comparing the circuit board to the layout, we can verify where the traces and components actually should go.

 

Both sides of the circuit board are now spotless.

 

The new components will be placed above the surface of the board to allow for cooling and to insulate each part from the live circuit traces below each component.

 

The copper around the mounting hole is gone, so the lead of this component will be sweat soldered to the surviving trace.

 

To bridge the gaps in the copper, bare solid wire will be soldered in place, and insulated from the rest of the circuit with this Teflon tubing.

 

Here is a picture of the Teflon tubing at work.

 

All of the components are installed now.  The Peavey Deuce uses four 6L6 tubes but only had two screen resistors; the other screens were tied to the plate voltage.  The Peavey Mace amp uses six 6L6 tubes and has six screen resistors.  I went with four screen resistors, as I believe that using two was an engineering oversight.  The original Peavey documentation was ambiguous on this point.

 

This is a nice side view of all the components, flying above the printed circuit board.

 

Subsequent testing showed that the original phenolic tube sockets were either contaminated or were carbon tracked.  The old sockets need to be replaced.  Here, we’re drilling out the pop rivets that hold the sockets onto the chassis.  Note the magnet, which attracts the steel shavings and keeps the mess to a minimum.

 

These tubes hang base side up, thus the base clamps keep the tubes firmly in the socket.  As we know, heat rises, so the sockets get hot during normal operation.  These were cooked.

 

The new sockets will be held in with #8 stainless fasteners.  Here, the chassis holes are enlarged to pass a #8 machine screw.

 

Likewise, the holes in the new ceramic sockets need to be enlarged to the same extent.

 

The sockets will be mounted from the chassis.  We won’t have any access to the bottom of the socket once the printed circuit board is soldered onto the socket pins, so I need some captive hardware permanently mounted to the socket ears.

 

These small outline nuts are brass with a silver plating.  They solder easily to the ears of the new tube sockets.

 

Once the nuts were tacked into position, a nice fillet of solder is run all around the nut to keep them in place.

 

These new ceramic sockets are ready to be tacked into place on the circuit board.  Pay attention to the direction of the index slot!

 

Here are the new ceramic sockets in place on the refurbished amplifier circuit assembly.

 

The new sockets and tube clamps were installed with the stainless steel hardware. To assure that the solder joints under the sockets will not be under stress during operation, a spare octal base tube is plugged into the socket to align the socket contacts before each solder joint on the amplifier pc board is made.

 

Here is the refurbished amplifier circuit board assembly in its final resting place.

 

All the high voltage power wiring, filament wires, and signal wires are in place and ready to be dressed.

 

The power supply circuit board had many bad parts.  The large square resistor seen here was open-circuit!

 

And, when that large resistor died, it got really hot!

 

These large round flameproof resistors are available online as a better-than-factory replacement.

 

Nearly every component on this assembly was replaced.

 

We have 488 volts DC with no load on this assembly.  That will be acceptable.

 

The power supply board is back in place, and the cabling dressed.

 

On the preamplifier board, many of the electrolytic capacitors were showing their age.  The one of the left vented.

 

To remove the preamp board, all the knobs come off (of course.)

 

The felt glued on the face of this socket protect the faceplate from damage.

 

The preamp board is free from its moorings.

 

There appears to be little wrong on this side of the board assembly.  There are no signs of prior repairs here.

 

The vented electrolyte from this electrolytic capacitor attacked the copper lead of this capacitor and ate through it.

 

These cement resistors were bad.

 

The new resistors were mounted off the face of the board, to improve reliability.

 

The controls were flushed, dried, and re-lubricated.

 

This hybrid amp uses solid state techniques for low-level amplification and tubes for high level amplification.  These two transistors were bad.  These driver transistors are part of the phase splitter circuit, and drive the tubes directly.  The replacement transistors have a different pin-out, so the circuit board holes were marked with the correct layout.

 

Some components in the phase shifter circuit were changed out to improve its performance.

 

Here’s one of the original loudspeakers.  Car audio?

 

The other loudspeaker was mildly interesting.  Unfortunately, it was completely destroyed internally and was scrapped.

 

The old loudspeakers were removed and the cabinet was cleared out.

 

The customer selected a pair of import Celestions.  These were modified for an eight-bolt mounting pattern.

 

A magnet was employed to keep the steel shavings away from the loudspeaker magnet.

 

That hand reamer comes in handy to tune up these holes.

 

Here is our loudspeakers in their new home.

 

The loudspeaker wiring on the old speakers was made from microphone cable.

 

We will employ these right angle quarter-inch plugs for the new loudspeaker wiring.

 

The only hard part of using these is getting the solder to tin the case, which is commonly used for the return circuit.

 

We can’t use the cheap stuff now that we’ve gone this far!

 

Sixteen gauge 300v wiring was used for the loudspeaker cables.

 

When assembled, the case forms a clam shell with an integral strain relief.  The extra black wire also serves as a strain relief.

 

Now, all the pieces can be put back together.

 

My favorite part:  First Light from these 6L6 tubes from The Tube Doctor.

 

This thing plays like a new amp.  From 1980.  Because for all practical purposes, it is a new amp from the 80s!

 

Here’s another happy Unbrokenstring Customer!!

 

Thanks for reading all the way to the end!

Contact – David Latchaw EE
Cell – 281-636-8626