Phonic 1500 Rack Mount Stereo Amp Repair and Update

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Billy was running this amp as part of his PA for years until one channel quit.  Could the Unbrokenstring Crew bring this inexpensive amplifier chassis back from the e-waste pile?
To some folks, Phonic and Professional do not belong in the same sentence.  But this unit has been working very well behind the scenes, for more years than many of its Brand Name Brethren have been working.

 

Simple controls on the front.  Cranked to eleven, of course.

 

It doesn’t get much simpler than this.

 

Billy asked to have an IEC power cord installed on this when (not if) we fixed it.

 

The Neutrik power connectors are a nice touch, almost required at higher power levels.

 

Inputs are either balanced quarter-inch TRS or XLR connectors.  The usual set-and-forget controls are found here.

 

The power transformer seems adequately-sized for this power level.  AC-to-DC duties are performed to the lower right, and one audio channel is seen between the transformer and the aluminum heat sink.

 

With the unit turned around, the other audio channel is found here.

 

The front panel controls are here.  These potentiometers will get a million-mile cleaning and lubrication.

 

The dark charcoal-colored ribbon cable connects signals to the two audio channels.  The flat ribbon is Just The Thing here, because it does not block the path of cooling air into the unit.

 

The electrolytic capacitors are bulged, which is not unusual for a unit that has seen this many years and this much use.

 

These electrolytics will be replaced.

 

These rectifier blocks are wired in parallel.  Yeah, Baby!

 

I was kinda wondering where the line fuse was located.  It’s under this bundle of cables.  Yes, that says 30A at 115VAC.

 

Before the unit comes apart, I need to document where all these cables go.

 

These cables need to go back where they started.

 

Behind the colorful bundle in the foreground is the circuit board that connects the rear-panel output connectors.

 

I need to remove the larger circuit board to get to the solder-side of the PCB.  The dirty little ribbon cable comes off first.

 

These output cables come off next.

 

And now, we begin.  There are about thirty screws that hold the circuit boards in place.

 

These little screws are everywhere.  Like that’s a bad thing…

 

More screws.

 

The heat sink is split into two sections, one for each channel.  They need to come loose from the chassis as well.

 

This aluminum block bridges the tops of the heat sinks to add strength and rigidity to this unit.  Nice!

 

OK, the main board is out of the chassis.

 

Interestingly, some other version of this amplifier uses more electrolytic capacitors.  In this version, the pads are jumpered.

 

Turning the board over, the solder joints to be cleared are ‘marked’ with some rosin solder flux so I can find them if I look away to grab the soldering iron and braid.  Yes, I’ve unsoldered the wrong solder joints in the past.

 

The old caps at the top of the picture are out and the holes in the PCB are cleared.

 

Meanwhile, back at the ranch, we will remove the captive line cord and add the IEC connector here.

 

This cord has 14AWG conductors in it.  A matching large molded IEC power cord will be supplied with this unit when it is returned to the customer.

 

Here is our new IEC male socket.  Some of these come with flange ears, but spacing on the rear panel is too tight to allow the use of one of those.  This one snaps into place.

 

This looks a little rough, but this is the approximate outline of the rectangular cutout for the new IEC connector.

 

While we are hatchet-ing on the rear panel, these magnets will catch any chips or bits of steel removed from the hole.

 

Here is the outside view of the first trial fit.

 

And this is what it looks like on the inside.  This IEC socket snaps into the hole, so the hole size needs to be right.

 

Here is the new IEC socket wired into place.

 

And this is the closeup of the finished installation.  That silver thing next to the CE mark is a ground point.  I temporarily removed the thumb screw while grinding on the chassis.  It goes back on the unit next.

 

Now it’s time to reassemble.  Screws, anyone?

 

Glad I took all those pictures of where these wires went!

 

The unit is now reassembled.

 

Here, the terminal block outputs are tested at 250 watts.  So far, so good!

 

The Neutrik connectors are tested next at 750 watts per channel.  All is well!

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

MIJ Fender Precision Bass Gets a New Nut and Setup

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Lisa’s marvelous Fender P-Bass needed attention.  Some of the open notes were dead, and the electronics needed some attention.  Could the Unbrokenstring Crew sort it all out?
I just love the pale yellow finish.  Except for a string, everything is here.

 

Yes, it really is Made In Japan.  Back in the day, ‘made in Japan’ was another word for cheap imported junk.  Nowadays, this is some of the better stuff, particularly in guitars.

 

Name, rank, and serial number, please!

 

An electrical test shows that we have no output.

 

A quick look under the hood does not reveal an immediate problem.  Hmmm…

 

Oh, this is it.  The ground point for the whole unit is this potentiometer body.  However, the ground wire to the output lead does not connect to the potentiometer body anywhere.

 

With that fixed, the remaining ground wires are cleaned up a bit.

 

The bridge ground wire made an intermittent connection to the bridge.  We need to remove the green corrosion.

 

OK, the electronics are now all up to snuff, and actually look pretty nice.

 

While we’re here, we’ll tighten the output jack and potentiometers a bit.

 

The knobs go on now.

 

Final test is performed with a signal generator and another bass pickup.  The signal generator excites the windings in a bass pickup from an Aria Pro II bass, which will be featured in a future blog post.  The test pickup is brought near the instrument’s pickups, and the magnetic field carrying a test tone is coupled into the instrument’s electronics.

 

The dead open notes are traced to a cracked nut.  Here, we’re cutting the finish around the old nut so that it can be removed cleanly.  The Exacto knife gets a new blade for this operation.

 

The old nut comes out in two pieces.  The crack expanded until the nut broke in two.  That’s why we’re replacing it.

 

Here is the new nut that the customer wanted installed.  Good stuff!

 

Oops.  Houston, we have a problem  This new nut does not fit the neck.

 

The new nut is just a tiny bit smaller than what is required for this neck.  What gives?

 

We can clearly see the difference in the sizes between the old nut and the new one.  This neck is the width of a five string bass, but it was delivered as a four string bass from the factory.  So, we will make a custom nut for this instrument.

 

A Tusq blank is radiused to match the radius of the fretboard.  I’m using an Exacto knife as a scraper.

 

The Tusq blank is cut to rough length with a fine saw.

 

It doesn’t take long to slice through the Tusq material with this blade.

 

This is a saw blade set that I use for sawing fret slots and general fine work on wood.

 

The blank is now shaped on the disk sander.  A piece of birch plywood serves as a raised table that can be placed very close to the abrasive surface of the disk, necessary when shaping small parts.

 

The blank is now pretty close to the rough shape we need.

 

The first trial fit shows that we haven’t cut it too small, yet.

 

This is a little better.  The ends are flush and smooth with the edges of the fret board.

 

In AutoCAD, a drawing is created showing the cross sections of the four strings and the width of the fret board in actual size.  The distance between the edge of the outside strings and the edge of fret board, established by factory specs, is drawn, and the position of the outside strings fixed.  We then subtract the diameters of the four strings from the width remaining.  This result represents the space between strings, which shall be three equal spaces.  This establishes the center lines of the inner strings.  The spaces between the strings are the same, not the center-to-center distance.

 

But, to cut the string slots, we need to know where the edge of the fret board is, and where the center lines of the strings fall.  These solid lines represent that information.

 

The lines which represent the centers of each string are transferred to the nut.

 

A shallow file cut is made at each string center.  Here, we are checking these cuts against the template.

 

These shallow cuts represent the eventual center of each string.

 

These cuts were made with a triangular mill file.  Nothing special, but accurate enough.

 

Here, we’re polishing up the sides and faces of the nut, in preparation for gluing the new nut in place on the neck.

 

The nut depth is established by the fret height plus a constant which is established by Fender (and can be adjusted a bit by a good luthier, like me, for best play-ability.)  This is the Secret Sauce of making an instrument a great instrument.

 

The slot depth is now established by this stack of feeler gauge shims.  They are held in place with rubber bands wrapped around the back of the neck.  I’ve taped off the head stock so that I don’t scratch it up with the end of a file.

 

When the file touches the stack of feeler gauges, continuity will be detected by this multimeter, and it will beep.  This is another check of slot depth, besides my eyeballs.

 

Here, the slots are cut.  With a little cleanup and polish, this will be a good nut!

 

The nut is all done and polished.  Looks good!

 

The action on this instrument at the twelfth fret is pretty high…

 

We have a metal neck shim between the neck and body, made from a piece of the machinist’s feeler gauge of the proper thickness to reestablish proper neck geometry.  The metal shim is the hardest practical material for this purpose, with an accurate thickness, and better mechanical stability and hardness for greatest vibration transfer between the neck and body than a guitar pick or a piece of business card.  This results in the best tone.  And a set of feeler gauges are less than five bucks.

 

A quick adjustment gives us just the right amount of neck relief.  (Sharp-eyed readers will spot the fact that the strings are off in this picture.  This is the only pic I took of the truss rod adjustment, setting the neck flat while the neck shim was being sized.  Who cares if my pics are out of chronological order?)

 

To set intonation, we needed to work on the bridge.  Here is the underside of the bridge, probably not seen for decades.

 

The intonation screws were dinged.  Here, we are chasing the threads with a die to clean them up.  Yes, they are English/Imperial threads, not metric.

 

The bridge is tightened down and ready to go!

 

The moment we’ve all been waiting for!  Add strings, tune up, intonate, and play!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

The Korean Squire ‘Blackout Strat’ Build

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Matt’s unicorn was to again own a Blackout Strat like the one from his youth.  Could the Unbrokenstring Crew make an equal-or-better unicorn?
The body is a Korean Fender Strat with genuine wear.  The new neck is in superb shape.  This will be the basis for a cool project, particularly since modern parts are available to upgrade this guitar.

 

The neck comes off and will be updated separately.  Say ‘Goodbye’ to the Squire neck plate.

 

The dot inlays in this neck are really spectacular.

 

The pictures do not do justice to the spectacular mother of pearl in this fret board.

 

First, we’ll clean up a couple of handling skuffs that occurred while this project was coming together.

 

Tinted polyurethane, various grades of fine sandpaper, and a buff polish gets us back to where we belong.

 

A set of vintage tuners will be fitted to this head.  Here, we establishing the center line of the tuners.

 

Screws for the tuners will go where the scratches cross.

 

The pin vise is pressed into service to bore the screw holes.

 

This drill bit will be used as a gauge to verify the diameter of the tuner holes in the neck.

 

Sure enough, this neck was pre-drilled for 3/8ths inch bushings.

 

The bushings are fitted tightly into the neck.  This is essential for good tone, as the bushing supports the capstan, which is one end of the support structure that establishes string tension.  If these are loose, your strings won’t work very well.

 

Turning our attention to the body, we find that the ground wire was not properly soldered to the string claw.

 

The interior routes of the body were painted with a conductive paint.  This just won’t do for the Unbrokenstring crew.  This screw ties the conductive paint to the rest of the ground circuit.  We can do better.

 

The output jack is liberated from the stamped ‘football’ socket plate.  We will rework the wiring with heat shrink support in order to support the wires and make it more durable.  And the Unbrokenstring Crew will redo the soldering job, because We Own It if it fails.

 

The neck pocket must be square and clean.  Any debris or finish will interfere with the transfer of mechanical vibrations from the neck.  For maximum sustain, we need to take this pocket down to the bare wood.

 

This side view shows that the corners are square and clean.

 

Copper foil will be used to create the cavity shielding.  We’re starting with the hard stuff, the output jack route.

 

Bottom and sides are done.

 

All of the interior routes are lined with copper.  The seams are tacked together with solder.

 

This is the bottom side of the new pick guard, also made in Korea.  The aluminum foil must go.

 

All of the holes will be de-burred with this tool.

 

We will have a nice flat surface to which the copper foil will adhere without voids.

 

Foiling the pick guard takes just a few minutes!  Again, the seams are tacked together with solder to form a plane.

 

The foil is trimmed away from the edges with an Exacto knife.

 

This build will use these pickups.

 

This set will be a very cool foundation for this instrument.

 

The controls are mounted right to the copper.  True to Fender specs, we are using surgical tubing for pickup springs.

 

The pick guard assembly is done, complete with Orange Drop tone cap.

 

Here is another view.  All that is missing is the output jack wires and the bridge ground wire.

 

Here, the body is going together.  The output jack wiring and bridge ground wire is routed through the body and soldered to the pick guard assembly.

 

This workmanship turned out pretty.  New Old Stuff (NOS) push-back wire insulation is used to complete the vibe.

 

Everything fits!

 

The neck gets a little prep before the strings are installed.

 

Remember the Squire neck plate?  This is the replacement.

 

Meet the strings!

 

This new neck has a new nut, which needs to be slotted and filed.  String spacing is established here.

 

Here, the slots are taken to the proper depth, calculated beforehand.  The stackup of feeler gauge blades establishes the bottom of all of the string slots.  When each file touches the feeler gauge blades, we’re done.

 

The string slot depth is where we want it.  Next is to file off the top of the nut and polish it, which has been shown in other blog posts.  When this is finished, the strings will protrude just above the top of the nut.

 

Now this is beginning to look like a guitar.  This is set up as a ‘hard tail’ so no tremolo bar is needed, but one is supplied.

 

We lost Matt somewhere in the bowels of Guitar Center.

 

This message came into the Unbrokenstring Global Command Center after this guitar made it back to the rehearsal room:

“BTW been meaning to tell you, OUTSTANDING job you did with the recreation of Crow’s Fender Blackout Strat!

     “I think can say, without any hyperbole whatsoever, that just plugged straight into an amp with no tweaking whatsoever, that is the BEST SOUNDING GUITAR I HAVE EVER HEARD IN MY LIFE.

     “Completely outshines even the original Blackout he was trying to unbury.

     “We’re still absolutely dumbstruck by clarity and full tonal range of it. Truly amazing work, sir!

     “Without question, your finest creation to date.”

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Ampeg BA115 Bass Amp Falling Apart (Literally)

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Back in August of 2017, one of these combo bass amps came through the Unbrokenstring Shop with cracked solder joints, which were probably a result of brittle lead-free (RoHS) solder on the circuit board.  The owner of this Ampeg saw that post and called up the Unbrokenstring Crew to ask if this amp could be fixed as well.  Here we go!

Disassembly and reassembly of the combo amp is exactly the same as was performed on the earlier post.  Here, we are starting this blog post with just the chassis on the bench.

 

Name, Rank, and Serial Number, please!

 

From above, everything appears to be as it should be.  No wires are hanging loose as was seen in the amp serviced in August, 2017.

 

But once the circuit board was removed from the chassis, this rotary switch came loose from the circuit board!

 

As was seen in the other repair, the metallurgy involved with the soldering process was to blame.  In addition, in my opinion, this switch was not the exact part that matches the footprint on the circuit board.  Note that the pins are bent inward to the center of the switch.

 

These pins are plated in gold.  This is a good thing for the component, but gold, in solution with molten solder (yes, the metals mix) makes the resulting solder joint brittle.  Here, some activated rosin flux is added to the gold plated pins to prepare them for a coat of tin-lead (non-RoHS) solder.

 

Tinning is complete.  In this picture, we can see the intentional bending of the legs to match the holes on the circuit board.

 

This is a high quality part, and works well in this application.  However, the manufacturing engineer at SLM was off his/her game that day.

 

Our new solder joints will probably outlast the amplifier.

 

No parts were required for this repair, only labor.  This unit plays 100% now!

See the previous Ampeg 115 post for reassembly.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Boss DD-20 GigaDelay Pedal Needs Direction

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Every function on this pedal works properly.  Except, the user cannot select menu functions without a lot of hassle.  And, the delay time can only be increased, not decreased.  Can the Unbrokenstring Crew decode the mystery and make this unit a little less of a pain in the rear to use again?

The unit turns on and works, but the delay time knob only increments.  This makes it almost impossible to select any function or mode unless the user spins the knob all the way around to wrap back to the beginning of the menu.  And there is, like, three hundred menu items.

 

Everything is accessible from the bottom.  The vertical strips are Velcro hook material, to hold the pedal in place on the pedal board.

 

The black plastic rectangle on the left is a battery box.  On the right is a stack of circuit boards.

 

Removing the circuit boards involves removing the nuts on these jacks.

 

The ribbon cables interconnect the two main circuit boards.

 

The metal piece around the jacks is really part of the noise shielding of the box.

 

We can tip out the bottom board now that the jacks and power adapter jack is free from the rear panel.

 

Interestingly, this little crescent moon-shaped piece is a bushing that makes a small hole out of a large one.  This may have been a design feature for another model of pedal, or, more likely, the result of an engineering change.  Or, my favorite theory, a design screw-up.

 

Back in the early days of electronics, stiffened cardboard was coated with wax to make insulating sheets called ‘fish paper.’  Nowadays, we have synthetic equivalents.  This insulating sheet separates the two circuit boards.

 

To gain access to the top circuit board, we need to remove the control nuts on the top side.  This is the knob for the delay select function.  The knob works fine, the function it performs does not.

 

This plastic bushing covers a larger hole and allows the knob to be recessed below the plane of the top panel.

 

These other controls are more conventional in their mounting scheme.  Those hex drive screws are for looks, and only hold the bezel down on the top of the pedal enclosure.  We don’t have to mess  with them today.

 

At long last, we are able to remove the upper circuit board.

 

We already have a pretty good idea that the rotary encoder for the delay function is not working properly.  To verify that, the unit is partially reassembled for electrical test.  The cardboard is more insulating material to keep everything from shorting out.  Kinda like home-made fish paper.

 

The unit is running and the oscilloscope can tell us if the two phases of signal are coming from the rotary encoder.  The answer is, one phase is missing.  Thus, the rotary encoder can only appear to electrically ‘spin’ in one direction.

 

So, it’s time for the old rotary encoder to come off the circuit board.  A soldering iron will melt the solder and this Solda-Pullit will remove the molten solder.

 

The new rotary encoder is installed, as shown.  This rotary encoder also has a built-in momentary switch that is actuated when the shaft is pushed down towards the circuit board.  Or, in use, when your foot taps the knob while playing.

 

Pretty nice workmanship, wouldn’t you say?

 

Re-assembly is the reverse of dis-assembly.  There are a lot of parts here, so this unit will go back together with the help of all the pictures we took earlier.

 

The insulative sheet goes here.

 

The bottom assembly goes here, with the requisite white cables.  Don’t forget to re-install the little crescent-moon-shaped piece where the connectors poke through the back panel.  I did, the first time.  Oh, yeah, and the metal shield, too.

 

Many of the Boss pedals use this barrel connector for power.  The shell of the pedal housing slide into the slots molded in the connector to firmly hold the power connector in place.  Note that not all barrel connectors have these slots.  If you ever replace a Boss pedal power connector, verify that the replacement part has the slots.

 

This is a little better shot that shows the proper orientation of the shield plate.  The little sharp fingers around the outside edge should point away from the middle of the pedal, so that they can ‘bite’ into the pedal housing.

 

The red and black cable carries power from the internal battery box that we saw in the picture above.

 

The unit is cleaned up and all the knobs reinstalled.  Ready for test!

 

All functions work flawlessly, PLUS the rotary delay time knob both increases and decreases delay time and allows the user to scroll up and down through the menus.  I think we’re done here!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626