Epiphone Dot Gets a Customized Bigsby Tremolo

Matt with My Twilight Pilot lusted for a black Epiphone Dot with a Bigsby tremolo assembly. Could the Unbrokenstring Crew rise to the occasion and make it happen?
It don’t get mo’ prettier than this!

 

The Bigsby needs a small modification, so that the tremolo bar can swing over the strings.  Here, the Bigsby is disassembled so that the modification work can begin.

 

This piece limits the swing of the arm.  So, we will carve on this to change the limits.  We don’t like limits.

 

After the carving, the metal is sanded to a uniform satin.

 

Successively finer grits remove the scratches from the previous operation.

 

This metal polish is made in Germany.  The Unbrokenstring Crew uses this stuff to polish fret wires.  Likewise, it will polish this Bigsby so that it gleams.

 

We’re just about there with the modification.  It can stand a little more carving.

 

The tailpiece is centered on the centerline of the strings, not necessarily the center seam of the lower bout.

 

When the body of the Bigsby is correctly positioned, there are two places where the metal touches the finish of the guitar.  While we are adjusting everything, some painters tape protects the finish.

 

We are committed at this point.

 

Two strings are installed to tension the Bigsby into position.  The location of the two additional screw holes are determined so that the whole assembly is straight with the neck before the holes are drilled.

 

This drill bit is in a spring loaded shell, which forces the actual bit to be in the exact center of the hole.

 

The strings are ON and now we can proceed with the setup of the Bigsby. See the small nylon washer where the spring fits?  This is an essential part of the setup.

 

String tension lowers the height of the tremolo bar.  We need to be in tune before doing much more of the setup.

 

The StroboTuner is spinning away while we tune and set the intonation of the instrument.

 

What is this?  We have a couple of screws that are having an acetone bath.  Why do you ask?

While we’re working on the guitar, these strap locks are going onto the guitar.

 

This is the half of the strap lock that goes on the strap.  Both sides are shown.

 

Those screws that were washed in acetone were painted with black lacquer, and cover up the inserts in the body where the original stop bar was held.  Pretty, isn’t she?

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

 

Peavey KB4 Keyboard Amp Disintegrating

This Peavey keyboard amp works intermittently, but the modular AC outlet has completely come loose from the rear panel. Can the Unbrokenstring Crew fix this?

This unit is pretty cool, with a built-in luggage roller and extendable handle as standard equipment from the factory.

 

The modular AC plug was covered in RTV rubber.  Was this a user ‘fix’ or did it come from the factory this way?

 

Fortunately, all the electrical conductors were insulated.  Otherwise, we would have sparks.

 

User controls are on the top.

 

The ground polarity switch is a throw-back to the days of two wire electrical cords.

 

A simple mixer is integrated into the unit.

 

Name, rank, and serial number, please.

 

The panel layout allows for some space for the handle.  Good Job!

 

We removed the head from the cabinet,

 

Here is our intermittent.  This power resistor had broken free from its solder pad.

 

A circuit board trace had broken.  An Exacto knife clears away some of the solder mask to allow for a repair.

 

This crack was very small, so a good solder jumper is all that is needed here.

 

This unit appears to have been wet.  Do you see the minerals left behind after the water evaporated?

 

Here is another little blob of mineralization.  This may have been from solder flux residue left after the assembly was manufactured.  Some fluxes turn white in the presence of water.

 

This screw was loose inside the grille.  This screw holds the loudspeaker in place.  This is not good.

 

Most of the screws were loose.  While we have this unit on the bench, we should be sure that the loose screw is not a sign of a more sinister problem lurking with this unit.

 

Here is the loudspeaker in this unit.  Nice!

 

My guess is, humidity has softened the baffle upon which the loudspeaker is mounted.  These Tee nuts will be removed and new holes drilled in the baffle in different locations.  The Tee nuts will be reinstalled and we should be Good-To-Go.

 

Some black nail polish will camouflage the new fastening hardware.  You do have black nail polish, don’t you?  Doesn’t everybody?  Hint: This also makes good thread locker.

 

OK, now for the IEC power jack.  This unit has mounting ears, so we won’t rely on friction or glue to keep it in place.

 

As an added bonus, this IEC jack has a built-in noise filter.

 

The hole in the chassis was enlarged to accommodate the new jack.  This hand grinder is adequate for the job.

 

Yep!  Just fits.

 

Now we will bore the steel panel to accommodate the mounting hardware.  This will be SO much better than glue!

 

This doesn’t look too bad, does it?

 

All of the original wiring goes straight onto the new IEC jack.  This is better than factory!  Hot glue was apparently used at the factory to secure the switch and the old IEC jack to the rear panel.  So that answers that question.  Shame on you, Peavey!

 

Here is one last look of the internals before we reassemble the head.  The mixer is at the bottom and the power amp and power supply is at the top.

 

This unit is literally ready to roll!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Behringer Ultrabass BX4500H Bass Head Needs a New Fan

Billy said that this unit worked very well until it overheated and shut down after about an hour and a half of use.  He also didn’t hear the fan run.  Could something this simple be all that is wrong with this bass head?

The Unbrokenstring Crew operated this unit for an hour and verified that it overheated and shut down, as it should.

 

Here is a tour of the rear panel.  The power switch is in the back of the unit.

 

I really like the Neutrik Speak-On connectors for power at this level.  The 1/4th inch plugs could be operated above their data sheet limits for current and voltage if used at the 450 watt level.

 

We’ve removed the top of the unit.  Removing the finger guard lets us verify that the fan itself is completely locked up.

 

To get to the fan, we need to remove the heat sink assembly, which is held in place with these screws on the bottom.

 

This pic just allows us to keep the wiring straight for reassembly.  This circuit board handles the power output duties.

 

The fan is bolted to the end of the heat sink assembly.  The power amp is also attached to the whole stack.

 

We need these specs in order to specify a replacement.

 

This power cable is specific to the mating connector on the circuit board that supplies the 24v for the fan.  We need to keep this and transfer it to the new fan.

 

There is a chance that we can remove the whole cable assembly from the old fan and move it to the new fan.

 

The new fan has three wires.  The third wire is probably a tachometer output so that the speed of the  fan can be known.

 

The wires on the new fan are soldered to the fan with lap joints.

 

So, we can make new lap joints when moving the old cable to the new fan.  I’ve got a good feeling about this.

 

The new fan is bolted back on the stack and the whole arrangement is put back together.

 

This amp ran for four hours continuously until the neighbors called the cops complaining of a noise ordinance violation.  I believe that the overheating problem is fixed!

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

 

Behringer T1953 Preamp Refurb

This preamplifier and microphone processor is in the audio chain of every hit produced by Majic$tyle Studios over in Houston’s 3rd Ward.  But it quit one day.  So the music quit.  Could the Unbrokenstring Crew help the Majic$tyle Crew bring back the music?
Like many recording facilities around Houston, the modest studio in the back room has a lot of history.  A lot of talent in the Houston Urban Contemporary and Hip Hop scene have stood in front of this box, layin’ it down.

 

This unit needs an overhaul.  A quick tour of the front panel reveals input level controls for two channels.

 

Phase and tone controls are here.  The tone control can be completely bypassed, which can be useful.

 

These inexpensive meters actually do a pretty good job of displaying dry level and processed (wet) levels for each channel.

 

The ‘warmth’ is another term for the wet signal, routed through a starved-cathode tube circuit.

 

Power input and device info is found on the back.

 

All of the inputs and outputs are on the back.

 

The cabinet screws are a different length.  This is noted.

 

The DC power management and toroidal line transformer are located in the center.

 

All of the rear panel wiring is done with a circuit board, with the wiring harnesses glued to the chassis.

 

We are missing some DC voltages, so our investigation will focus on this circuit board.  What a heat sink!

 

Toroidal transformers are the way to go in high quality audio equipment.

 

Behind the front panel, we see a pair of 12AX7 dual triodes living on this circuit board.  The incandescent light bulbs are burned out, but are for aesthetic purposes only.  We will have some fun with these!

 

The rest of the front panel circuit board is almost entirely encased in sheet metal, as shielding.

 

I documented these cables, in case I needed to disturb them.

 

Can you see the bulged electrolytic capacitor?

 

Hot glue is used to keep all the connectors connected and all the big parts from moving around.

 

With the wiring harnesses removed, this circuit board is held in place by screws that fasten the power semiconductors to the heat sink.  When those screws are removed, the circuit board assembly can be repaired.

 

Remember the bulged capacitor?  It leaked out the bottom.

 

This power supply will get a full cap job.

 

The electrolyte from the leaking capacitor has chewed away some metal from the circuit board and a couple of the solder joints.  This will need to be taken into consideration when the new parts are installed.

 

The blue jumper wire restores continuity on the corroded trace seen in the previous picture.

 

New caps are installed!

 

Here is another view of the new skyline of our power supply board.

 

Heat sink grease is used to insure thermal transfer from the power semiconductors to the aluminum heat sink.  This tube of compound will probably live longer than I will.

 

The tabs of the power semiconductors SHALL BE isolated from the heat sink.  Here, an ohm meter checks for isolation.

 

Those incandescent light bulbs are not part of the signal chain, but just serve as a back light behind the vacuum tubes.

 

These are 24v bulbs.  I could just replace them, but why would I pass up a little fun with just a stock replacement part?

 

The tip of the base of the bulb is actually a specially shaped bead of tin/lead solder, which holds one of the terminals from the filament of the light bulb.  So, I desoldered it.

 

One of the leads from the filament goes to the side of the metal base of the bulb.  So I unsoldered as well.

 

This is a better view of the base, showing the central terminal.

 

The glass bulb comes out.  Besides the broken glass, you can see bits of glue on the rag.

 

The plan is to add LED lights in place of the bulb.  This is the series limiting resistor for 24 volt service.

 

It hides inside the base, as shown.

 

These red LEDs are built on top of the resistor and are supported by the bulb base.  This is gonna be cool.

 

The meters are also internally illuminated.  These meters will get yellow LEDs.

 

The front panel will get a million-mile cleanup and polish.  Off comes the knobs.

 

These bushings keep all the knobs turning smoothly.

 

Now that the front panel is free, it can be cleaned and polished with Gibson Guitar Polish.

 

Removing the knobs is the only way to get the grime out from down around the knob shafts.

 

The clear windows are polished on both sides.

 

Behind the aluminum panel, we have rotary encoders for the wet level control duties.

 

All the low level audio is contained on this double-sided circuit board.  Any copper that isn’t signal is audio ground.

 

The switches are cleaned and lubricated.

 

With the front panel removed, this chassis is just floating.

 

Let’s take a quick peek underneath the audio shield.

 

Audio gain duties are handled by TL074 opamps.

 

Here are the microphone preamp chips.  These are surface-mount JRC4580s.

 

Here is the other mic preamp 4580.  OK, we’ve done our gut shots for the day.

 

Now let’s get these meters done.  Hot glue is everywhere.

 

Once these meters are removed from the front panel, they can be easily serviced.

 

The lens and front bezel come right off.

 

So far, we haven’t destroyed anything.

 

The meter scale just lifts out of the body of the meter.

 

Here is where we are so far, keepin’ it real.

 

If we peer inside the body of the meter, we can see a ‘grain of wheat’ lamp.  These are all burned out.

 

I will attempt to reuse the leads to each of the bulbs.  The burned out bulb is removed.

 

To get a little bit more room to move around, I’ve removed the meter movement.  Be very careful!

 

The meter bulbs are 12 volt units, and two of them are in series because only 24 volts is available.  This is the LED current-limiting resistor for one of the meter illuminating LEDs.

 

I purchased a bunch of these 1% resistors years ago from Texas Instruments (TI) and so these are probably collectors items.  Why not use them on this project?

 

Here is the yellow LED and resistor installed inside the meter body.

 

Now the meter face is reinstalled.

 

The clear lens is popped out of the bezel for cleaning and polishing.

 

Everything is reassembled and the meter movement is zeroed.  Ahhh, that’s done!

 

This shield was loose under the circuit board assembly that held the tube sockets.  What’s up with that?

 

Turns out, these standoffs were unscrewing from the bottom.  More LokTite, please.

 

This is going to take three hands.

 

Unfortunately, I only have two hands.  But The Unbrokenstring Crew can make it happen!

 

Let’s reinstall the front panel and do a final assembly.

 

The meters are gently torqued onto the panel.

 

The knobs and bushings are restored.  This is starting to look nice!

 

The red and yellow LEDs are doing their job.  Unfortunately, the .jpg engine in my phone camera is making the red LEDs behind the vacuum tubes look a little gaudy.  Oh, well.  More importantly, this mic preamp works well and is DEAD SILENT with no input signals.  I think the rebuild went pretty well!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

KRK Rokit8 Powered Studio Monitor Repair

Steve used these monitors in his studio constantly. Unexpectedly, they broke. Can the Unbrokenstring Crew bring them back to life?

These monitors have no grille so the raw loudspeakers were given a little protection for the trip to the shop.

 

These are entirely self-contained, with power supply, amplifier, input attenuation, and high frequency controls built in.

 

Name, rank, and serial number, please.

 

The high frequency control rolls off audio energy going to the tweeter, thus acting as a simple tone control.

 

The balanced input line is a nice touch!

 

The fuse is located in a drawer built into the IEC power jack.  This fuse is open.  What caused this?

 

Let’s take a look inside.  I like all the acoustic attenuation batting inside this unit.

 

The swollen electrolytic capacitor is highly suspect.  Let’s test it.

 

This Chinese capacitor meter shows that all is well.

 

This kit ESR meter shows a very reasonable number for internal losses for a capacitor this size.  What gives?

 

The old 1950s technology Heathkit capacitor checker tests this unit at its working voltage.  Only at the full working circuit voltage does this capacitor fail.  Old Stuff Rules!

 

A couple of matched electrolytics are installed.

 

These monitors live to rock it another day!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey Mark III Bass Head – Busted Controls

These sturdy old bass heads turn up in pawn shops and Craigslist from time to time. They soldier on for years, sometimes making music for decades. This unit came in for some minor repairs and a Million Mile Checkup. Let’s get to work!
This unit has LEDs for the power indicator and a clipping indicator, or something called “Compression.”

 

From the school of ‘crank it up and rip the knobs off’ we have a knob that has been ripped off.

 

Most of these Peavey heads usually consist of a preamp, mixer, and/or EQ assembly behind the front panel, and a rear panel that holds an amplifier and power supply.  The transformer is bolted to the case in the middle.

 

We need to get to the circuit board, so all the knobs and nuts come off.

 

This unit was built in the era of the Plastic Potentiometer Shafts.  Grrr…

 

A few screws keep the panel and circuit board flat.

 

More screws go into the magnetic parts holder.  We are almost there.

 

Here, at last, is the circuit board.

 

Potentiometers that stand off the circuit board like this are sometimes called ‘spider’ pots.

 

These other controls are fine.  The values 10K and 50K refer the resistance, and the letter ‘B’ implies that the taper is linear.  An ‘A’ letter implies an audio taper control.

 

The Alps company made these in Brasil.

 

Here is the new replacement part, with a metal shaft!  If this were my unit, I’d replace them all with metal shafts.

 

The plastic shaft broke off inside this knob.  A few minutes with an Exacto knife is all it took to reclaim the knob and cut my thumb..

 

The new control is soldered into the circuit board.  The factory workmanship on this assembly is pretty good.

 

Back together it goes.

 

With the knobs in place, you can’t tell that anyone has been here.

 

One last look as we reassemble the unit…

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey Bandit Combo Amp Refurb

David found this fine old Peavey combo amp in a pawn shop for almost next to nothing. Could the Unbrokenstring Crew resurrect it?
This saga begins on a cool Saturday morning in the Guitar Center Pasadena parking lot.  If you look closely, you can see right through this unit.  Literally.  What are we getting ourselves into this time?

 

Electrically, this unit is mute.  No sound comes from it.  But, as the story is told, this amp languished in the barn for years so we have to expect the worst.

 

Name, rank, and serial number, please.

 

This part number goes to a Peavey Spider loudspeaker.  This is gonna be fun, as you shall soon see!

 

The grille is off.  This poor thing is a mess.

 

If a rat gets hungry enough, it will eat anything.

 

Perhaps it was not tasty enough, as the rats left plenty of bits of uneaten loudspeaker cone behind.

 

The brunt of the rat nasty-ness was taken by the reverb tank cover.  We need to clean this up right away!

 

The spring reverb tank is in perfect condition.

 

The tank cover did its job and kept this reverb tank in a pristine condition.

 

Pledge furniture polish contains solvents and emulsifiers that are excellent for cleaning Tolex.

 

The nice lemon-y scent is just the thing to counteract the barn smell.

 

The bottom of the unit is a mess, though.

 

The amplifier electronics are on the bench.  The unit is upside down.  The reverb tank connector is to the left.

 

The bottom cover comes off.  We need to go over this unit with a close eye for damage due to humidity and dust.

 

Perhaps moisture (rat urine? mouse-ture?) has seeped under the solder mask and attacked the copper on the circuit board.  Everything gets a bath.

 

The top side of the electronics is pretty filthy, as we might expect.

 

The circuit board is free of the chassis.  Everything is being scrubbed with water, alcohol, and compressed air.  The switches, jacks, and controls are cleaned, flushed, and lubricated.

 

Tracing signals, this guy is bad.  This IC is replaced from stock.

 

I want to clean up around the cases of the transistors.  The heat sink is at zero volts while the transistor case is at +70 volts or so.  The dirt may bridge across the insulator.  Not on my watch!

 

These power transistors are just fine.  They are also super-rare.  Luckily, I won’t have to replace these.

 

But they sure do clean up good!

 

The transistor on the left is already installed.

 

We use Wakefield heat sink grease on power devices at the Unbrokenstring Shop.  Old Skool.

 

The original mica insulator is reused.

 

This is actually quite messy, but is a thing of beauty when I do it.

 

Both sides of the washer are coated with compound.

 

The whole stack is installed as shown.

 

While we are here, some of the solder joints could use some attention.

 

This amp is now delivering 75 watts into 8 ohms.  This chassis is Good To Go!

 

Back in the case it goes.

 

These jacks are on the rear of the unit.

 

The knobs press on.

 

This guy goes in the trash.  Wait?  Aren’t we going to recone it?

 

No.  We don’t have to recone.  Peavey Spiders are available without the magnet.  No tricky alignment issues here!

 

When the magnet was removed from the old loudspeaker, the magnetic gap was cleaned and then sealed shut with tape.

 

Once the old magnet is aligned, three screws hold it in place.  We’re done!

 

This is the new loudspeaker, seen from the front.

 

A rear view shows us the cleaned cabinet and reverb tank cover.

 

The loudspeaker looks new, because it is new.

 

The grille was cleaned up and is seen here, reinstalled.

 

The refurbed chassis slides in from the rear.

 

These washers go underneath the trim plates thru which the chassis screws extend.  Don’t leave these out!

 

These big trim pieces are metal, coated black.

 

And these, boys and girls, are how the chassis is suspended inside the cabinet.

 

Installing this trim piece is all that is left to do.

 

David is really tickled with his like-new Peavey Bandit!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

 

Marshall AVT50X Combo Amp Has Issues

The customer was headed to the recording studio with his favorite amp.  However, our patient has two issues. When the unit operates, the fan is noisy and the output signal comes and goes when you tap around the headphone jack. When you don’t hear the fan anymore, the whole thing has quit. Can the Unbrokenstring Crew make this work AND make the fan quiet at the same time?

This hybrid combo amp has a lot going for it. Besides being Marshall loud, the unit is light-weight and has some moderately cool features such as a lead and rhythm channel, solid state reverb, a 12AX7 preamp tube for that tube sound, a headphone output, and a CD input so that the aspiring bedroom rock star can play along with their favorite real rock star.

 

An unexpected bit of excitement is, this is actually a Bletchley-built unit, factory-built for export to the United States.

 

Removing the chassis from the cabinet is straightforward.  Here is a view from behind the front panel.  The power switch is to the left.  The headphone jack and the CD input jack are in the center.

 

The front panel controls are those green vertically-mount potentiometers.  The input jack is on the right.

 

Looking inside the rear panel, we see the IEC power jack.  The power transformer and heat sink dominate the center of the chassis.

The rear panel jacks on the rear panel have their own circuit board.

 

We need to remove the main circuit board, as The Unbrokenstring Crew has quickly identified many intermittent solder joints that have caused this unit to quit.  We are going to be giving the soldering iron a workout today.

 

These pictures that follow document where the cables plug in.  All of the connectors are polarized.

 

The blue wires go to the final amplifier, which is a high power integrated circuit.  Power for the amp is carried by the white wires.

 

These cables carry the output signal to the rear jacks.

 

More cables.  The white wires are the AC mains cables.

 

These cables go to and from the power transformer.

 

The knobs and now pulled and the retaining nuts are removed from everything.

 

The main circuit board is free!

 

Now that all those knobs and jacks are out of the way, we can give the front panel a good cleaning.

 

The Gibson Guitar Pump Polish does a good job of removing grime and fingerprints.  And it smells nice!

 

The headphone jack has a wide circuit board footprint.  Here is the replacement jack compared against the output jacks.

 

The original shield from the bad headphone jack is moved onto the new part.  We’re changing the headphone jack because the preamp output signal goes here and either passes thru to the output amplifier, or goes to headphones.  When NOT using headphones, the signal to the output amplifier was intermittent, because of the dirty switched contacts on the original jack.  These cost less than a dollar so replacement is faster and better than cleaning the original.

 

The headphone jack is 100% now!  Because it’s brand new.

 

Some of the controls needed to be changed out.  The new parts arrived today!

 

Have you ever seen a four-terminal potentiometer?

 

These are all changed out.

 

Next, we need to look at the noisy fan.  This is the power amp / heatsink / fan assembly.

This is an inexpensive 12vdc fan, similar to the ones used in personal computers.

 

Perhaps we can oil this guy and shut him up.

 

Well, that didn’t work.  It’s worn out.  Off it comes.

 

We can salvage the electrical connector and install it on a new fan.

 

The new ‘silent fans’ came as a pair, so this heat sink is going to get two fans.

 

This fan is oriented to move some air across a power resistor on the main board.  Because I’m an Engineer, that’s why.

 

The other fan is oriented the same way as the original fan.  IMHO this makes a little more sense than the original setup.

 

This is the top view of the final setup.

 

When removing cables, a lot more than the cable came loose from the circuit board assembly.  See the hole?

 

This pin used to be soldered to the board through that hole.

 

These pins are tin plate over steel.  They have a larger thermal mass than the other components on the circuit board, and therefore MAY have not been at a high-enough temperature long enough to make a good solder joint.

 

These pins will be re-tinned with tin/lead solder.  The entire circuit board will be reworked to make it NON-lead-free.

 

This stuff is rosin-activated solder flux.  This enables good ‘wetting’ of the tin/lead solder joint.

 

This is a beautiful NON-lead-free solder plate over steel.  The whole amp gets this treatment.

 

Lots of those pins came loose when the cables were removed.

 

Those bad solder joints go a long way in explaining why this amp became intermittent.

 

This amp was manufactured while the world was converting to lead-free solder.  So, the process guys were still learning what worked and what didn’t work.  These didn’t work.

 

All of the solder joints in this amplifier were reworked by removing the tin solder and reflowing with tin/lead solder.  The Unbrokenstring offers this service for those who wish to have the MOST reliable gigging and recording equipment.

 

Lead-free solder is NOT reliable.  Exemptions from the lead-free directive (RoHS) have been issued to automotive, avionic, energy/down-hole, and medical electronics precisely because it has proven to be unreliable.

 

Here is our new headphone jack again.

 

We are ready to assemble the unit.  Now where did I put all those knobs?

 

Four hours of continuous testing proves that this combo amp is in top shape!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626