Peavey MX Combo Amp Rescued from a Bad Amp Tech

This combo amp had lived a hard life and had finally quit.  Grandpa wanted his grandson to get the amp fixed so that they could jam together again.  Could The Unbrokenstring Crew bring this unit back to life?
 We begin with a quick tour of the rear panel.  The ground switch is a tip of the hat to the Old Days of two wire AC.

 

 The foot switch plugs in where the REMOTE SWITCH jack is coming loose.  This gets fixed.

 

 I’m surprised that this hadn’t ripped loose.  The whole connector wil be replaced.

 

 

 Name, Rank, and Serial Number, please!

 

 What have we here?  We found grandpa’s stash.

 

Let’s get this line cord wired correctly.  Do you know what’s wrong?

 

 The black wire goes under the brass screw. “Black on brass will save you ass.”  You’re welcome.

 

 The reverb tank connects to the main circuit board with this connector.

 

 Even after all this time, the high voltage capacitors are still charged.  Woah!  This is my discharge wand at work.

 

 Our first mystery… where does this nut go?

 

 This is a fuse.  No, you think that it is a piece of 16AWG wire, but it is a fuse.  Or, it is where a fuse goes.

 

 And here, someone was tired of the fuses falling out of the holders, or what was left of the holders.

 

 The heat from the flow of current has wreaked havoc on this solder joint.

 

 This probably smelled bad when it was hot.

 

 Now that the introductions are out of the way, we need to start replacing this nonsense.

 

 These are commercial fuse holders.  These will replace all of the preceding nonsense.

 

 The plan will be to install these new fuse holders at a spot in the circuit where they will be functional, yet out of the way.

 

 The new fuse holders are held down with a screw.  This hole is where the screw goes.  Here goes!

 

 Another hole is drilled for another fuse holder.

 

 This hole is in the center of a trace.  We won’t miss that copper.  Much.

 

 Insulating nylon nuts and bolts are used to keep the new fuse holders in place.

 

The traces in the burned circuit boards are replaced with this Teflon-covered wire.

 

 Everything is now stuffed back into place.  Not too shabby, if I do say myself.

 

Turning our attention to the rear panel, your sharp eyes may recognize this connector as a MIDI female panel connector.

 

 To keep the connector hardware in one place, some of this Thread Locker is all we need.

 

 We have the original foot switch.  It needs a new cable, with a connector to match what we just installed in the amp.

 

 This MIDI cable will be repurposed to replace the cable on the footswitch assembly.

 

 We don’t need this connector.  Instead, this end of the cable will be wired to the switches themselves.

 

 The new cable is soldered directly to the switches  Note the strain relief installed to the right of the picture..

 

 This pedal is ready for action once again!

 

 The amp is reassembled and is ready to go!

 

 The four hour burn-in test is underway.  I think we have rescued another vintage Peavey amp!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Crate Vintage Club 30 Recap

Robin had another of these wonderful Crate combo amps for servicing.  The Unbrokenstring Crew knew just what to do!

Robin’s 50 watt combo was serviced earlier, in this blog post: https://www.unbrokenstring.com/crate-vintage-club-50-amp-repair/

 

This is a 30 watt unit, with very similar construction.  Look at all that St. Louis Music goodness!

 

The loudspeaker is 100% and made in U.S.A.

 

This pic just documents where the wires go.

 

These wires go to the reverb tank.  They are marked with a magic marker so that they can be correctly re-installed.

 

The tubes need to come out as we are removing the printed circuit board from the chassis.

 

These numbers correspond to the tube numbers on the schematic.

 

While we’re here, we’ll make a check of the condition of each of the tubes.

 

Restoring the washer stack is essential to keep the strains on the printed circuit board to a minimum when reassembled.

 

So we are now able to remove the main circuit board.

 

 

As we did with the 50 watt unit, we are taking this opportunity to clean up the front panel.

 

Sure enough, the capacitors have reached End Of Life.

 

This circuit board is now recapped!  Pretty!

 

After reassembly, this quick check shows us that we can drive 30 watts continuously into eight ohms with no problem!

 

This unit has an easy bias setting arrangement.  Here, we’re using the 4 1/2 digit Fluke meter to measure current.  The AC power is supplied by the Variac on the shelf.

 

Some of the cabinet screws were cross-threaded at one time in the past.  So, we can chase them with a die.

 

The captive nuts are cleaned up with a matching tap.  We’re good for Final Assembly!

 

Another fine combo amp is ready for the next million miles!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey 260 Booster Amp Needs a Boost

A local church has a rack of Peavey gear that drives their public address system.  This unit had failed.  A phone call to The Unbrokenstring Crew was all that was needed to get this unit onto the operating table.
Unlike a traditional guitar amp or stand-alone amplifier, this unit takes high level signals and buffers it to the loudspeakers.  They are often used as a means to fill in or expand the coverage of an existing system.

 

And the inputs are daisy-chain-able.

This is, of course, the volume knob.

 

Here is a tour of the rear panel.  The power switch is ‘center-off’ and, on this unit, does not switch the chassis ground to connect to one side or the other of the AC power line.

 

No tour of a rear panel is complete without a high-rez pic of the power cord.

 

And the name/rank/serial number part of our tour.

 

The AC line fuse was popped.

 

This is, correctly, a five amp slow blow unit.

 

Removing the front and rear panels permit access to all the electronics.

 

The chassis is attached to the rear panel of the unit.  Surprisingly, the power transformer is NOT fastened to the wooden cabinet, but is also attached to the rear panel.

 

These filter capacitors are good and will not be replaced.

 

This unit has a shorted rectifier.  New parts were secured, and are shown here.  Although only one rectifier is bad, the other three are the same age and have experienced the same abuse, so they will all be replaced.

 

The new rectifiers are installed.

 

Funny thing is, the new fuse looks a whole lot like the old fuse, except it is not all exploded and burned and stuff.

 

The output from my radio is amplified to a larger signal using my Marshall Stack, to drive the input of the amp at the proper level.

 

This was the point when the neighbors called the cops to complain of the noise.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Jasmine Classical Guitar Needs a Setup

Sophia’s classical guitar had developed a buzzing string.  Perhaps the humidity changes had changed the geometry of the instrument.  She immediately called The Unbrokenstring Crew to take a look at it!
This instrument has a truss rod, something a little out of the ordinary for classical guitars.

 

This is an Indonesian instrument, with lots of mahogany throughout.

 

Using the truss rod adjustment, we make the fret board as flat as we can get it.

 

My Super Duper Absolutely Flat sanding bar goes to work on some high frets.

 

Can you see where the metal has been removed?  This is near the tenth and eleventh frets.

 

One side of the fifth fret was particularly high.  This was the source of the original buzz.

 

The wreckage was bad over the body of the guitar, where the fret board was probably glued straight to the sound board.

 

Before we crown the frets, the fret board will be taped off.

 

To clean and condition the fret board, Dr. Duck’s Axe Wax goes to work.

 

These strings are going back on the instrument.

 

The strings are nicely tied off at the block here.

 

The stringing process is documented in earlier blog posts.  Everything is going well here!

 

Sophia performs live while standing.  So, we’ll add a strap button to the instrument lower bout here.

 

The hole for the pin clears the body of the screw.

 

This strap pin is just above the center line of the instrument. so that it will hang flat against her body.

 

This strap pin is from an inexpensive electric guitar that had strap locks installed.  It goes nicely with the binding.

 

At her favorite venue, Dunn Brothers Coffee in Friendswood TX, Sophia tunes up with an app on her phone.  Life Is Good!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Mesa Boogie Bass 400 Repair and Refurbishment

Craig was very proud of his rack-mount tube-powered bass amp.  But there were crackles and pops while playing.  He wanted to have everything looked-at.  Could The Unbrokenstring Crew get it tuned up?
This is a heavy monster.  Yes, that implies 400 watts out.

 

These are the tubes that were still alive.  We’re missing a couple of pairs.

 

Some contact cleaner/lubricant will be applied with a pipe cleaner into each socket pin.

 

The sockets are in good shape, but just need some chemical attention.  You can still get pipe cleaners, but instead of checking the tobacco aisle, you might look at the gun cleaning supplies section of WalMart to find them.

 

The individual pins on each tube are wiped with the contact cleaner/lube before they are inserted into the unit for testing.

 

The push pull class AB1 pairs are oriented from front to back in the chassis.  Once the operating point was established, pairs of tubes were sorted that had similar characteristics.  So we matched six pairs of tubes to within 2% of each other.  Power and output transformers are seen in the foreground.

 

The amp was driven by a sine wave and operated at 60 watts into a dummy load.  Each pair of tubes were verified in sequence.

 

And this is what six pairs look like while putting 400 watts into a dummy load on a messy workbench.

 

Craig gigged with this amp.  When someone asked what he thought of his rig, he said, “It’s brutal.”

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

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