vintage

Peavey Artist Combo Amp Refurb

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This Peavey Artist combo amp was WAY too distorted to suit even the most extreme metal head. Could the Unbrokenstring Crew look into this and put this unit back into service?
First, a tour.  You can have two channels, or a mix of the two ‘Automix’ inputs.  This was a ‘thing’ back in the day.

 

The other controls are straight-forward.

 

The standby switch is in the front, whereas the AC power on is in the back.  Actually, I like this because if both power switches are in the back, half the time I switch the wrong one.

 

The AC line duties are all squared away on this side.  The City Of Los Angeles has their own version of UL.  That’s the yellow and red sticker.

 

The right hand side of the rear chassis has the ins and outs for this amp.

 

These are all Peavey-branded tubes.  They are all in good shape and will stay in this amp for now.

 

So we put a clean sine wave in, and this is what we get out.  The positive power supply is weak.

 

An overall gut shot shows power on the left, preamp on the right, and power amp on the bottom.

 

These capacitors have begun to swell and push the seals outwards.

 

We have signs of overheating.  These resistors handle power distribution and are somehow related to our problems.

 

The other power supply has a cooked resistor as well.

 

Here I am just documenting all the plugs and wires so I can get them back in the same place.

 

These capacitors are also bulging and will be replaced.

 

Time to remove the power supply board and work it over.

 

This circuit board holds the tube sockets.  We have an intermittent short to ground under this assembly.

 

At first I thought that the short was under the tip terminals of these jacks, but that was not the case.

 

I am going to pull this assembly out and look it over as well.  The blue, red, and brown wires are high voltage.

 

The blue capacitor in the upper right is the ‘death cap.’  If it shorts, 115vac is connected to the chassis.  Not good if you ever touch the amplifier.  Fatal if you touch the amplifier with one hand and grab a microphone with the other hand.

 

This circuit board is supported by the tube sockets.  All four sockets will be unsoldered.

 

Out this guy comes.

 

Here is our short circuit.  These are component leads from parts installed on top of the circuit board and soldered from the top.  I guess if the excess length is out of sight, then it is out of mind.

 

Here are some of the parts on the top side.  I don’t think these were replaced in the field, but rather it came from the factory with the untrimmed leads.  Sloppy.

 

However, wires that are too long are easier to deal with than wires that are too short.

 

I am cleaning up the bits of crap in the bottom of the chassis, using some sticky tape as a way to capture the crap.

 

I have installed new bleeder resistors and new capacitors on this assembly.  That big blue resistor is a high voltage dropping resistor.  This part is fine and will not be replaced.  However, those are not made anymore, and I have some of the last remaining stock of the OEM resistor.  You’re welcome.

 

Everything gets trimmed and cleaned up before reassembly.

 

The power supply board has new caps everywhere.

 

The Ty-Wrap was my idea.  These big parts need some mechanical support, but I’m not big on lots of hot glue.

 

The original power resistors were way out of spec, so these new parts are higher wattage to take the abuse.

 

These caps on the preamp board were replaced.

 

As was this guy.

 

These were the overheated resistors that we saw earlier.

These new resistors are actually more robust than the parts they replaced.  And they are flame-proof.

 

We are back on the air!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Gibson Les Paul De-ghetto-izing and Rewiring

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Quyen found this 2013 Les Paul Custom Lite on Craigslist, and purchased it from a church musician.  The instrument was everything he wanted, except the controls were not the traditional Les Paul controls that he was familiar with.  Could the Unbrokenstring Crew make the control configuration a little less ‘ghetto’?  Let’s see what we can do!

This instrument is really top-drawer.  Bound neck and headstock, gold hardware, totally pimped out!  The body is thinner, partially hollow, and the back of the instrument is contoured to feel a little more comfortable.

 

Here’s the rub.  The coil splitting duties are performed by this mini-toggle switch, which takes the place of the second tone control.  The owner wanted the ergonomics of the Custom Lite instrument but with two tone controls.

 

The control cavity is under this plate.

 

To simulate single coils, one side of each pickup coil is grounded.  The ground wire has yellow insulation.

 

With one tone control to rule them all, this little blue capacitor and the control under it is the whole tone circuit.

 

This pic just documents the bridge ground wire.  None of this needs to be altered.

 

If you compare this control cavity with other Les Pauls, you can see that it is much shallower, because the body is thinner.  Otherwise, it is not hard to visualize a two-volume, two-tone setup here.

 

To preserve the ability to go single-coil, a tone control with a push-pull pot is evaluated.  Here, a drawing is made of the actual cross-section of the guitar body at the instrument cavity, and the exact dimensions of the push-pull pot is recorded in the notebook.  Making this modification fit properly will require some careful gun-smithing.

 

The center of the hole where the mini-toggle switch originally lived is in the correct location for the second tone control.  Here, a stepped bit is used to carefully enlarge the bore in the body to accommodate the push-pull pot.

 

The inside of the hole is cleaned up a bit with the stepped bit.  The loose wires are taped out of the way and the finish on the guitar is protected with blue low-tack painters’ tape.

 

The bare maple exposed by the stepped bit is refinished with some appropriate stuff.

 

Pushing on the felt tip of the pen dispenses black lacquer.  This step is not entirely necessary, because none of this will be visible, but there is no sense in leaving bare wood exposed to Houston humidity.  And I’m OCD.

 

The threaded portion of the push-pull pot is long enough to work in a Les Paul body.  However, the switch portion is nearly too long to fit under the control cover.  Likewise, we will use another nut to get the control knob height to match the height of the other three knobs.  We have some fiddlin’ ahead of us.

 

The three bent leads on the potentiometer were bent a little more to keep them away from the edge of the spot-faced recess in the body.  You can see this by comparing this picture to the previous one.

 

This tang provides mechanical support for those instances when this particular switch is used on a printed circuit board.  But, it extends past the end of the switch.  So, it has to go.

 

The tang is gone.  The two fingers that hold the end of the switch housing in place were pressed down.  This is about as compact as I can make this assembly.

 

The paint is dry!

 

The switched potentiometer is mounted, and the height of the shaft is adjusted so that, when the knob is installed, it will appear at the same height as the rest of them.

 

The owner wanted the vintage ’50s wiring for this instrument.  Already, we are about 3/4ths there.

 

This horrible solder joint was almost certainly done at the factory.  I can’t let this go out of the shop like this!

 

These orange 0.022uF caps are just the thing to match the 50’s wiring esthetic.

 

To tie the switched potentiometer body to common ground, I am soldering the ground wire to the interface between the pot and the switch.  That is a weak place that can use all the support it can get.  Just looking at the long haul, folks.

 

The yellow ground wire is added, and the original switched wires from the pickup windings were transferred to this switch.

 

Now this looks more like a 50’s Les Paul wiring cavity.  No circuit boards here!  The cavity cover fits without interference.

 

More OCD-ness.  The output jack stuck out too far.  These washers were redistributed and a nut added inside.  I feel better now.

 

Does this look factory?  Mission accomplished.

 

This Cragislist instrument is a real winner!

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Crate Vintage Club 50 Amp Repair

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St. Louis Music produced this handsome tube-based Crate amp. Robin said that this unit is ready for some service.

I just love the blonde Tolex and clean appearance of this unit!

The front panel reveals that this is a single input, two channel unit.  The clean channel sports conventional treble and bass controls.  Bringing the effects loop to the front panel is a nice touch!  This jack is stereo, for in/out with one jack.

The gain channel has separate gain and volume controls, and a mid-tone control.

The foot switch is wired in parallel with the channel select switch seen above.  Note the separate controls for reverb for each channel!

These loudspeakers are original and still in great shape.

Taking a tour of the bottom of the chassis, this unit has an attached line cord.  The power transformer is affixed at an angle, as seen on the left.  The grey wires go to and from the reverb tank.

Five minutes of visual inspection is worth thirty minutes of troubleshooting.  Can you see the problem?

To remove the chassis, we need to unsolder the cables to the reverb tank.  I marked this one so that it can be reinstalled in the proper place on the circuit board.
The loudspeakers are wired in parallel.  This pic documents the wiring colors.

The chassis is now free of the cabinet.  A quad of EL84 tubes serve as the class AB push-pull output circuit.

The preamp tubes, in the foreground, and all 12AX7s.

Removing the brackets on the output tubes is a little gnarly, though.  This screw will be replaced.

Silicone foam cushions the envelope of the tube from  the stresses of vibration and thermal expansion.

To do any soldering on the main circuit board, all the controls need to be disassembled.  Marshall knobs, anyone?

We have three jacks to disassemble in order to liberate the electronics.

The primary and secondary wiring is easily disconnected from the circuit board, as shown here.

One more chance: Do you see any electrical issues?

We have the main circuit board well in hand.

Some glue was used to support the large mechanical parts.  This stuff needs to be chipped away.

Here, we are checking the caps in-circuit with the Heathkit capacitance checker, just to verify that they are toast.

My only criticism of this amp (and others) is that the tube sockets are directly attached to the circuit board.  The thermal expansion is more than the average solder joint can stand.  Wire those sockets by hand; don’t mount on a PCB!

Here we are with new capacitors.  All of the solder joints have been inspected and reworked as required.

With the unit disassembled, this is a good time to clean up and polish that gorgeous front panel.

The wrong sized fuse was installed.  The Unbrokenstring has a complete stock of fuses, for such a moment as this.

Remember the broken screw?  Here, we are drilling and tapping the chassis for a new screw.

The blue masking tape was deployed on the inside of the chassis to catch all the metal shavings.

The screw fix worked!  As we knew it would.

The reverb tank wires are fished back through the hole in the chassis and refastened.

Everything has been reinstalled and we are ready to test.

This cover conceals the bottom of the amplifier chassis.  The amp was a little noisy.  Could some shielding help?

Aluminum foil tape is added to the cover.  This will be electrically in contact with the chassis of the amplifier.

Aluminum shielding goodness at its finest!  This is all on the inside, so no one but you will ever know that it’s there.

This Crate is very versatile – the clean channel is almost hifi quality, while the gain channel is a vast territory of sonic goodness to explore.  And LOUD!  Pick one of these up if you find one.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Professional Advice for Those Caring For Flood-Damaged Guitars

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Roy Bowen and Scott Leedy of RS Guitarworks in Winchester, Kentucky had it in their hearts to reach out to Houston area musicians, documenting their experiences gained while working with flood-damaged guitars during the big flood in Nashville back in 2010.  The Unbrokenstring Crew is honored and blessed to be able to publish this document for your edification.  This supplements my previous post, which was general in nature and included electronics.  To wit:

 

Guitar Flood Triage

Below is a list of steps that we developed while restoring guitars from the Nashville flood and Hurricane Sandy.  The following steps should be taken as soon as possible.

 

  • First remove strings.

 

  • If truss rod is not removable, loosen truss rod ½ turn, add a couple of drops of penetrating oil on the threads then tighten ¼ turn.  On vintage Fender or Gibson guitars where the truss rod can be removed, remove truss rod lug. Oil the threads then re-install snug but not tight.  You want to make sure threads are lubed so the truss rod does not rust in place.

 

  • Remove all parts from body including hardware and electronics.  Wood will try to move and solid items like screws & tuners will not let it move.  This can cause cracking of the wood, as well as cause rust staining in the wood.

 

  • On bolt on instruments remove the neck.

 

  • Take all hardware and place it in bags filled with uncooked rice and leave it until you are ready to re-assemble.

 

  • All electronic parts such as pots and switches should be cleaned with a quality electronics cleaner and then stored in uncooked rice as well.

 

  • Air-drying of the wood is best after parts have been hand dried inside and out.  Keeping guitars in a small room with a dehumidifier can also help.

 

  • Do not try to use any cleaners on the finish or hardware of the guitar until it is properly dried and stabilized.

 

  • Do not try to tape down loose binding or wood coming un-glued as it may damage fragile finish loosened by water

 

  • Get the guitar to a qualified repairman as soon as possible.

 

Be sure to visit http://www.rsguitarworks.net to tell them THANKS and to check out their restoration work with other water-damaged guitars.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Fender Rhodes Electric Piano Amp Refurbishment

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A Southeast Texas area church had a wonderful Rhodes Piano that needed some attention.  Whenever the unit was powered on, a loud hum was all that came through the speakers.  Time for the UnbrokenString Crew to go to work!

The Fender Rhodes Piano consists of a keyboard section, containing keys, action, and tuned rods that work in the same manner as tuning forks. The rods vibrate when struck, and the motion is sensed by a coil not unlike a guitar pickup. From there, the signal is sent to the other section, a powered stereo loudspeaker assembly seen here.

 

The speaker cabinet is two-faced e.g. loudspeakers fire from both the player’s side and the audience’s side.  A pair of loudspeakers are assigned to each output of a stereo amplifier.  The pairs of loudspeakers are across from each other in the cabinet, one firing forward and the other one back.  This enhances the swirly, phased sound of the instrument.

 

These loudspeakers are Fender branded CTS units.  The metal box in the background contains power and input circuitry.

 

CTS built these loudspeakers in June of 1975.

 

This part number indicates that these are 32 ohm AlNiCo loudspeakers.  This is a standard-issue Rhodes Piano unit.

 

This voice coil is totally cooked.  The motor drags badly in the magnet.

 

This voice coil is open-circuit but moves smoothly in the magnet.  Is a repair possible?

 

There is the broken voice coil wire.  This wire is really cooked, so we will elect to replace the loudspeaker with a pair of modern 8 ohm units wired in series, to yield the proper 16 ohm load to the amplifier.

 

We have removed the panel at the end of the cabinet.  The power transformer is visible to the right.  Each channel has a separate input here.  Also, a special cable from the Rhodes keyboard attaches here.

 

The power cord for this unit is no different than an extension cord.

 

Instead of a regular extension cord, we will use a SmartPower unit to power-up the unit and protect it from surges.  Think ‘mini-Furman unit.’  I also sell these, BTW.

 

These transistors read as short circuit.  I think we now know everything we need to know to make an intelligent quotation.

 

Name, rank, and serial number please.

 

One output of the power supply assembly is 25vdc for the keyboard section.

 

The keyboard voltage is set by a potentiometer accessible through this hole.

 

The power supply filter cap is in great shape for its age!

 

Likewise, these guys look great and test good.

 

Everything here is as it should be.

 

Steven removed one of the damaged loudspeakers.

 

Over the years, the gasket glued itself to the cabinet.

 

A little extra cleanup won’t hurt a thing.

 

The circuit board for the power amplifier is a hand-drawn affair, typical for the 1970s.

 

The board designer was nice enough to add is some text that would help the amp tech find his/her way around.

 

Some power resistors were burned up.  All of the components to the right of the transformer were replaced.  The transformer is for inter-stage coupling, not power.

 

Some power transistors were hand-selected for duty in this amplifier.

 

The repaired amplifiers are re-installed in the bottom of the cabinet.

 

We are ready for final test!

 

Amplifier design has certainly changed over the years.  This is a unique design that has withstood the test of time very well!  The customer was VERY pleased with the finished job.  Weather Report Cover Band, anyone?

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626