Keyboards and Synths

Peavey KB4 Keyboard Amp Disintegrating

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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

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

Kurzweil K2500XS Synth Repair and Recondition

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The youth worship leader called The Unbrokenstring Crew, having exhausted all other avenues available to him to get their synth repaired.  Could we step up to the plate and fix the various issues with this unit?

If you look closely, you will see a bit of green tape on the E key just above middle C.  That key plays loudly, or not at all.  There is no in-between, which is a problem while playing an offeratory.

Houston, we have a problem.  These batteries have taken their leaking game to a whole new level.

Fortunately, this battery box can be replaced.  But how much other stuff has been contaminated with electrolyte?

We need to split the case open to look at the wiring and circuit boards.

These screws hold the key bed in place.  This unit is so heavy that I decided to get some of these out while I was here.

This is what the inside of a synth looks like.  The battery box is to the left of center of the picture.  Everything is wet.

The ‘green tape’ issue is resolved by disassembling the key bed.  Out it comes, and it is heavy!

The screws holding down the pitch wheel assemblies are located on the bottom of the chassis.  This allowed all the battery electrolyte to be neutralized and cleaned out.

This key bed is from Fatar.  It is ‘semi-weighted’ and, to me, feels just like a real piano.

The key bed is tilted up to gain access to the bottom of the assembly.

I labelled these two ribbon cables B for bass and T for treble.  This corresponds to the left and right half of the keyboard, respectively.

These short jumpers go between two long circuit boards.  Note the white zero-ohm jumpers on the PCB.

Now we remove a whole bunch of screws to gain access to the other side of these circuit boards.

The long thin circuit boards are now free from the key bed chassis.  We see the objective of our mission!

These rubber strips are push buttons that close a pair of electrical circuits when a key is depressed.

There are two push buttons per key.  One button plunger is longer than the other.  When a key is depressed, the amount of time between one button and the other is used to derive velocity information for that note.

And here is our ‘green tape’ problem.  The longer button is torn, so the keyboard scanner only saw the shorter switch closure.  In a normally-operating keyboard, the longer button closes first, followed by the shorter one.

New keypad strips are available for this unit.  After inspecting the entire keyboard, I replaced two of the keypad strips for the octaves around middle C.

All of the strips were removed and the circuit board cleaned.  Note the pairs of carbon contacts on the PCB.

The key strips must fit securely in a precise orientation.  Note all the little alignment nubbins fitting into the PCB holes.

The key bed is ready to be reassembled.  Note the black plastic strips that serve as spacers between the metal cabinet of the key bed and the circuit boards.

This is the treble circuit board going back into the unit.

All those little screws seen earlier are re-installed.

The small ribbon cables is reinstalled.  Those big ribbon cables go back on next.

This is actually a ground conductor for the pitch wheel assembly.  I found it easier to reinstall the pitch wheel assembly now, before the key bed is placed back into place.

Some of the keys have an audible squeak when they are depressed.  OK, let’s fix this too!

I’ve modified a bamboo chop stick to reach into the center of the spring, allowing me to unhook the bottom of the spring.

Springs for the white keys are on either side of the spring used on the black keys, seen in the center.

Once the spring is removed, the key can be un-clipped from its hinge.

Very super-secret, highly-technical precision formulated compound is used to lubricate felts and friction points here.

A little bit of the lubricant is sparingly applied to where it needs to go to kill the squeaks.

The long tab actuates the weighted action of each key, giving this key bed its semi-weighted feel.  The shorter tab actually depresses the rubber key pad strip switch seen earlier, to signal the synth to make a sound.

Dr. Shoen gives the completed synth a trial run in piano mode.  He is very pleased with the keyboard action.

Jacob explores the synth presets.  I think everyone is satisfied with the repair!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626