repair

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

Peavey Citation MK IV Two Channel Guitar Amp Head Repair

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Rod had this Peavey head kicking around and thought it was time to put it to good use. However, it didn’t work at all. Could the Unbrokenstring Crew work its magic and bring this road warrior back to life?
A quick scan of the front panel shows that the input circuit sports the sort of flexibility that the Peavey Marketing Department loves to explain to anyone who would listen…
Each channel has independent gain, and a master volume to Rule Them All.  Effects can be inserted via the front panel.
On the rear panel, we have parallel speaker jacks and the usual ground/no-ground power switching.  Peavey often married different front panels, which contained preamp circuitry, to different rear panels, which carried power and audio amplifier components.  The ‘series’ number goes with the power amp, not the front panel.  We Got This.
Name, rank, and serial number, please.
Pulling the front panel, we see that all of the components are mounted on one circuit board.
I took a few pictures to be sure that the wiring and cables were returned to the same spot when we are through.
The cable to the right is just wired to the power indicator.  The other two carry signals.
This is a better view (to be sure that they cables are properly oriented on their pins.
The front panel is free of the rest of the unit.
All of the controls and switches will be cleaned so this assembly comes completely apart.
We can now clean and lubricate everything now.

 

Can you spot the broken solder joints?
Someone has been here before!  This needs to be cleaned up, too.
The Blue Shower is a good cleaner.  The DeoxIt contains a lubricant for the potentiometers.  Good Stuff!
Back Together it all goes!
This screw hole was stripped out.  First, we will soak the stripped hole in the wood with this wood hardener.
Next, a birch dowel is cut to partially fill the hole.  The dowel reduces the apparent diameter, allowing the screw to hold.
With the stripped hole repaired, we are back in business!
This unit plays very well, and all the controls and switches are Like New!

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Blackstar HT5RH Guitar Head is Dead

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This five watt head had gone nearly silent. The national retail chain that sold it told the owner to purchase a new one, because that’s their business model. Could the Unbrokenstring Crew repair this unit and bring it back to life?

At five watts, this head is bedroom-friendly, yet there are plenty of opportunities for distortion and tone shaping.

 

Interestingly, there are three options for outboard speaker cabinets as well as all of the usual in’s and out’s.

 

The back comes off with these screws.  But it appears to be stuck in place!

 

I speculate that when the unit was built, the Tolex glue was still fluid and squeezed out between the back and the frame, sticking the rear cover in place.

 

The chassis is held in with these screws.  No surprises here.

 

This is a hybrid solid state / tube unit, with a 12AX7 triode pair in the preamp section and a dual triode 12BH7 pressed into service as a push-pull tube output stage.

 

There are two different versions of the schematic available.  They can be easily identified by checking the number of conductors in that big ribbon cable that connects the rear panel circuit board to the main circuit board.  This particular unit uses the cable with 21 conductors.  Thanks to Armando Garcia at Mars Electronics who furnished this schematic!

 

In the foreground are the big heat sinks for the voltage regulators.  The rear panel wiring board is in the background.

 

Viewed from the other side, the main circuit board contains the preamp circuitry.

 

The preamp uses DSP techniques to create the reverb effect and the tone-shaping functions of this unit.

 

This is the preamp tube.  It’s fine.

 

This is the 5 watt output tube.

 

This part checked OK but it is a little weak.  We will continue to use this unit for troubleshooting purposes.

 

This is the pin-straightener from my tube tester.  The Chinese tubes have a little larger envelope than the JAN versions of these tubes, so the tube is a tight fit in the straightener.  If the tube envelope is too large, the pins can be straightened using the same tool, but the pins are inserted from the other side of the straightener.

 

So the output drive signal is split into an in-phase and out-of-phase copy, and applied to the grid of the output tube.  But the plates of the 12BH7 are stuck at +300 volts.

 

The primary windings of this output transformer are shorted to each other.

 

Let’s get this transformer off the chassis and take a closer look at it.

 

These Chinese transformers are usually not worth fixing, but this transformer is hard to find.  Some exploratory surgery shows us that the problem is deeper in the windings and not readily repairable.

 

So, out it comes entirely.  The search is on for a replacement.  The original manufacturer has no stock.

 

Hammond makes a versatile aftermarket unit that is available through distribution.  We can make this work!

 

The frame of the transformer is just a little bit larger than the original part.  One of the mounting holes is being moved.

 

Placing the magnet near the site where the drill is working helps keep those pesky metal shavings under control.

 

The new transformer is bolted in place and some Thread Locker is applied to the bolts to keep it in place.

Here the output leads are threaded through the insulating grommet in the chassis.

 

The input wiring is soldered into place, so the leads are trimmed to length.

 

To preserve the original wire insulation colors silkscreened on the circuit board, short pieces of the original wiring were left in place and butt-spliced to the new output transformer harness.

 

This unit is ready to test.  Pretty neat looking!

 

Surprisingly, this unit has a bias pot and a balance pot.  However, there is no commonly-available information available for the technician to set these.  So, I wrote this procedure:

BALANCE ADJUSTMENT

  1. Set DMM to mV range
  2. Affix DMM probes to TP6 and TP7
  3. Switch POWER ON; switch STANDBY ON after five minute warmup.
  4. Adjust BALANCE pot for 0V display on DVM
  5. Switch all power OFF

BIAS ADJUSTMENT

  1. Affix black DMM lead to ZD2 cathode (banded end) near Input Jack. This is a convenient GND.
  2. Affix red DMM lead to D20 anode (not banded end.)
  3. Switch POWER ON; switch STANDBY ON after five minute warmup.
  4. Adjust BIAS pot so that DMM display is 46mV.  This puts about 10mA of idle current through the 12BH7.

Recheck BALANCE and BIAS adjustments as they are slightly interactive, particularly if the internal sections of the 12BH7 tube are not matched.  Follow all the usual precautions of removing power and working safely around high voltage!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Sovtek Small Stone Pedal Refurb and Update

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The modulation rate control on this wonderful Sovtek Small Stone phaser effect pedal had broken. While the unit is in the shop, could the Unbrokenstring Crew also add a 21st century DC pedal power jack to the unit?

Like a message in a bottle, this pedal has the look and feel of a relic from another planet. Even the switch looks like alien technology.

Removing the top cover reveals a heavy steel plate that holds the major components.  Look at the LED holder!

The control for the phase modulation had disintegrated.  Not much was left holding the shaft in place.

The back side of the modulation control was not out of the ordinary beyond the Cyrillic alphabet. Perhaps it could be rebuilt using parts from another similarly-sized potentiometer.

We have removed the old potentiometer from the circuit.

The tabs on the back cover of the potentiometer can be peeled back in order to disassemble the unit.

Interestingly, the internals of this control are completely different than what we might expect from a domestic control.  This potentiometer is a ‘reverse audio taper’ component.  The Russians achieved this by mounting the resistive element on the opposite side of the main wafer of the control, effectively reversing the direction of the taper.

So, it appears that we need to find a control that is close to the physical size of the old part, so we can reuse the knob.

We are working in millimeters here, in case you are wondering.

An aluminum bushing allows this smaller shaft to fit in the Russian knob.  Perhaps we have another degree of freedom in our search for a proper replacement.

This bushing can be removed…  a good thing that will allow us to do some gun-smithing if we need to do so.

The knob is not quite big enough to allow a quarter-inch shaft to be substituted.

So, we located a reverse audio taper control custom-designed for Neve recording consoles.  Yeah, I got connections.

This part has an appropriately-sized shaft that will permit us to use the original knob.  Good news!

The new control is wired into the circuit in the same manner as the old one.  Teflon spaghetti tubing handles the high-temperature insulation duties here.

These little spacers were rattling around in the enclosure after the circuit board was removed.  Where do these go?

Turns out, they are spacers that go on top of the cast bosses in the bottom of the original box.

The new power jack is mounted and wired into place, along with new steel Switchcraft in and out jacks.

The whole arrangement is now fitted back into the case.

An internal nine volt battery is used for powering this unit for checkout.

We have a winner!  Time to tighten down the screws and button this unit back up.

Here is the top cover with the new control installed.

The case cover is now back on.

The owner wanted to leave no question regarding whose pedal this was.  Mine!!!  Mine!!

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