Sovtek Small Stone Pedal Refurb and Update

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


Kurzweil K2500XS Synth Repair and Recondition

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