Thursday, April 26, 2012


The bender on my Roland D-50 Synthesizer was crapping the bed.  In test mode, the bender values at center detent were fluctuating data values all over the place.  Upon opening the case, removing the bender assembly, and taking a good look at the potentiometer this is what I saw....

A mini-pot in extreme oxidation mode.  Nonetheless, it was worth a try to clean the pot w/ several doses of contact cleaner.  No luck.  Needs replacement, no problem....wrong.  The bender pot is an extremely rare vintage 'center tap' variety.  Normal pots have three terminals and use a wiper that rotate around a resistor track.  'Center tap' potentiometers add a fourth terminal to the middle of the track.  This is done in order to obtain a near-zero-ohm reference point with respect to the wiper at center detent.

I searched many forums and blogs for solutions.  There has been some suggestion to take a regular 3-terminal pot and add a small wire to the track and use a 'solder paint pen' for the connection.  Admit it, this is the most ridiculous idea.  No way it works...right?  I couldn't find any sites that documented this process anywhere.  Bearing astute craftsmanship, I decided to give this monkey-business a whirl.

First, separate the pot mechanisms from the metal case by prying up 4 small metal tabs just enough to get the pieces apart.  Next, the shaft needs to be separated from the wiper assembly.  I did this by resting the pot, shaft downward, between the slightly opened jaws of a bench vise.   Then I used a nail set and small project hammer to lightly tap the shaft out of the wiper assembly.  At this point the shaft, wiper assembly, and resistor track are separated.

I have a small manual hobby drill, with a teeny-tiny drill bit just the same size as my 24-ga bus wire.  I carefully hand-drilled a very small hole just adjacent to the top of the track.  Then I passed a small bus wire through.  To help stabilize the wire, and prevent soldering activity from disturbing the solder paint connection, I used 5-minute epoxy.  I applied a drop of the mixed epoxy at the hole on the side opposite the track (the outside) where the wire passes through the hole.  After curing overnight I snipped the bus wire on the track side flush with the track.

I found this 'solder paint pen' product at radio shack.  It's called Circuit Writer and cost $20 which seemed high, but if it gets you out of these impossible jams...I'm starting to think it's worth every penny.  I had to shake it for a very long time to get all the juices mixed properly.  I tested several times on a piece of cardstock, waiting each time for the stuff to dry, and tested with a Digital Volt-Meter (DMV) continuity-check each time.  After 5 or 6 times it finally turned a bright silver color, and gave a near zero-ohm reading on the DMV.
I applied a very small amount of the Circuit Writer to the top of the track where it meets the wire.  Its tricky not to get this material on the part of the track the wiper passes over, or the wiper might bind there and deform.  That would be bad.  After the material dried I tested it using the DMV with one lead on the wire, one on the track.  It beeped right away and gave a 0.6-ohm reading...success!

The VERY IMPORTANT next step was to test fit the wiper track and case, and make sure the bus wire didn't ground-out against the metal case.  Unfortunately it did.  So to remedy that I applied a small amount of clear finger nail polish to the inside of the case in the vicinity of the bus wire.  After drying and testing the fit again, there was no problem.

After the finger nail polish had dried, the most difficult part of this project was to carefully assemble the parts back together and tap the shaft back onto the wiper assembly.  Then I tested the terminals on the DMV by first testing the three main terminals for ohms.  Second I checked the wiper terminal against the new center tap bus wire at center detent and it still read 0.6-ohms...success!

I soldered the new pot in place of the old, put the bender back in the synth, and it actually works!  Unbelievable!  If your craftsmanship skills working w/ very small parts are up to par, then I could definitely recommend this solution.

Friday, April 6, 2012

K5000S: It Lives!

Previous owners cracked the LCD and spilled red bull inside a lot of momentary switches. The new LCD came in today and looks great! After some diligent work it is producing wicked sounds once again, can't wait to start programming this beauty. See Original post for some dis-assembly pics.

Monday, April 2, 2012

Yamaha TX81Z FM Synth Demo 1

Behold. This is the TX81Z FM rack mount synthesizer. Truly an undervalued piece of gear, and a powerful secret weapon for the most adventuresome tweakers. I've been going full-on honey badger w/ FM synthesis programming my own sounds into this little gem. And I absolutely love it!

Some patches are going through a Lexicon MPX550 (offscreen) set on Gothic Hall reverb preset. And some are just bone dry.

List of patches:
0:17 DrtySecret. Very unique FM pitch interval sound.
0:44 Rhodes. Delicate electric piano tones.
1:01 81Z State. Old school acid house bass. 808 State.
1:27 Wowocaster. Notes played fast legato (upper) it sounds like a fake strat. Held notes (bass) swell and growl. Wow, man.
1:47 Filthy FM. Raunchy, dirty, crunchy lead sound.
2:03 Harder FM. Old school hardcore rave sound.
2:27 DrkMachine. Brooding Sci-fi horrorshow sound.
2:49 AttackBass. Conventional bass w/ quick attack transient.
3:14 WhurlyEP. Electric Piano. Queen, and Supertramp.
3:44 AuroraBlue. Gorgeous atmospheric pad.
4:10 PaddedEdge. Digital, Edge-y pad.

Sunday, April 1, 2012

TX81Z, K5000S

Been programming the TX81Z a bit. About a dozen patches or so, very surprising sounds. So I'll probably share a demo this week.

FM synthesis has really got my digital mojo going lately, such that I picked up a Kawai K5000S in dire need of restoration. The K5000S is known for sounds using more pure Additive Synthesis methods. It is also capable of some nice choir sounds and has a superb multi-stage bandpass formant filter for doing just that! Known functional issues of my instrument:
- cracked LCD (queue the Ladytron music)
- previous owners spilled red bull or something on it resulting in multiple sticking buttons with goo inside them.
So I'm disassembling the entire unit, pulling the cards w/ button switches, meticulously cleaning the PCBs of sticky goo, and removing/cleaning/replacing the small momentary switches. I ordered a new LCD that should be here later this week, it's going to look spiffy gray graphics on a white CCFL backlight...can't wait to see how that all goes.