Oberheim OB-SX Vintage Analog Synthesizer - OB-X/Xa Sounds - Compact Preset Synth w/ Limited Editing

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St. Petersburg, FL, United States
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Vintage Oberheim OBSX 6-Voice Synthesizer

Matrix Synth's favorite synth!

Fully refurbished and in excellent condition

THE BACK STORY:

I’m not the original owner of this keyboard, the previous owner had it for over 25 years. For most of that time, it had been in storage.

The keyboard was manufactured in 1981 and originally had 4 voice cards. It has Oberheim's original "black and gray" color scheme on the top panel. At some point after March of 1982, two extra voice cards were added and the keyboard was upgraded to make all 56 presets available using combinations of the "ABC" and 1-8 keys. (That feature was standard on the later "blue stripe" versions of the OBSX.) This upgrade was probably done by a factory approved service center.

After those upgrades, but before 1987, someone performed additional modifications which don’t look like the work of a factory tech.

I bought the keyboard after all the upgrades, modifications, and complete overhaul were done.

The previous owner wanted to make this OBSX work and look like new again.

THE REPAIRS:

1) The Keys

All 49 of the keyboard bushings were replaced with new, lubricated bushings. All the keyboard contacts were cleaned and deoxidized with DeoxIT and DeoxIT Gold. One broken J-wire was replaced (with correct wire) and another broken J-wire was repaired. The keyboard now plays and feels like new. No horrible "clack-itty-clack" noise as you play.

2) Electrolytic Capacitors

All electrolytic capacitors connected to power supply rails were replaced. That included nine on the power supply board, two on each of the six voice cards, five on the processor / control board, three on the mother board, and two on the pot board. Wherever possible, the replacement caps were rated for higher voltage and operating temperature than the originals. These changes were made to ensure that all DC power supply levels would be clean and stable.

3) Sample and Hold Capacitors

All 32 of the tantalum sample and hold capacitors on the control / processor board were replaced with polypropylene film caps. Several timing caps on this board were also replaced with PP film caps. All of these replacement caps were rated for higher voltages than the originals. These changes were made to reduce leakage in the sample and hold circuits and to stabilize the synth control voltages.

4) Calibration Adjustment Pots

Most of the original miniature adjustment pots in the OBSX were a type that had exposed resistor elements and contacts. These had become oxidized and dirty over the years and as a result were noisy and unstable. In all, 79 of these miniature adjustment pots were replaced: Eleven on each of the six voice cards, ten on the control / processor board, two on the power supply and one on the mother board. The new pots were all sealed. Multiturn pots were also replaced with similar pots that had a 25-turn range instead of 15-turn. All of these pots were replaced to make the calibration of the OBSX rock-solid.

5) Wiring

All of the internal wiring (except for ribbon cables) was replaced with new 20-gauge wire. The old wiring had become dry and brittle. The original color coding was maintained.

6) Molex Connectors

Every single Molex connector in this OBSX was replaced. Yes, all 94 of them. The old connectors used round tin-plated pins to connect to flat tin-plated spring contacts. These were all oxidized and some of the spring contacts didn't have much spring left in them. Connections were poor, some were loose and intermittent. On top of that, there were lots of cracked solder joints on the mother board where most of these connectors were mounted.

The new Molex connectors all have gold-plated contacts. The pins are square, (not round) so there is much greater surface area in contact with the gold-plated spring contacts. There are no bad connections anywhere throughout the synth!

7) IC Chips

Fortunately, all 30 of the original Curtis CEM chips in this OBSX have no faults however some of the generic TLL and CMOS chips have been replaced.

All 74 series ICs were replaced. Most of these were on the control / processor board. Texas Instruments chips were used wherever possible and to lower the drain on the power supply the 74 series chips were swapped with the “74LS” equivalent.   

None of the chips on the voice cards were changed to ensure the vintage Oberheim sound was retained and not altered in any way.

 

8) Modifications 

The mystery tech who modified this OBSX added two adjustment knobs and a toggle switch to the pitch bend assembly… and didn’t label them. It was fun figuring out what they do.

The new knob to the left of the modulation lever allows you to set the minimum mod level. Normally, you pull the modulation lever toward you to get the amount you want. The lever is spring-loaded, so when you let go, the modulation level goes back to zero. The new knob allows you to set modulation independently of the lever and it stays where you put it. A good idea, but if you forget and leave this knob turned up, it interferes with auto-tune.

The new toggle switch to the right of the pitch bend lever has no effect in the "UP" position. If you move it to the "DOWN" position it disables the pitch bend lever and has the effect of pushing the pitch bend lever all the way forward. There’s probably only one use for this. It would allow you to bend a pitch (or chord) down an octave using the lever, then flipping the switch would make it stay there. It would be a sneaky way to make the OBSX sound like it had an extra octave without having to abruptly shift up or down. Maybe that’s a good idea... but again, if you leave the switch in the "DOWN" position, auto-tune won't work.

The second new knob to the right of the pitch bend lever and above the toggle switch allows you to attenuate the level of frequency modulation (and/or pulse modulation) being applied by the selected preset. The normal position of this knob should be fully clockwise. That gives you the full amount of freq/pulse mod intended by the preset. Turning this knob counter-clockwise allows you to back off the modulation level all the way to zero if you like. The knob does nothing if no freq/pulse mod is used by the preset, but it is cool to have control over it when it's there. Unlike the other two modifications, this one doesn't interfere with auto-tune.

The good news is that these three mods can all be disabled if you don't want them. Turn the left pot all the way to the left and the right pot all the way to the right and put the toggle switch in the "UP" position and all three mods are off.

9) Miscellaneous Repairs

The pitch bend pot was replaced as it did not return back to center reliably.

Each oscillator on the voice cards has a pair of "matched" resistors that ensure that various control voltages affecting pitch all have the same volts/octave scaling. Eight of these resistors - four on card #5 and four on card #6 were replaced. The old resistors on card #5 apparently didn't match closely enough and somebody (at the factory?) jury-rigged an extra pot between them to make them match. That turned out to be a really bad idea. The extra pot was intermittent, which made oscillator #2 on that card generate random pitches. The bad pot and resistors were pulled and replaced by properly matched versions. The second two resistors on card #6 also didn't match and were replaced.

All PCB boards were cleaned and excess flux removed.

 It’s common for HI TRACK adjustments to be adjusted at the end of their range. All original 10K pots were replaced with 20K pots to be sure they all had enough adjustment range. It turns out that the Curtis documentation for the CEM3340 chip recommended a 20K pot for the HI TRACK adjustment, so it’s much better now.

CALIBRATION:

This OBSX was fully calibrated to factory specifications using proper calibration and test equipment.

Some of the filter adjustments on the voice cards are made by ear, so those might not match another OBSX exactly, but they do match each other!

There is a 12th pot on each of the voice cards that is not on the Oberheim schematics. (On the schematic, it is shown as a “selected” resistor.) It’s understood that this pot is intended to match the high frequency resonance of the cards to each other, but there isn’t any procedure for making this adjustment. So, those six pots were the only ones that were not replace nor adjusted. All 6 cards sound identical, however.            

COSMETIC STUFF:

The whole keyboard got a thorough cleaning, inside and out. All the keys were removed and washed. The wooden end panels were removed, cleaned and waxed. The mounting hardware for the side panels was replaced with brass finish oval head screws and washers. The adjustment knobs were cleaned and the white triangle pointers that had nearly worn off were repainted. Numerous small scratches on the top panel were touched up with black semi-gloss enamel paint.

HOW DOES IT SOUND?

It sounds like new! Having all the voice cards calibrated and matched make a huge difference in the consistency of the voices over a wide range of patches. Auto tune works every time and (once the keyboard has warmed up) it can go for hours without retuning.

You’ll be especially impressed with UNISON mode. When all twelve oscillators fire together you get the very definition of vintage FAT analog sound.

This item is sold As-Described

This item is sold As-Described and cannot be returned unless it arrives in a condition different from how it was described or photographed. Items must be returned in original, as-shipped condition with all original packaging.Learn More.

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