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© 2013 Benden Sound Technology

Made by Serif


This synth had been newly-acquired by a client, but failed to self-calibrate the VCOs and VCFs. The Xpander, being one of the last analogue polysynths, was highly advanced compared to its predecessors and made the most of the microprocessor power by then available. Rather than rows of preset resistors requiring tedious adjustment by a technician, the Xpander firmware could automatically recalibrate the analogue voices on demand, not only retuning the oscillators but matching filter cutoffs and resonances across the six voices. At least, it could if all the analogue circuitry was operational...


It was immediately clear that the power supply had been worked on previously and that the standard of workmanship was questionable. In particular, certain capacitors had not been replaced when they should have been, and this had led to excessive ripple on the voltages supplied to the analogue circuitry. No amount of self-calibration could counteract the instability thus caused.

A full recapping of the power supply restored reliable self-calibration. For added future-proofing, I also changed all electrolytic capacitors in the rest of the unit, replaced the lithium memory-backup battery and silenced the annoying hum from the mains transformer!

New Development

My client then asked whether it would be possible to do anything about the fading vacuum fluorescent displays (VFDs). This is a well-known problem with Xpanders and Matrix-12s (which share much of the same circuitry) - VFDs, being related to thermionic valves, eventually lose their luminance and are also vulnerable to physical damage, and this has left many synths impaired or, at worst, inoperative. The original displays are long out of production and their modern equivalents are far from being a drop-in replacement.

I can't resist this sort of challenge. It took a lot of effort, but I was able to design a total replacement for the original Xpander displays using modern parts and the results are stunning. The clunky old "starburst" displays are replaced by clear dot-matrix lettering which enhances an already very desirable instrument (see photo).

This display upgrade is available to purchase and is a fairly straightforward retrofit - please jump to my product pages.


Just running through the factory presets on the Xpander gives a hint of how deep its capabilities are. Yes, there are sounds there which one has heard on almost every synth, but there are also others which make you want to grab the manual just to work out how they were done. In particular, the complex modulation opportunities available (and only made possible by software calculation) enable rich sounds which would require extra offboard effects to equal on less advanced instruments.

While not without limitations - the envelopes are not among the fastest known, for example - the Xpander is a great argument for owning fewer, deeper synths.

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Xpander voice card. All six voices share one huge PCB.

Xpander CPU board. Main processor is a 68B09.

Xpander front panel PCB with display board removed. Rotary encoders are at the top right.

Brand new dot-matrix displays courtesy of the OB-VFD upgrade.

Xpander PSU BEFORE repair. A mixture of original and replaced capacitors.

Xpander PSU AFTER repair. All new low-impedance electrolytic caps. Bias mod for OB-VFD display upgrade is just visible.