EMS Synthi AKS – Extensively Serviced & Modified by Portabellabz

Rare EMS Synthi AKS in excellent condition and perfect working order. This early transitional example was extensively serviced, modified and professionally calibrated by Constantin at Portabellabz in March 2024, expanding its sonic capabilities while retaining its unmistakable original character.

It has been carefully kept in a smoke-free studio and is ready to play.

Instrument details

The serial number, 4355.KS, appears on the KS section. The corresponding sticker on the Synthi is no longer present.

The KS is a standard early revision featuring MF50052T shift registers and the dynamics function.

The Synthi A is a particularly rare early transitional example. Board A was designed for the PA234/GEL234F1-based output amplifiers and reverb driver used in Mk I instruments, together with an early power supply capable of powering the KS like a Mk II. It was subsequently modified at the EMS factory with regular Mk II output amplifiers—using the 741 and germanium-transistor daughterboard—and an MFC6070 reverb circuit.

Original EMS modifications

• Oscillator synchronization: each oscillator has an on/off/on switch allowing it to be slaved to either of the other two oscillators
• Oscillator stabilization: all three oscillators are fitted with metal-can supermatched pairs
• Oscillator 3 selectable high- and low-frequency ranges
• Oscillator 3 calibrated to track at approximately –0.32 V/octave, like oscillators 1 and 2
• Portamento control with rotary switch, slewing the KS sequencer CV on row 16

Portabellabz modifications and service — March 2024

• Temperature compensation added to the supermatched pair in each oscillator
• Oscillator 1 low-range mode, selected by pulling the sine control
• Oscillator 2 low-range mode, selected by pulling the square control
• Sample-and-hold modification using push-pull noise controls
• Extended attack and decay times, enabled by pulling the attack control
• Invertible trapezoid polarity, enabled by pulling the trapezoid control
• Selectable sustained or transient gate operation
• Two inverter pins positioned to the right of the pin park
• Two attenuverter pins positioned to the left of the pin park
• Buffered rows 8 and 9, maintaining stable signals when multiple pins are inserted

Provenance

According to the previous owner, the instrument was purchased from Rod Argent’s shop on Denmark Street in the late 1970s. It was subsequently used professionally in theatrical productions, including Howard Barker’s “The Hang of the Gaol” at the Royal Shakespeare Company’s Warehouse Theatre—now the Donmar Warehouse—and an RSC regional tour of “Henry IV, Parts 1 & 2.”

Condition

The instrument is in excellent overall condition and fully functional. Its front panel is very clean, with only minor signs of wear, while the KS and case remain in good order.

Voltage is switchable for worldwide operation. Please contact me before purchasing to confirm the required voltage and plug type.

This listing is for the EMS Synthi AKS only. No additional sequencers, cards, books, cables or other accessories are included.

This is an exceptional and highly capable example, offering a broader range of modulation and sound-design possibilities than a standard Synthi AKS.

Due to the age and nature of vintage electronic equipment, the instrument is sold as-is with no returns. Please ask any questions before purchasing. It will be packed extremely carefully for shipping.