EMS Synthi AKS

AKS has serial No. 4551 which means its date of manufacture was about September 1972.

We got this unit back in 2013 as non functional, and contacted with EMS. Robin Wood at EMS has been so kind and do the proper repairs and added some useful modifications.

You can read my email converstions with Robin during the reapair, and you can check the full list of repairs and mods took place.

List of mods:

• Osc stabilisation
• Oscillator sync (Osc2)
• VC Shape (Osc2)
• Hi/Lo range (Osc3)
• Patchable inverter
• Filter 18/24 dB/oct switchable
• Filter unslew switchable
• Portamento/gliss
• VC Filter response (VCQ)
• 10-turn pot to Input Ch1
• External Gate Input function

Robin comments on modifications:

Note that it is not always a good idea to have all possible modifications fitted. For a Synthi A there is very little in the way of free panel space for adding many more switches, and especially potentiometers. A balance between panel clutter and additional functionality is important to bear in mind. For example fitting oscillator sync on all three oscillators would add three pots and three switches but you would never normally use all three syncs together at once and the panel would be instantly very cluttered. I usually suggest fitting sync to one oscillator - say oscillator 2, which will allow Osc 2 frequency to be slaved to either Osc 1 or Osc 3.

If you want Hi/lo frequency range fitted to the oscillators bear in mind that Osc 3 will also need stabilization and a dual-transistor upgrade. Only Osc 1 and Osc 2 are stabilised as standard on new Synthis.

VC shape is good for thickening sounds - principally through Pulse Width Modulating Osc 2's square wave (also Osc 3 if Hi/Lo has been added there). Triangle/ramp shape mod is also available simultaneously. Note that VC shape for Osc 1 will just produce a crunchy distorted sine-wave and Osc1 ramp wave is not shape-controllable.  The VC shape input is created by fitting a 3.5mm jack socket to the control panel to the right of the matrix (one per oscillator) with connection made direct to the matrix by a pin-to-3.5mm jackcord.

The switchable Filter 18/24 db per octave mode and switchable Filter Unslew are both recommended and good value.  Unslew allows much faster modulation of the filter from a CV and hence an extension to the range of sounds available. You may have noticed how there is a slight portamento when you use the filter as an oscillator. The CV breakthrough in the 'unslewed' position also gives a nice click for percussive sounds when using a fast CV.

The patchable inverter is a very handy tool as it simply turns a voltage upside down. For example if you have an ascending sequence of notes and feed this into the inverter you will hear a descending scale instead. Input and output are via a single pin-to-3.5mm cord providing the function at one pin location. This function also facilitates VC panning, inverted filter sweeps and many others.

Portamento needs an additional panel potentiometer and switch so for a Synthi A consideration is needed (especially if you definitely want Oscillator Sync on one oscillator, which will already have added one potentiometer). The only obvious panel-space for the Portamento rate pot is down by the joystick.

VC Filter Response allows the Filter Response (resonance) to be voltage- controlled. Connection to it is made via a pin-to-3.5mm cord.

Note that on a Synthi A you can only fit a 10-turn pot to Input Ch1 due to clearance problems to the AC mains transformer for Ch2. These make for easier adjustment of the tuning spread when using the Synthi with a MIDI/CV converter or external CV input.

If you are intending to use a MIDI/CV interface it is also important to have an external gate input socket for triggering the Synthi envelope. This is standard on new Synthis and is done by stealing one of the mic input sockets for the function.

Extra input channels are a problem on a Synthi A as these require extra Jack sockets and there is no free panel space (again this is better on a VCS3). The microphone input jacks could be sacrificed and the extra inputs switched-in to the joystick rows, but then these cannot be used simultaneously with the joystick. Bear in mind you can also use special pin-ended cables to inject external signals direct into the matrix if you run out of input and output channels.

Here you can find some comments from Robin regarding some of the repairs:

{Problem}
Broken right-side front corner of KS sequencer top plastic cover:
[ROBIN] Should be OK to repair.

{Problem}
Missing case female latch:
[ROBIN] No new replacements available. Used OK but this means it will look a bit rusty.

Your Synthi AKS repair is now complete and I am attaching our detailed invoice for the work done.

I hope you will not find the cost too shocking. The electronic problems with the KS sequencer keyboard were as numerous as any I can remember before. Usually I expect to find one or two ics that have died and that is all, but this one was much more complicated. A previous repairer had created some short circuits through poor soldering and exchanged some components for incompatible alternatives. After this a whole sequence of failed functions was revealed. Then there was the plastic cover repair, which I have learned from experience cannot be simply glued.

The KS casing is rather fragile and in the past I have found that simply glueing it does not work well. So I need to use some screws and aluminium pieces to make it strong.

So that makes it quite a precious KS now. But they are very much in demand since we stopped making them over 20 years ago.

I could not include all the detail of the work carried out on the invoice, but please feel free to ask if there is anything which doesn’t look right.

We only supplied the direct-to-matrix cables to you, and these do not have the 100K resistors in them. You can try to use them but you will need to accurately attenuate the incoming CV at its origin (as these bypass the Synthi’s input channels, where attenuation and correct tuning is normally done).

As per the notes I sent I suggest you make up a special jack cable with a 100K resistor in series, and keep this especially for when you want to use an external 1V/octave input to the Synthi.

The total cost of repairs were 795.65 GBP exVAT which includes 12 different repairs. I can send you a copy of the invoice if you are interested.

The cost of the mods were 605 GBP exVAT which includes 11 different mods.

We can send you a copy of the invoice if you are interested

HQ Photos link:

https://www.icloud.com/photos/#07ekMn6T1_hJEuSsCvlDpX6Gw