PCBs for EMS
Synthi A / AKS / VCS3
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PCBs come unpopulated and without parts. All
modifications are at your own risk and I assume no
liability for personal injury or damage to equipment
or loss of use caused directly or indirectly by the
use of any of the PCBs. Although the installation is
quite simple it should only be performed by those
experienced in electronics.
From the factory, the matrix is not buffered.
For example, if you scale a CV to control
oscillator 1 and you then want to patch the same
CV source to oscillator 2, normally oscillator
1 will go out of tune when you insert the pin
into the row.
The row buffer will prevent this and keep a
stable pitch regardless of the number of pins
present in the row.
If you now decide to add an extra modulation
from another source, normally the
oscillator scaling will go out when you
insert the pin into the column.
The column buffer will prevent this and keep a
stable scaling regardless of the number of
pins present in the column.
Buffering the matrix entirely is not needed.
This will also affect the Synthi's behaviour and
character and might cause some problems.
But buffering a few rows and columns only is
useful to keep stable voltage where it's needed
: oscillators and filter frequency columns and
input channels, joystick, oscillator 3 rows. To
buffer the trapezoid row, the polarity
control push-pull pot is a better
The PCBs I offer are a modular and easy to
install solution. They mount directly to the
matrix and need no other support or screw or
No pin resistor value modification is needed.
The mod is 100% reversible.
The PCBs fit both Sealectro and Ghielmetti 621
Each small PCB for rows is able to buffer 2
adjacent rows and can be used for 1 only if
The large PCB for columns offers up to 4 buffers
directly connected to the oscillator and filter
frequency columns. Each buffer is independent,
so you can use all 4, or 3, or 2 or even 1 only
to match your own needs.
procedure and BOM
Duophony PCB for DK1 and Cricklewood
This PCB is
based on the one EMS used to add the DK2's duophony
to the monophonic DK1 and Cricklewood keyboards.
The installation is easy and doesn't need any
permanent modification or intervention to the
original PCB, the pic speaks for
"ch2 sw" goes to the ch2 switch, disconnect
the former grey cable, insulate it's end and
connect the PCB's pin 5 "ch2
in its place.
The schematics and calibration procedure are
in the service
The PCB comes with a copy of the original
installation notes by EMS.
MFC6070 replacement PCBs
The MFC6070 reverb driver in the MK2 Synthi A and VCS3
is getting increasingly rare and expensive.
The reverb circuit using it is the less good sounding
despite very common.
It's fragile and a frequent cause of reverb issues.
Rather than pay unjustified price for such a crappy
part, why not replace it with the better sounding and
more reliable MK1 or MK3 driver ? The three circuits are
very similar and the MFC6070 can be replaced with a
small PCB without any modification of the original
The MFC234 replaces the MFC6070 with the PA234 or
GEL234F1 (same part renamed) used in the MK1 units. GE
ceased production of this early audio amp IC around
1971, it is thus hard to find today. Many see it as the
most desirable output amps and reverb driver in a Synthi
or VCS3. It's the essential part of the nice MK1 sound.
The MFC741 replaces the MFC6070 with a 741 and a
AC153K/AC176K germanium transistors pair, easier to
source than the PA234. This is what I call the "MK3"
reverb circuit used in the Synthi A and VCS3 since the
late 70s and still today.
installation is straightforward and doesn't cause any
damage to the original PCB :
- remove the
2 resistors and 3V6 zener diode above the MFC6070
the 3V6 zener diode with an insulated wire
- remove the
polystyrene cap to the right of the MFC6070
- remove the
- solder a
piece of component leg in the holes of the MFC6070
emplacement, the top right one can remain empty,
it's not used and a corner of the AC153K obstructs
the hole on the daughter PCB
- thread the
MFC741 or MFC234 populated PCB onto the component
- solder and
cut the exceeding pieces of component leg