Download This Australian synthesiser was designed by Trevor Marshall and

Modifications to “Beastie”, the ETI 4600
1. Introduction
This Australian synthesiser was designed by Trevor Marshall and Barry Wilkinson.
The magazine Electronics Today International ( E.T.I. ), printed the schematics and how to build this
analogue synth in 1974. It was available through Jaycar as a kit.
The manual for this synth is not a user manual but a " how to build, adjust and Align " this synth. It is
a step by step guide for each module including schematics and parts lists which makes things a lot
easier. Also it has the plans for the construction of the cabinet. The original manual is included.
The modifications are shown in order of the date of implementation.
2. Original status of the synthesiser
The PSU has been rebuilt, chips & caps replaced and the PSU is stable with the correct voltages .
All 4 VCOs have been rebuilt ie. re-chipped & recapped, new transistors etc. The VCOs are working
but there are still some gremlins. They need aligning and trimpots adjusted etc, but they do sound as
good as any Oberheim or ARP.
The 5 mixers are working, The 3 E.G.s are working, 2 filters are working, one is a 24Db filter and the
other is a 12Db filter, 2 amps work, EQ and reverb work.output stage works, input needsa new pot (
supplied ) also the keyboard controller module has the modification PCB fitted.
I would replace all capacitors (ecxept for the ones I've done of course), this will make it a stable
synth. The keyboard controller and joystick modules need an overhaul.
There are still some other faults and bugs in there but again quite easy to trace.
The keys are held up by rubber bands, these have collapsed hence the uneveness. I would put a new
manual in or use the other manual which is in good solid condition with good springs.
Parts that I will include are:
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8 spare 4416 CMOS chips, hard to get and 1 is essential for each VCO.
A 2 pole 12 position switch for VCO 4.current one is damaged. Impossible to get.
Over 20 pins included for the matrix.
A Doepfer Joystick to replace current broken one.
I'll throw in some LM301 ICs with seats.
3. Zener Diode Protection for +7, +5 and -7 Volt Supply Rails
Three zener diodes were added as protection for the lower voltages as this was not originally
included in the design. It is a recommendation in theETI4600 construction manual however on page
47. Date implemented 30/08/2012.
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4. Synchronisation of VCO 2 and 3 to VCO1
The ETI5600 has a simple circuit to allow resetting of the waveform of slave oscillators. Whenever a
pulse is given by the master, the slave will reset and begin its cycle again. The ETI5600 had separate
switches for each slave oscillator. However I did not want to change the front panel. A previous
owner of this particular ETI4600 had put in a switch for unknown purposes and this is used to switch
oscillators 2 and 3 into slave mode. It is the middle switch between the Glide switch and the
modulation switch.
The ETI5600 had separate switches for each
slave oscillator. However I did not want to
change the front panel. A previous owner
of this particular ETI4600 had put in a
switch for unknown purposes and this is
used to switch oscillators 2 and 3 into slave
mode. It is the middle switch between the
Glide switch and the modulation switch.
The sync output is implemented in VCO 1
only. The components are mounted on the
waveform select switch. See diagram
below.
Sync is switched by the switch mentioned
above and is designated SW3 in the circuit.
The sync input is implemented on VCOs 2
and 3. They cannot be switched
individually. The diodes (D9) are soldered
directly onto the 2 pole 2 position switch.
The circuits are kept independent.
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This is a photo of the waveform selection switch on VCO 1.
Date of implementation is 18/09/2012.
5. Add MIDI Keyboard Control
After several attempts at trying to get the original keyboard controller to work, I gave up. It “works”
but does strange things when two keys are pressed simultaneously. The issue is caused by the circuit
that detects whether one or more keys are pressed. Sometimes it would create a trigger pulse and
sometimes it wouldn’t.
Using MIDImplant technology, a MIDI to control voltage and trigger was added.
MIDImplant is a special circuit needed when you want to control your older analogue
synthesiser with MIDI. You can use this little board if your synth works with V/oct or Hz/V
control. It takes about 2 seconds (or 4 keystrokes) to switch between V/oc and Hz/V ratios.
It's calibrated at 1V/oct, but any ratio is possible, from 0 to over 2V/oct. Hz/V are available in
5 flavours, covering from 1 up to 5 octaves. Basic soldering skills are needed as well as a
bit of knowledge about your synth. The service manual of your instrument, or at least
schematic is most helpful.
Both CV outputs are monophonic, that means each of them can control one oscillator. They
use last/top note priority. It means that whatever new note was played, it's immediately
converted into proper voltage, regardless of the keyboard state before hitting the new key (if
there were other keys pressed or not). If some of pressed keys are then released, CV output
changes to represent the highest remaining note currently played on the keyboard. If the last
key is released, CV remains at the level as before the release. Although this description may
not seem obvious, described mode of operation is probably the most intuitive way of
controlling pitch for monophonic analogue synths.
Refer to http://www.midimplant.com/ for more information.
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Twin shielded cable connects the MIDI in to the
MIDImplant. +7 volts are brought in from the
keyboard controller board and the CV and trigger are
supplied back to the controller.
A 2 position, 4 pole switch allows the user to select
the original circuit using the keyboard on the ETI4600
or the MIDI input. The trigger connects via the switch
to the transient and envelope generators. The CV
(control voltage) connects into pin 3 of IC 9.
A filter capacitor is added to the power rail. The
power to the MIDImplant is connected via a
protection diode.
A 4 pole 2 position switch is used to switch between
the ETI4600 keyboard and the MIDI implementation
and switches +/- 7 volts as well as trigger and CV.
Switch wiring:
 Orange +7v from controller to MIDI yellow
 Green CV from MIDI to blue controller
 Trigger (grey) selects MIDI (purple) or
keyboard controller (white)
-7v power switching was added later for portamento.
See section 6.
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There was an issue with the trigger sometimes being unreliable. The output from the MIDImplant is
the standard 0 to +5 volt trigger as used by Moog and many other synthesisers. However, the
ETI4600 requires a -7 to +7 volt transition. This was fixed by adding a clamping circuit. More on this
later in section 7.
Date of MIDI implementation 22/03/2013.
6. Adding Portamento (Glide) to the MIDI implementation
One facility that was not available in the MIDI implementation was Portamento commonly called
Glide. This was implemented by adding two TL071 operational amplifiers with a resistor/capacitor
timing circuit to slow the rate of change of the voltage after a key is pressed. This was also
implemented on the veroboard used for the MIDI controller. This gives an exponential rate of
change. Here is the circuit.
In order to maintain the integrity of the original
design, a 4 pole 2 position switch is used to
select between the ETI4600 keyboard and the
MIDI implementation. Most of the complication
comes from switching the portamento pot
located on the keyboard controller.
Green and yellow connect to the MIDImplant.
Red and orange connect to the pot.
Brown and black connect to the keyboard
controller. The pot is shorted to maintain
continuity if MIDI is used but not portamento
(blue wires).
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Switching for -7v was added for the operational
amplifiers. (Brown and Red)
The cable between the MIDI board and the keyboard controller is:
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Brown
Orange
White
Grey
Red
Black
-7v
+7v
0v (ground)
CV
trigger to transient generators
trigger to keyboard controller
Portamento was added 23/03/2013.
7. Creating a Reliable Trigger from the MIDI Using a Comparator
The MIDImplant trigger voltage of 0 to +5 volts is converted to -7 to +7 volts using a single TL071
operational amplifier clamping circuit. A simple resistor divider circuit provides a 3.5 volt reference
and switches to either the + or – supply voltage rail depending on whether the voltage from the
MIDImplant is less or greater than the reference. This was implemented on the same veroboard as
the MIDImplant.
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This is the final implementation of the entire MIDI to CV and trigger conversion…
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8. Adding External Control Voltage and Gate Trigger Input
a. Control Voltage
In preparation for devices such as sequencers or CV control from the Moog, additional inputs were
added. The intention was not to add anything to the front panel or the patch panel. The strategy was
to use the existing external input 2 to double up as a CV in. Then the audio or CV would appear on
the patch panel for external input 2.
All circuit diagrams refer to page 42 of the ETI4600 construction manual.
External input module has a switch to change
sensitivity to the incoming audio. It rarely gets
used, so the original single pole switch was
replaced with a 2 pole 2 position switch. One
pole is used to maintain the existing circuit and
enable restoration of the original function. The
other pole is used to switch between CV in and
audio in.
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Wiring of the switch. Left side
shows the new wiring. The right
side shows the original wiring.
b. Gate Trigger
An additional ¼ inch socket was
mounted on the rear
The shielded trigger cable
terminates on the switch in the
external input module 1 trigger
circuit as seen below.
The switch selects either
external trigger from the
external input module or from
the external gate trigger on the
back panel.
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