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.
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:

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.
Page 1

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.
Page 2

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.
Page 3

This is a photo of the waveform selection switch on VCO 1.
Date of implementation is 18/09/2012.
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.
Page 4

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.
Page 5

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.
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).
Page 6

Switching for -7v was added for the operational amplifiers. (Brown and Red)
The cable between the MIDI board and the keyboard controller is:

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.
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.
Page 7

This is the final implementation of the entire MIDI to CV and trigger conversion…
Page 8

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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Page 10
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.