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On 23rd April 2009 I gave a talk at Anglia Ruskin University about my
modifications to the Jen SX1000. You can now
download the slides (PDF, 1.7MB).
This page documents my restoration/repair project on a Jen SX1000,
henceforth called "Jenny". I acquired Jenny from Tony Allgood at 2002's UK
Synth-DIY meeting, with the proviso that it needed a lot of work, not least
being a replacement M110 chip.
Undeterred, I brought Jenny back to Cambridge and started looking around
for a replacement M110. I found plenty of hens teeth, rocking horse manure
and cockerel eggs, but no M110's (they seem to be pretty rare these days).
It dawned pretty quickly that a simple plug-and-go repair was going to be
out of the question, so that lead me to start thinking about a more complete
restoration and modification project.
The result is described on this page, Jenny being completed at the end of
July 2003. The results are shown in the following photos:
||Jenny enjoying the sun in the garden.
||The enhanced rear panel, with new mains connector, headphone socket,
signal interface, analogue control interface, and MIDI socket. The
labels were laser printed onto paper, laminated in
our office laminator, and then the labels carefully cut out with a
scalpel and glued onto the rear panel.
||The M110 replacement oscillator module installed inside Jenny.
||Inside shot showing the additional circuits and wiring to the
front panel PCBs.
Thanks to the very helpful Bill Fox I now have a scanned copy of the SX1000 user
manual in PDF format. It is rather basic, but it does come in Italian, English, French and
Jen SX1000 User Manual (PDF, 2.4MB)
Now you can here Jenny in action. The following are a few attempts at
capturing some of the fun you can have with a Jen.
||A few notes with varying amounts of PWM, LFO and filter
twiddling. The only processing done was a bit of
compression to even out the levels a bit.
||Something a bit more together. Two tracks of a soft
brass-like sound, plus some drums and reverb.
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Jenny was in a bit of a poor state when I got her. The photos below show the basic
layout. Pretty obvious are the missing coloured knob caps, which all Jen's
suffered from due to weak glue.
||Jenny's front--you can clearly see the three rows of knobs, some of
which are missing their coloured key caps.
||This shows the back end of this little monosynth, rather devoid
of any connections other than a single mono output jack.
||The left-hand side controls the single oscillator, noise source, LFO
and, just at the bottom right-hand corner, the output volume.
||The right-hand side controls everything else---the VCF and the VCA.
||The upper PCB is the noise source (switch selects off-white-pink
and the pot sets the level mix) and Glide pot. The larger PCB is the
oscillator waveform shaping and the LFO. At the top-right is the volume
pot on the VCA board (see below).
|[Click] ||The upper
PCB is the VCA, which is mostly the ADSR for the VCA. The lower PCB is
the filter, which is a 4-pole low-pass filter based on two LM13600
dual-OTAs. The resonance control almost takes the filter into
oscillation. The top four pots control the filter's ADSR. The bottom
four mix the various control sources (manual, LFO, envelope) and the
|[Click] ||This is
about as simple as a mains PSU as you get: transformer, rectifier, a
couple of caps and three voltage regulators (+5V, +12V, -12V).
||And here it is folks---the very rare M110 from SGS-Ates, very rare,
and in this case, half dead.
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The knob caps are often missing on Jen's due to glue that over time
seems to weaken and let go. The table below lays out the cap colours
on the original machine.
The colour scheme is as follows:
- red for VCO,
- yellow for LFO,
- white for VCF,
- blue for ADSR,
- gold for noise,
- deep red for master volume.
The Jen originally came with a set of large patch sheets that went over the entire front panel, with
red and blue marks to set the controls for a particular 'patch'. Someone scanned it and posted it to
Vintage Synth Explorer.
Here is a
blank one (800kB)
you can print out and use for your own patches.
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Here are scans of the original (pre-modded) schematics (copied from Jezz's site):
- Page 1 (833kB) - Master oscillator, voice generator, PSU
- Page 2 (793kB) - Waveform generator, LFO, glide osc.
- Page 3 (529kB) - Filter, envelope generators
- Page 4 (659kB) - Filter (continued), noise generator, VCA
A single-page version is
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Ok, so you know what she looks like, now for the technical stuff. I
catalogue here what I see as problems that need rectifying to get Jenny into
a decent state of operation.
||To begin with, all the pots are way too noisy and
scratchy to be useful, so they will need relacing.
||I hate permanent mains leads, for two reasons:
So, plan to fit a standard IEC mains socket on the rear.
- They are usually too short most of the time (Sod's Law)
- Makes carrying equipment around more hassle, as you end up tripping
over the lead, or treading on the plug, getting it caught it
||The main issue is the half-broken M110 master
oscillator. I say half-broken, because the keyboard scanning and
trigger/gate works, but its the oscillator outputs that seem dead
(this might have something to do with the range switch being a
make-before-break, so shorting together two outputs momentarily...something
else to replace).|
Since the oscillator also resets the sawtooth at each cycle the sawtooth
output does not work.
|Lack of "grunt"
||Many people have commented out the lack of
"grunt" or "punch" that the Jen has, often citing the digital oscillator
as the cause. However, the EDP Gnat also has a digital oscillator, and
that seems to pack a powerful punch, so I think there is hope for Jenny.
||The missing knob caps look ugly, so replacements
must be found somewhere, or a completely new set of knobs.
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This section catalogues the various modifications I have made to Jenny.
They are not in any chronological order, so you may find some of the
time-related references in the wrong order!
Please note that these modifications were made to my Jen for my interest
and amusement only. If you try any of these mods yourself and damage or
break your Jen (or someone else's Jen) then I cannot be held responsible.
The details are presented purely for discussion.
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Previous sub-oscillator designs have constructed dividers from JK
flipflops, limiting any practical size to 2 octaves (divide-by-4). I chose
to use the 4024 counter, giving up to 7 octaves below the output of the main
The circuit is quite
simple, taking input from the main oscillator square-wave output, and
selecting one of the outputs of the divider stage. Which one you choose is
more a matter of personal taste, and you have lots to choose from! I left
myself enough free wire to reconfigure the output if I get bored with the
Note: I could have used a rotary switch to select the divisor, but there
isn't any room on the front panel.
Now that the level knob for the noise/sub-oscillator is going spare, I
decided to add a sawtooth frequency multiplier
It consists of two parts:
- Variable Threshold Comparator --- when the sawtooth level reaches a threshold
set by the front panel control then the sawtooth waveform is reset;
- Variable Gain Block --- resetting the sawtooth early will obviously
reduce its amplitude, so this gain block keeps the output
at a constant amplitude.
The current circuit is configured to double the frequency when the control
knob is at halfway, then all the way up to about 10 times the input frequency.
Update: A small
mod to allow the LFO the gently modulate the saw trigger point. It uses
the PWM LFO pot, marked "P.W.M.", to inject a small amount of the LFO signal
into the sawtooth multiplier bias circuit, modulating the position of the
the sawtooth reset point. It is subtle, but gives the sawtooth a little extra
movement to the sound, thickening it up.
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Filter Pedal Input
The same mod as for the swell pedal input (see below) but this time
increases the filter cutoff frequency.
Normalised Mod Wheel Input
If you fit the MIDI board then you might also want to make the
normalised VCF control input be the Modulation Wheel control voltage. In that case use
a jack socket with normalising contacts and connect it up to the appropriate output
from the MIDI board.
Nothing original here--its an inverting amplifier wired
across the envelope amount pot, as described with schematic and board layout
on Bluebear's site (link below).
Remove Input Filter Capacitor
I had read on Håkan's page that the VCF suffers from input
high-pass filtering by its input capacitor. Closer inspection of the input
stages shows the 1uF input capacitor forms a high-pass filter with the
220-ohm input resistor, whose cutoff frequency (-3dB point) is around 720Hz.
The solution is remove the VCF input capacitor (replace it with a wire
link) and to fit both the VCO output and the noise output with their own
DC-blocking capacitor and output resistor.
Another BIG change to make is to replace the filter integrator
capacitors. The original ones are cheap'n'nasty disc ceramics---fine for
decoupling, but bad for audio. These have been replaced with 270pF
polystyrene capacitors (the closest I had at the time) and sound much nicer.
I have also tweaked the offset and resonance trimmers to improve response
of the filter. Unfortunately the design of the filter is such that the Q
seems to be frequency-dependent (greater Q at higher frequencies). I may
look into the maths of this filter and see if there is a way to correct
this (perhaps turn it into a state-variable design).
I also note that on the schematic, the output of the VCO level pot goes
direct to the VCF's input capacitor. In practice this is not
the case---there is a 47k resistor in series with the wiper.
The circuit for the
VCF input stage is a two-input mixer---one input from the VCO and one from
the noise generator (see elsewhere). Operation is quite simple: the dual
linear pot mixes the two input sources (VCO and Noise). The 33k resistors
bend the linear response to almost-log, and is a cheap way of making a
log/anti-log pot. The 47k resistor and 4u7/10n capacitors couple the two
DC-biased signals into the VCF input.
Filter Response Analysis
Online (especially eBay auctions :-) there is a common misbelief that the
SX1000's filter has a 12dB/octave response - the usual kind of response for a
2-pole filter. However, the schematics cleary show four filter sections,
so the obvious conclusion is that this ia a 4-pole filter, with a 24dB/octave
response. In theory.
I have run an experiment. Using the white noise source to provide a flat
spectral input to the filter I have measured its response. The result
confirms that the filter exhibits a response far steeper than -12dB/octave,
confirming the suspicion of a 4-pole design. The analysis was done using
The graph shows a slope of about -20dB/octave. This is slightly
shy of the -24dB/octave you would get from an ideal filter, but I believe
this discrepancy can be explained by component tolerances between the four
filter stages - you would only get the full 24dB/octave slope if all four filter sections were
tuned to exactly the same frequency. In the SX1000 the resistors around
the filter are all 5%, and while the capacitors in my test SX1000 have
been upgraded to polystyrene (from the original ceramic) the tolerances of
those capacitors is also 5%.
So, to reiterate - the Jen SX1000 has a 4-pole -24dB/octave filter. Yay!!!
Extra Filter Modes
The filter in the SX1000 is a simple four-stage cascaded integrator design, with overall
feedback path for resonance. Simple, and you get a strong -24dB cutoff. Which is nice.
However, many analogue synths offer more than just a low-pass response from their filters.
Could it be possible that the SX1000 can also be made to produce other filter shapes?
Well, thanks to my friend Emilie, who
has done the tedious maths for me, the answer is yes. You can read her paper on
for the details. Note that this paper is based on the hugely popular (in synth modules) SSM2164, which makes
use of the inversion you get with integrators made with 2164 gain cells. The SX1000 filter uses
non-inverting integrators based around LM13700 OTAs. Fortunately the filter goes directly to a
CA3080 OTA for volume control, which has both inverting and non-inverting inputs.
With a bit of
careful arrangement we can use the various intermediate outputs of the filter, together with the
VCA, to build up other filter shapes. Emilie's paper lists the various modes that are possible.
In this project I decided to limit the scope of changes, so no change to the fourth path ("d = 1"), and no messing
around with the first filter stage ("Enabled"). These limits bring the number of possible modes down to two:
|4-pole BandPass (BP4)
||-0, +1, -2, +1
|3-pole HighPass and 1-pole LowPass (HP3+LP1)
||-1, +3, -3, +1
Note: if you're prepared to use a bigger switch to allow gain setting for the fourth integrator then
there are a few more filter modes you can play with: LP2, BP2, HP2+LP1, Notch+LP1 and Phaser+LP1.
If you do decide to try this then I recommend you break into the "D" path at the input to the VCA, and
leave the resonance feedback path alone.
The implementation is quite simple: one rotary switch and a few resistors and capacitors.
The numbers in the table above relate to the relative gains that need to be applied to each
tap, where the first tap is the output of the first filter stage, and so on. In both cases
we already have the fourth tap as that on its own gives us the LP4 output.
The gains are easy to work out. We start with the fourth tap, which always has a gain of 1 and is fed by a 100k resistor.
For the other taps since the VCA inputs are pretty much current inputs (not quite, but close enough)
we increase the gain by reducing the tap feed resistor, thus feeding more current into the VCA.
For a gain of 2 we divide 100k by two, and get 50k - I use a 51k resistor as the nearest preferred value.
Likewise, a gain of 3 can be done with a 33k resistor.
Add some DC blocking capacitors and we're all done.
Connections to the filter board (match corresponding points to the diagram above):
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The first mod to make is replacing the fixed mains lead and rear rocker
switch with an IEC socket, and to replace the 10k LIN output volume pot with
a 10k LOG switched pot, thereby (a) replacing the nasty mains lead with a
nice IEC socket, (b) bringing the power switch to the front panel, and (c)
fitting a proper "audio" curve volume control.
I have also added a blue LED to provide visual indication
of when it is turned on, now that old neon rocker switch has gone.
[The blue LED in action!]
Fortunately, the 30mm x 22mm hole occupied by the old mains rocker switch
is just the right size for a standard IEC socket. Well, I'll qualify
that---the one I bought from my local Maplin shop was 1mm too big in both axes, so a careful
bit of trimming with a Stanley knife (0.5mm off each edge) and I have a
mains socket that is a perfect fit.
This leaves a round hole to the right of the rear panel, which is used for the
headphone socket (see below).
HAZARD: This modification involves Mains Wiring. ONLY attempt it if
you are competent with mains voltages, or get an electrician to check your
work before you connect it to the mains!!
[View the new switched pot on
the VCA board]
Swell Pedal Input
Adding a swell pedal
(increases volume on top of what is set on the front panel) is relatively
simple---we just need to feed in some more current into the emitter of the
CA3080's current converter transistor.
[View the VCA board mod]
Again, another simple
addition, but quite useful for solo playing when you don't have an
amplifier to hand. We tap off the audio signal from the wiper of the master
volume pot and feed this to a stereo low-power amplifier based on a TL072
(Ok, not HIFI but sufficient for this application).
In case you're wondering why I chose to build a stereo amplifier, rather
than just connect left and right phones to a mono-amp, its to allow future
expansion to stereo (perhaps a panner or chorus unit?)
The additional take-off lead from the VCA board can be seen in the mains
switch picture (link above).
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LFO Wave Switch
The standard LFO generates one waveform, a triangle wave. Not happy with
this, I have modified the LFO to both operate over a wider frequency range,
and also do rising and falling ramps.
To select which waveform is generated I had to fit a switch to the front
[This is what it looks
The circuit for the modified LFO
shows the components that have been added or changed (the oroginal components
are left unmarked). The switch determines the output waveform:
- A - rising ramp
- B - triangle
- C - falling ramp
The estimated frequency range of this LFO is 0.2Hz to 212Hz, enough to
have plenty of possible uses.
For the basic modification (excluding the wave shape switch) just remove
the switch and any corresponding components. Then the modification to the
original Jen LFO (see schematic
page 2) is made with the
following five steps:
- Remove 22k resistor from pot track (live end) to ground;
- Replace 150k resistor with 4k7 resistor;
- Add 100R resistor from pot trace cold side (CW end) to ground;
- Insert 47k resistor into pot wiper track;
- Replace pot with 100k log pot.
External Audio Input
A simple modification allowing external signals to be fed into the VCF
requires the addition of a mono switched jack socket.
The circuit is simple,
adding the socket to the sub-oscillator feed to the noise selector switch.
Analogue Noise Generator
The original noise source in the Jen is an MM3857N digital noise generator.
It sounds horrible, more like a steam engine chuffing along!! So, this is
soon replaced with a true analogue
noise generator, based on a reverse-biased transistor junction.
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Here is the circuit (corrected,
revision B) of the first version of the M110 replacement module. I've
squashed it onto one page, so it fits within the limitations of the free
version of Eagle.
Note: Missing from the schematic is the information that IC6
is a ULN2803A octal driver.
Pictures of the M110 replacement module:
- top, showing all of the passive
components fitted, a few chips, and waiting for the micro and DACs,
- bottom of this compact PCB,
showing the 40-pin DIP header ready to sit in place of the dead M110.
The additional connector is for additional power supplies, the MIDI
input, and the CV/gate/modulation outputs.
Schematic and PCB
Eagle Schematic and PCB files available to download:
Whichever one you choose, they both expand into a subdirectory called
CAUTION: I have been made aware that these files are incomplete and
contain errors. Use these files instead:
If you want to make your own board, I suggest you download one of the
above files and then extract the .brd file and send it off to
who will turn it into a PCB for you.
I used them for my board without any problems. You might wish to add
a solder mask and/or silk-screen position print. I kept my board simple
and cheap :-)
Software is available:
The archive expands into a sub-directory called
jen1k.hex binary file is suitable for programming into an
Note: this archive does NOT include the
AvrX free open-source
RTOS which is needed to compile the code. I suggest you install it in
You will also need a copy of AVR-GCC to actually compile the code; I use
3.3. You might need to frig around with directories in the makefile for the
Jen software (especially where the compiler can find the AvrX libraries and
Alternatively, just use the
jen1k.hex file to program an
Atmel ATmega8L. The AVRDUDE program included with WinAVR is pretty good.
I use an STK200-style programmer that was kindly donated by
Note: the ATmega8 fuses need to be programmed with the following
- Low Byte Fuse:
lfuse = 0x1F
- HIgh Byte Fuse:
hfuse = 0xD9 (no change from factory default)
If you find any bugs in the code, please do let me know so that I can fix
Building this board will be slightly fun due to some areas being a little
squeezed together. If you use a through-hole-plated PCB then assembly will be
easier. The only trick to be wary of is fitting all components before
fitting the 40-pin turned-pin sockets, which otherwise get in the way of
Depending on the size of your programming header you might have to attack
it with a knife to shave off the top millimetre of plastic, which would
otherwise foul the bottom of the case. Just don't cut your fingers off!!
Installing the Processor Board
Three changes to the main oscillator board before fitting this board into
the M110 socket. These changes are:
- Remove the resistor-capacitor pair that drive the base of the
- Remove the PN2222A transistor and link the B-C pads;
- Remove the 560R load resistor connected to the collector.
These changes remove the level converter on the output of the master
oscillator. I found this gave so much distortion of the 2MHz clock signal
that it was better to run the signal at 5V direct from the 74LS221.
Several changes are necessary for the keyboard contacts PCB. They are:
- Remove the 5k6 pull-up resistors;
- Remove the redundant caps and trimmer;
- Replace the old M110 socket with a new turned-pin type.
You need to use 40-pin IC sockets to space the processor board away from the
keyboard PCB. It is best to use turned-pin IC sockets. Fit turned-pin SIL
headers to the PCB, and then carefully push a 40-pin socket onto the header.
You can see in the above photo the header strip and the IC socket.
One final modification to make is a hard link from the Modulation CV output to
the normalised input of the
VCF pedal input. This allows you to control the filter
cutoff from the Mod Wheel of your MIDI keyboard.
The only calibration on the board is the DC out scale trim. This affects the amplitude
of the sawtooth waveform and the filter keyboard tracking. If you do not have an
oscilloscope available then a crude method is to adjust it for equal volume of square and
sawtooth waves (play a key, flip the wave switch until they sound about the same loudness).
The MIDI channel is hard-coded to MIDI channel 1. You cannot change this without
modifing the firmware.
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After some concerted panel-bashing I now have enough new jack sockets to
support all the additional connections into the system.
[This is how I fitted them all in]
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Here are details of suppliers for various parts used during Jenny's
- A good supplier, used for many of the other parts, including
the replacement mains transformer (part no. 88-0300)
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The SX1000 is a fairly reliable machine. However, as well as the M110 dying
there are a few other common faults that are quite easy to repair.
- Broken glide oscillator
- Symptoms: You play a key, and the note sounds. Then you play a different key
but it sounds the same pitch. This fault even occurs with the MIDI retrofit.
Cause: Both the M110 and the MIDI retrofit need the glide oscillator
to move from one note to the next. If the glide oscillator has stopped working
then the pitch will not change.
Fix #1: The glide pot needs cleaning. First try turning
the glide control full circle a few times (first left, then right, then left, etc)
to see if that helps. Otherwise open the synth and squirt some switch cleaner into the
pot. If that fails, replace the pot.
Fix #2: The glide oscillator is a simple circuit based around
a single op-amp. It is located on the small PCB at the lower-left of the front panel,
together with the noise generator (MM3857N).
The original was a LF351. A good replacement is the TL071 or TL081. Both are readily
available from online suppliers or your local electronics store (e.g., Maplin in the UK).
Just remove the old chip and fit the new one, making sure it is the same way round.