MIDI for the Synthex

The later Elka Synthex models had a small MIDI interface built in, consisting of not more than a serial interface device (Motorola 6850), some glue logic and the obligatory photocoupler. The original boards are quite rare, and therefore rebuilds showed up over the years.
I’ve also designed a small board, using SMD components where possible for the ease of assembly and availability of components. Since the firmware used is still the unmodified original, the modern MIDI board suffers from the same problems as the original does – more messages than note on/off would probably screw up the small CPU of the Synthex.

Enough requests provided, a replacement CPU board allowing to MIDIfy the whole synthex (except the LFO section, because it’s strictly analogue) would be a possibe future products. So come on guys…

Oh sure, you want some tech stuff. Here it is, small enough to hide below the 24 wire strip cable:
MIDI board for the Elka Synthex

A Mega Cartridge for the DK Synergy

This Synergy ROM cartridge does not only behave like an easter egg, it also has some eggs inside

Megacartridge for the Synergy

Here the Mega Cartridge resembles the VCART6 cartridge, but that’s by far not the end. The initiator of this nice little project, Fabian Draeger, has just made a short video showing the prototype in action:

Right after inserting, the Mega Cartridge says hello and then immediately switches to the module last used.
In the video the VCART6 and the Wendy Carlos 1 are shown, but the memory inside is big enough to keep all official cartridges ever released. Depending on your interest, the Mega Cartridge will soon be available in quantities as fully built PCB. To compile your individual ROM, I would need proof of your ownership of the original cartridges for copyright reasons.

For the tech guys here's of course a photo of the inside

Mega Cartridge inside view

Old, new, white, blue

Something old – a PPG Wave 2.2 or 2.3,
something new – a 40×2 LCD,
something white – the LED backlight,
something blue – the background color

That’s the recipe for replacing a fading or just boring display in a PPG Wave, and here is how it looks like:

This is a drop-in replacement for all 2.2/2.3 that are already using the new HD44780-compatible display. It can be easily recognized – it is somwhat smaller than the old ones, revealing the metal frame on the front panel, and is mounted on some metal. Old 2.2 with the TAS82 require the on-board controller to be removed and some additional circuitry installed. The firmware is aware of both types.
Even refitting of Wave 2’s will be possible soon, I’m working on a firmware patch to allow the new type to be used with the Wave 2 as well.

And as a special gift, you get rid off that squealing noise of the inverter šŸ™‚

To give you an impression how it would look like in ePaper-style – black on white – I’ve mountedĀ  another display in the Wave. This type requires drilling some new holes, otherwise the LCD appears shifted to the right:

Wave 2 with backlit display…

A How-NOT-to-do-it guide

This Wave 2 owner decided that his PPG also requires a backlit display.
No problem so far. But the datasheet of the EL foil backlight stating that it need 200 to 250 volts AC obviously led to a very wrong decision:

What we see here is an AWG28 2-wire cable ripped off from some flat cable, soldered to the primary lugs of the mains transformer -or, in other words – directly to the 230 VAC mains.
The other end is connected to the EL foil behind the LCD. Even the cheapest alarm clocks from the 70’s running EL illumination from the mains had at least a protective resistor in series.

 

PPG Waveterm B

Aside from some routine maintenance, this Waveterm B had a little devil sleeping inside for 25+ years now, showing that intuition and being insistent sometimes beat circuit analysis skills.
This is the inside of the Waveterm after completion:

What we don’t see:

  • complete rewiring of the mains supply to prevent short circuits or even electrocution of poor service personnel
  • new distance bolts and thermal grease for the voltage regulators
  • interconnections between the two computer boards soldered directly to get rid of the flaky connector
  • additional bolts to improve mechanical stability of the PCB sandwhich (see photo below)

What we do see:

  • new electrolytics in the power supply – yes, I dared to re-fit them the PCB, as they are much smaller than the orgininals and won’t get toasted by the DRAMs in full
  • EPROMs had their contents re-written from a known good Waveterm, as one had another checksum as a good dump of the same firmware version
  • All DRAMs at their places

Huh? Where else should they be if not in their sockets?
Hmmm.. let me think… lying in the dust, being stamped on by some heavy state elephants before real pain is being introduced to them?
Something else adequate to compensate for fooling me three hours? No understand?

Look here:

Actually I don’t have much reason to complain. It worked for 25 years this way, much more one would expect from some modern high technology product. And it did work – sometimes, most of the time. But slight knocking would make the 68000 CPU part crash. After bending the DRAMs leg back everything is fine now for the next 25 years.

The two computer boards are stacked using some DIP40 sockets below the 6809 CPU and were only secured to each other on one side. To prevent this connection from becoming intermittent, I installed two additional bolts to hold them together on all 4 corners:

 

Finally I can show you a good reason to replace an electrolytic capacitor (no, this is not some kind of glue):

 

Rhodes Chroma – Troubleshooting the voice boards

When a Chroma sounds oddly or does not auto-tune reliably, 16 oscillator, filter and VCA circuits along with a handful of CMOS ICs are waiting for a check-up. Although the Chroma’s firmware has functions to support troubleshooting, several devices are hard to diagnose without the possibility to test the voice boards outside of the instrument.

That’s why I built a simple test jig which allows to set all functions and parameters by means of DIPĀ  switches and potentiometers.

With this set-up, verification of flawless function takes not much more than half an hour for all eight boards.
The board shown here has several faults – from two intermittent electrolytics over a broken 4556 CMOS 1of4-demux to a dead 4051, the latter two having been identified using the test jig.

Although this jig is rather primitive compared to my later design (Wave 2.2/2.3 test jig), it is still very helpful.

Wolfgang Palm : Der Kleine

(The Small / Tiny / Little One)

This blog is about the restoration of a very rare synthesizer: Der Kleine made by Wolfgang Palm, only three were built in the early 1970s.

It came to me together with the remainings of something which would eventually have been a case once upon a time. AtĀ  the moment, a new housing is being made from medium density fibre board (while chipboard is wood particles with some glue, MDF is just the opposite). Have a look at an early stage here:

In the mean time, the keyboard has been moved up about one centimeter, the spaces left and right from the keyboard are covered and a bottom cover is being cut and drilled these days.
After that, the case will be wrapped in some kind of artificial leather, just like the original.

[Update June 9: Der Kleine getting its new dress]

The red arrow i pointing to the only part taken from the old case for good karma: the wooden strip that supports the operating panel.
All potentiometers and switches will be replaced, a new power supply featuring a toroid transformer and for the first time compliance with the local electrical regulations will be built.
Several capacitors and resistors will be replaced, missing parts added, followed by a first test and calibration. Some more photos of the electronics will follow.

I’ve reverse engineered the circuit, a small service and usage manual will be available for download here soon. I’ll give some details on the circuit as well as on the functions and ranges of the knobs, so it would probably worth to read for musicians as well as for technicians.

The VCO of Der Kleine is somewhat different from the majority of other analog synths: it needs a linear-in-frequency control voltage, the slope will be around -118Hz/V calculated from circuit components.
No, no negative frequency, but negative CV šŸ˜‰
To allow for external control, I’ll add a trigger input and for compatibility reasons a log-to-lin converter translating the usual V/oct control voltage into the linear scale needed by the oscillator.

Here’s a screen shot of some calculations done based on the values of resistor string on the keyboard, comparing the frequencies generated by Der Kleine to those of the actual note values (covering the range from F1 to C5)

 

Solina String Ensemble

These days a Mk.I String Ensemble made its way on my table.
While working perfectly without the ensemble effect, it showed some heavy noise and reduced volume in ensemble mode.

The noise turned out to be a defective TCA350Y BBD chip. While I’m waiting for the replacment part, I checked the other two delay circuits and found one showing no output at all. Fortunately only a transistor in the astable circuit driving the BBD’s clock inputs was intermittent, so all transistors of this type will now be replaced to prevent further trouble.

There were several types of this instruments available under different manufacturer labels – Solina, ARP and Eminent. The earlier models used TCA350Y BBDs while the latest had TDA1022s.

Consumer Information

(for the case your String Ensemble is missing one or more certain notes…)

At least three different top octave synthesizer chips (TOS) were used: M087, SAA1030 and TMS3616. All of those could be replaced by my universal replacement module RPLTOS or RPLTOS+.

Read more about this little board here http://huebnerie.de/index.php?article_id=14 (in German language)

Wave 2 DRS Adapter

The PPG WAVE2 DRS ADAPTER – demasked

First a short summary of the jacks and switches:
On the left we have 6 phone jacks labelled A to F, one named START/STOP
and a 5pin DIN connector for TAPE SYNC.

Three switches allow to enable a click sound (there’s a small speaker in the DRS box), one to choose between internal or external clock and another to select whether the clock outputs shall be 3 or 4 times the internal timebase.

The phone jacks on the bottom invite the user to feed either a contact-closure or positive voltage trigger signal, the third one labelled INPUT needs to be connected toĀ  the trigger in jack on the Wave 2 – there’s no trigger input pin on the 14pin Amphenol connector, only an output!

Before I start with the description of the 5 right side jacks named CL.1 to CL.5 I’d like to tell all those people who can’t wait to hook this adapter to their Wave 2’s not to have too great expectations, or probably learn some 6809 assembly language.

No, let me start with the bad news instead of the clock outputs. The inputs A-F on the left are not connected within the box. The resistor pack which would close the circuit between the jacks and some port pins of the Wave 2’s VIA chip (which are, for those who really want to hack the firmware, the pins PA4..7 and PB0..1) is not fitted and has never been there.

Furthermore, no known firmware for the Wave 2 – and we know three different versions at the time of writing – has a single line of code that communicates with those port pins in any way.

Now let’s have a look at the circuitry that actually could work.
For understanding it is helpful to know that the Wave 2 has a programmable triple timer chip, a Motorola 6840. Timer #3 generates a clock signal that is normally fed to the clock input of Timer #1 through a 10k resistor. This allows to either tap the internal clock, or to override Timer #3’s output to force an external clock to become the clock source of Timer #1 and thereby control the clock for sequencer, arpeggiator and maybe other functions.

This mixed output/input is connected to the DRS box and directly available on the record out pins of the 5pin DIN jack for tape recorder connection. This allows to record the internal clock to a tape or cassette. In addition the clock signal is routed to two frequency dividers: one, dividing by 8, is selected by the clock switch in the x4 position. By using some advanced mathematics, I was able to calculate that the clock signal generated by Timer #3 is 32 times of … something.
For the x3 position of this switch, Wolfgang and his guys made quite some effort so the x3 clock scaling must have been very important. The clock signal is divided by a binary counter that resets itself when reaching a value of 11, but this would be a little bit too slow to reach 3 times the base clock (32/11 is less than 3, q.e.d.), so the counter is additionally reset whenever the counter of the by-8-divider reaches a value of 32. This way a mean frequency of 3 times something with some jitter is achieved.

Whatever the x3/x4 switch selects is buffered and shows up on the jack CL.1 at 5Vpp. CL.2 to CL.5 carry the clock from CL.1 consecutively divided by 2, 4, 8 and 16. That’s all. It’s up to your imagination what you could do with 3 or 4 times the internal sequencer clock. The CL.4 signal, namely 1/2 or 3/8 of the sequencer clock, makes the speaker click unless told not to do so by the CLICK switch.

If you have recorded some signal to a cassette, you would be able to send it back as the sequencer clock through the DRS box.
The playback pins of the DIN jack are followed by a detector (as simple as a CMOS Schmitt trigger) and a buffer which overrides the clock input of Timer #1 when the switch is set to EXTERNAL.

Finally there’s a START/STOP jack. It’ driven by an inverted version of the CA2 pin of the Wave 2’s VIA chip. When the sequencer is stopped, this pin goes high, disabling all the internal circuity of the DRS box – the counters of the frequency dividers are kept in reset state, and all outputs are forced low, including the tape recording output and the clock jacks.Ā  The START/STOP output is low in this state. It goes high when CA2 becomes low, which is whenever the sequencer is started.

For those who still want to know everything about this little box, here’s the mapping between VIA pins and jacks:

A -> PB0
B -> PB4
C -> PA7
D -> PA4
E -> PA5
F -> PA6

Creamware A16 DC modification

Honestly – serious production with Midi-to-CV to control analog synthesizers seems to be impossible. 7 bits translates to quite large steps in the control voltage. But how about using an studio grade 18 bit A/D – D/A box driven via ADAT?
If a Creamware A16 comes into your mind, you’d better think twice about it. There are some reasons converters like this are hard to modify for precise DC:

  • First of all, the inputs are capacitively coupled. But of course it can’t be as easy as just bypassing the caps…
  • Both the ADCs (Philips SAA7367) and the DACs (Philips TDA1305) use single +5V supply. This implies that the analog signals need to be centered around an internally generated reference voltage
  • The reference voltages are not quite precise. Although their long-time stability is very good, the initial accuracy is 5% or worse. Therefore also the signal range varies from chip to chip.
  • The ADCs have an integrated high pass filter to remove DC offsets, but fortunately it can be easily disabled by lifting up one pin and strap it to ground

The engineers saved some parts by not subtracting the DAC refence voltage from the outputs, but lifted the ground potential of all 32 jacks by the reference of the first DAC (channels 1 & 2). For this purpose, a high power voltage follower with massive decoupling on the output was designed. As the reference voltages differ from DAC to DAC, this approach would not work for DC, so I removed the circuitry and connected the jack ground to circuit ground.

When I was working on the PSU – a quite strange design by the way – I modified two regulators to supply +/-7.5 volts to the op amps because in the original design the op amp supplies were a) somewhat too low and b) shifted by the same amount the jack grounds are lifted beyond ground.

A last modification to the PSU applies to precision and stability: the analog supply which influences the reference voltages was derived from a LM317’s output by means of a voltage divider and a power voltage follower – including thermal drift. To improve the absolute precision of the converters, I replaced the resistive voltage divider with a TL431 shunt regulator.

But things became even worse…

While doing some measuring with the ADAT in and out jacks connected I found that the outputs are inverted with respect to the input voltage. With an ADAT box and appropriate software I was able to determine that the inversion takes place between ADAT-In and the analog output of the DACs, so another inversion in necessary here, including subtraction of the reference voltage.

This was accomplished by inverting the reference output of each DAC chip (there’s one reference per chip), but due to the rather high impedance of the reference pin, a voltage follower was necessary too. The inverted reference and the DAC output voltage are then summed up with an op amp. This op amp drives the output jack’s tip connection via 300R for protection and EMI purposes, while the feedback to the adder is connected after the protection resistor so that the input impedance of the connected load does not influence the output voltage up to a certain grade.

Both the reference and the DAC output are summed up via a series connection of a fixed and a variable resistor to allow to compensate the differences between the DAC chips.

By cutting several traces and soldering some components to the IC pins, not to mention the tons of RTV used to mount the variable resistors, it was possible to build both adders, the voltage follower and the inverter stage around the op amps previously used to drive the outputs.

Really…?
Actually – not. Originally NE5532’s were used, but as I neededĀ  to add another 8 op amps (read on to learn why), the current draw would probably have blown the PSU some day. The whole 1.3 amps were already fed through one single LM337, so I decided to remove all 5532’s (offset voltage: 0.5mV typ) and replace them by MC33078’s, which offer an improved offset of 0.15mV typ. at less than half the power consumption.

A similar treatment applies to the input channels. In the original circuit, the positive input (on the ring of the phone jack!) is capacitively coupled and mixed together with the inverted tip signal using the input op amp of the SAA7367 ADC as summing amplifier. For the DC modification it is required to add the DC input signal and the reference voltage, which is generated individually for each ADC input, and feed the result without inversion to the ADC.

Unfortunately this requires two op amps per channel – but there is only one NE5532 for two channels. Besides replacing the 5532 with the above mentioned MC33078 I piggy-packed another 33078 which inverts the inverted sums for two channels.

Obviously this is only possible with some additional wiring and rather bohemian component placement.
Not to mention another ton of RTV to keep the potentiometers in place – one for the input gain and one for the offset voltage, as for the DAC channels.

After all this makes 24 ICs and about 250 resistors to be removed and then about 200 new resistors, 16 capacitors, 32 ICs and 64 (as in six-ty-four) multi-turn potentiometers to be installed, not to mention the modifications on the PSU board.

After initial calibration, the zero offset of the DACs was kept within 1mV over several days. The gain was calibrated so that 5.0 volts input results in a reading of -1dBFS. This was recorded as a reference signal and used to adjust all output channels to 5.000 volts.

Stay tuned for some more photos…