Author Archives: StefanHuebner

SE1 goes modular

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Studio Electronics’ SE1X is a nice 19″ synth module featuring 3 VCOs based on a Moog concept, one Moog-style 24dB ladder filter and a 12dB Oberheim-like switchable lowpass / bandpass filter. The VCOs are capable of sawtooth, triangle and variable-width pulse waveforms; with the addition of a triangle-to-sine converter according to Middlebrook & Richer, the SE1X version generates a sine output instead of triangle on VCO2. Everything  is mixed together by classic LM13700 OTAs and finally controlled by two cascaded discrete Moog-style VCAs.

So far everything was strictly analog. The control of all the analog circuits including LFO and envelope generation resides in the digital domain of the central microcontroller. Although the D/A converter used features rather high resolution for internally generated signals, MIDI-only control restricts the actual precision to 7 bits for external control.

Needless to say that the SE1 needs an option for external CV control, stricly analog. Once we started drilling some holes, why not making the whole thing modular? To cut the story short, after all the component count of the modified SE1 has doubled, a new rear panel was necessary to allow 43(!) jacks to be fitted and 7(!) additional circuit boards to be installed.

Ready for a peek inside?

Two out of 43 1/4″ jack holes in the new laser-cut aluminium rear panel grant a peek on the old circuitry

How modular it finally got is probably best described with some photos – stay tuned!

1. The controller board

The controller board for the additional circuitry sitting on the SE1 analog board, waiting to be fitted.
By chaining into the 40 conductor flat cable between the SE1 digital and analog boards, the new board has full control over all analog control voltages and digital switch controls signals.
27 OP Amps and 18 CMOS SPDT switch functions take care of proper signal routing. Almost all analog CVs are buffered and routed to output jacks. Each CV has an input jack assigned, followed by protection circuitry and an input buffer. The mechanical contacts in the jacks do not get in touch with the CVs, but control the CMOS switches. Each VCO has an additional CV input which allows to override the internal waveform setting. The same applies for the VCF; a dedicated CV determines whether to use the internal setting or override to one of the three filter types. For ease of A/D conversion of those latter four CVs, the controller board features a small Atmel AVR microcontroller.

 

2. The VCO jack board (1 out of 3 identical boards)

A VCO jack board in a not fully populated state. Most parts reside on the bottom side.

Each VCO gets one of these jack boards with the following features:

VCO CV OUT – a buffered version of the internally generated VCO CV (pitch, frequency, whatever you like)
VCO CV IN – override the internal VCO CV by an external 0..5V control voltage
WAVE SELECT – override the internal VCO waveform setting by an external CV. With this jack board, all three VCOs now offer saw, tri, pulse AND sine and any combination thereof!
WIDTH CV OUT – the internal version of the pulse width control for rectangular wave form. Dim your studio light with it under MIDI control…
WIDTH CV IN – externally control the pulse width of rectangular wave
VCO RAW OUT – the raw output of the VCO, buffered to 0,775Vrms into 10k ohms in triangle mode
MIX IN – disconnects the VCO from the internal mixer and allows an external signal to be inserted (or an effects enriched version of the raw out)
AMOUNT IN – an external 0..5V CV overrides the internal VCO amount setting

There’s no amount out as this control is not modulated or influenced by the envelope generators, but only a representation of the 0-to-63 value set in the SE1’s menu. Too low resolution to be of real use, sorry.

Important: all intermediate outputs (read: all except the main output) are trimmed to 0,775 volts RMS with a triangular signal at 440Hz.
The inputs are adjusted the same, to a patch wire from any intermediate output to its adjacent input will not change the overall signal amplitude.
This allows for transparent insertion of effects, assuming they provide a 0 dB gain.

 

3. The RMOD/Noise Jack Board

This is the smallest out of the six jack boards of the mod. It provides the following features:

RMOD RAW OUT – the raw output from the ring modulator, trimmed to 0,775 volts RMS with an 440 Hz square wave (50% duty cycle)
RMOD MIX IN – allows to insert an external signal to the RMOD amount VCA, instead of the ring modulator
RMOD AMT IN – override the internal RMOD AMT control by a DC CV of 0..5 volts
NOISE MIX IN  – disconnects the noise generator from its amount VCA and replaces it with an external signal (like the external input on a stock SE1X)
NOISE AMT IN – override the internal noise amount control by a DC CV dod 0..5 volts
MIX OUT – a buffered copy of the “master mix”, the sum of the amount VCAs of OSC1..3, RMOD and noise, right before it enters the VCF

 

4. The VCF jack board

Here’s the board with all the filter controls and signals, look here for its capabilities:

VCF IN  – remove the VCF input from the “master mix” and insert an external signal – 0,775V RMS, input impedance 10kOhms, nothing new here…
VCF TYPESEL – a control voltage on this jack selects the VCF operating mode, one out of 12dB LP, 12dB BP or 24dB, overriding the internal setting
CUTOFF OUT – a buffered version of the internal cutoff control voltage (in the range -5..+5 volts!)
CUTOFF IN – overrides the internal cutoff control, same voltage range as for the output of course
RESO OUT – a buffered version of the internal resonance/Q control voltage (0..5 volts)
RESO IN – overrides the internal resonance control
VCF OUT – the buffered and normalized output of the VCF before it enters the VCA

 

5. Final! The VCA board

Although two parts are missing, here’s the VCA jack board! Featuring those features:

VCA IN – disconnects the VCA input from the VCF output and allows to insert an external signal. 0,775V RMS, you may have guessed it.
FUZZ AMT – something new: the SE1X’s Fuzzzz has gone variable. Apply a 0..5 volts CV here to control the amount of distortion
VCA1 CV OUT – the internal control voltage for the 1st VCA (controlled by ADSR)
VCA1 CV IN – overrides the internal VCA CV for envelope control
VCA2 CV OUT – the internal control voltage for the 2nd VCA (which is mainly the volume knob’s position)
VCA2 CV IN – overrides the internal volume control
OUTPUT – finally, here’s what you are listening to. The level remained unaltered, which means it is normally somewhat lower than 0.775V RMS.

 

White and Blue for the Wave Two

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Recent HD44780 based LCDs fit directly to Wave2.3s and can easily be installed into Wave 2.2s. By removing the old HD43160A display controller and adding a 7421, a modern display will work just fine. The 2.2 and 2.3 firmware already have the proper intialization procedures and only need to be convinced to use them by a simple jumper wire.

PPGs Wave 2 was excluded from this upgrade so far because the EPROM code can only deal with the original HD43160A LCD controller. So the old chip has to go at first

HD43160 removed from Wave 2 TAS board

Although the addressing and chip enable signals to the TAS board of the Wave 2 are somewhat different from the TAS82/83 in the 2.2 and 2.3, a valid enable signal for the LCD can easily be generated by addition of a 7421 chip as well.

Wave 2 LCD Wiring

But there’s still the problem with the firmware…

Fortunately, the HD43160A as a predecessor of the HD44780 uses a subset of its command set. Assuming the power-on-reset circuitry of the HD44780 on the new display works well, it would require one additional initialization command to be issued and another command to be modified. The space required for this modification including another delay loop was gained by removing the memory size checks for 6kByte and 32kByte borders, as all Wave 2s known to me have 12kBytes of RAM (excepting the sound RAM). So the memory test has been modified to check whether 12kBytes of usable RAM are available, otherwise throw a ‘9’ to the display to indicate an error.

There are two more locations in the firmware causing trouble: the HD43160A obviously needs the cursor to be re-enabled after some other operations, which is a command 04hex for the 43160. For the 44780 this means to write the characters from right to left, so these commands had to be found and were simply removed for now, as it does not seem necessary for the HD44780 to re-enable the cursor anymore.

Finally it was worth the trouble:

Wave 2 with new display

 

 

 

Kleine Hummel

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Recently this nice amplifier from Klein + Hummel found its way to me.
I have no idea when I will find some time to clean and fix it, but this should no mean to keep some photos from those who are interested in interesting gear:

Klein + Hummel Telewatt Ultra front view

Front view with top cover. The Telewatt ultra was also available in a build-in version without the cover.

 

Klein + Hummel Telewatt bottom view

The internals: first the legendary metal-paper capacitors will be checked and probably replaced, same for the electrolytics.

 

Klein + Hummel Telewatt Ultra top view

Top view. Yes, it does need some cleaning. Once cleaned, two EL34s will show up delivering 40 watts RMS into the speaker.
I’m quite curious how this little beast will sound…

 

 

MIDI for the Synthex

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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

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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

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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 🙂

To give you an impression how it would look like in ePaper-style – black on white – I’ve mounted  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…

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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

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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

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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 DIP  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

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(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. At  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 😉
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)