OB-Xa auto tune demystified

Recently a half repaired OB-Xa hit the workshop. The main trouble was complete failure to auto tune. After fixing some obvious faults (damaged vias, poor quality trimpots) the auto tune issue remained. Although the serial number indicated “old” auto-tune the program LED advanced during auto tune, so the firmware has been upgraded to a revision C already.

Until now I have always ignored that the service manual does say that the Revision C software has a new auto-tune circuit – so I had to learn it the hard way. I measured the gate time of the period meter circuit (which is around 3.82ms for a well tuned VCO, 4 periodes of a high C at 1046,5Hz), measured the clock frequency which is half the crystal clock or 2.4576 MHz. This gives a period count of around 9400.

Now I disassembled the firmware and saw that the target count is not in the 9400 range, but twice of that – 18,788 ! I verified my assumptions by feeding a 523Hz clock from a DDS generator which instantly let auto-tune pass.

During this, I also used the great “XACA2” patch EPROM from Ricard Wanderlöf (read here http://butoba.net/homepage/synthhacks.html). We had an email chat for some days trying to evaluate what is happening and came to the conclusion that all out assumptions must be right and that there must be a hardware change as well.

But first my comparison between the XAAD firmware (the last of Revision A) and the XACA disassembly:

While the Revision C compares the period count with 18,788, the old revisions indeed had a target value half as high!

So the only solution to this can be that the period meter is clocked from the full crystal frequency of 4.9152 MHz for Revision C and higher. Half an hour(!) I made this conclusion I found the ECO #119 which explained how to change the upper board for use with the Revision C firmware.

Some times we learn it the hard way. Lots of techs I asked during the process weren’t aware of the hardware modification – or just forgot about it, 40 years later…

Interesting enough: the original schematics of the later mainboard type for the Revision F and G firmware still show the period meter clocked by 2.4576MHz, while the target value in the firmware remains at 18,788. So this must be an error in the diagram, probably not the only one.

Here’s the ECO #119:

ProPSU – a PSU Upgrade for the Prophet 5

The SCI Prohet 5 Synthesizer has thermal issues in the power supply. SCI’s attempt to make do with a single CT secondary winding results in a voltage of up to 25VDC before the 7805 series regulator. The lack of a decent heatsink makes this even worse. Several techs have been rebuilding the stock PSU with either an additional tranformer or by replacing the transformer with a new one with an additional winding, supplying around 9 volts to an additional rectifier.

This modification also requires some work on the reset circuitry, otherwise the RAM contents might be damaged on shutdown. Other typical work covers replacing the thermal interface material, the big filter capacitors and the tantalum capacitors on the PSU board.

I decided to somewhat standardize the PSU maintenance of Prophet 5’s by designing a drop-in replacement PSU board I called the ProPSU. In order not have to temper with the name plate I made a aluminium heat spreader that is tightened to the back (use of some thermal compound is highly advised!) by re-using three of the original screws. The ProPSU board itself will be, after the cable harness is soldered to the ProPSU, bolted to the readily attached heat spreader by means of 3 or 5 M3x16 machine screws.

The ProPSU can be used for both Rev.2 and Rev.3 Prophet 5’s, with either the +12/-5 volts rails for the 2708 EPROMs provided or missing. Both 15V supplies can be adjusted by means of multi-turn presets. An on-board reset circuit switches the “+20V” rail to the mainboard’s reset generator, simulating a quickly discharging filter cap when the +5V supply drops below 4.75 volts. On turn on, the delay of the reset circuit ensures that all voltages are stable.


Ready to use ProPSU for late Rev.3 or modified Prophet 5 (new EPROMs, no +12/-5V required)


Mounting detail: the heat spreader bar is attached to the bolts of the old PSU’s regulators, after that the ProPSU
PCB is mounted to the heat spreader (isolation washers not shown here)

APR Odyssey: CV/Gate for the White Face

Several approaches of adding CV/Gate to the Mk I Odyssey consist of not more than 3 or 4 1/8″ jacks with switch contact that interrupt and hook into the internal CV, gate and trigger lines. While the CV uses standard 1V/octave, the gate and trigger signals require a rather high amplitude of about 10 volts, with the additional impact that the gate level has a direct influence of the achievable output level of the AR envelope. Furthermore, both gate and trigger signals are needed for most modifications.

By adding a simple transistor circuit that resembles the levels generated by the Odyssey’s A board this ARP now works perfectly even with 5V gate and trigger levels and does not require the separate trigger signal anymore, although the trigger input jack is still provided for special purposes.

Odyssey Gate CV

The prototype for the improved CV / Gate modification in action


PPG 390 Drum Unit

One of the most rare PPG units found its way to my workshop recently: the 390 Drum Unit.
PPG 390 Drum Unit

Basically, this is a very rudimentary 8 bit sample player. It features 8 drum sounds in EPROMs on 4 voice cards. Two clock sources drive the address counters on the boards, which means that Sample Freq 1 defines the sampling rate of the odd channels, while Sample Freq 2 is in duty for the even numbered instruments.
[Block diagram to follow…]

Upon arrival one card was missing, instead I found a non working wire-wrapped vero board with a prototype for a single 4kByte instrument in two 2716s, and two 2732 EPROMs with instruments samples that won’t fit anywhere. I designed a PCB to replace the missing voice card, providing sockets for 2764 EPROMs so experimenting with other samples woule be easier as they can be programmes with cheap USB programmers.

PPG 390 Voice Card

Once completed the design I realized that I cloned the original cards so perfectly that I still don’t have a use for the 2732s – but my layout provides an easy solution, with two piggy-packed 4040 counters and some wires I modified my new voice card to work with the Noise and Hi-Hat EPROMs. Some more wires would allow to experiment with 2764 or even bigger EPROMs.

The clock frequencies are in the range of 13 to 30kHz, so the sound duration is between 70 and 160ms for the originally used 2716 EPROMs and 140 to 320ms for the 2732 sounds and would scale accordingly when using a larger EPROM.

PPG 390 Overview

Apart from this addition, the cards got guides (the blue things) to protect the connectors, new capacitors, additional capacitors in the output lines (originally it had a 3V DC offset), a new transformer, IEC mains jack, a mains fuse and a mains wiring according to current standards.

It is probably not an impressive drum unit sound-wise, but an interesting collectors item and I somewhat like the shaker … 😉

Posted in PPG

Advancing the Synthex

Four years ago I wrote “except the LFO section, because it’s strictly analogue”, but this did not really let me rest. So I started designing a new LFO board with some special features that can be controlled by the new processor board that is currently in development.

While the processor board design is near completion, a first prototype of the Advanced LFO has just entered the programming phase. Here’s a peek for those who are following the project for some time now. It’s alive!

Advanced LFO Board

PPG 1020: Digital keyboard?

The PPG 1020 is known as the stable successor of the 1002 synthesizer. This was achieved by use of a special oscillator architecture that combines digital octave division and a closed-loop approach including a VCO. The VCO covers the whole range(4 keyboard octaves + approx one more for the “digital modulation” feature), but being within a closed loop, it is stabilized to some degree. The details will be topic of a later blog post. For now we only need to know that we need a digital word that determines the octave and a control voltage to drive the VCO to one out of 12 semitones.

Even without the use of a processor (the 1020 does not have a computer of any kind) it would be straight forward: scan the 49 keys, somehow divide the number of the pressed key modulo 12, the result will be the (already digital) octave number, the remainder drives a DAC to generate the CV.

This “somehow” was actually done by using synchronously counting registers – a chain of 2 binary counters, one turning over at 12, the other incrementing whenever the first one turns over, and a chain of shift registers scanning the keys. The first “high” (pressed key) in the serial pulse train from the shift registers will latch the counter outputs in the next cycle, so we have a low-note priority here.

The counters mentioned above drive the octave control word and a digital-to-analog converter built from two 4051 multiplexers and a chain of accordingly weighted resistors. The reference voltage to that chain is generated on the main board and derived from the setting of the tune knob and the pitch bend slider.

For modulation purposes, an offset is digitally imposed. Since the modulation source from the main board is an analog control voltage, analog-to-digital conversion is necessary. A discrete A/D converter was built from a constant current source generating a linear voltage ramp on a capacitor. This slope is than compared with the modulation voltage in order to reset the above mentioned counters, thereby shifting the generated octave control word and the control voltage. With this approach it is possible to modulate the pitch across an octave border, whereas the analog part of the oscillator control would be limited to variation within an octave.

The comparator in this A/D converter is built from discrete transistors, yet integrated into a CA3086. The same circuit, but under processor control, can be found on the analog panel board (ANIN) of the Wave Computer 360!

Here’s the component side of the keyboard PCB:
PPG 1020 keyboard PCB components side

While the copper areas in the right top and middle right are of no electrical use, the long copper stripe on the lower edge is actively driven by a delayed clock signal. One might wonder what the reason for this could be – a view on the solder side and some circuit analysis reveals the secret:
PPG 1020 keyboard PCB solder side

Every key has a 2x1cm big copper area connected to the J-wire on the opposite side. Electrically, each key is capacitively coupled to a delayed clock signal – a signal that is logical “0” whenever the shift registers sample the key lines. This loose coupling is easily overriden by a J-wire touching the bus bar, which is connected to the positive supply voltage by a resistor of just 1k.

The last question about this is: was this only done to eliminate the need for 49 pull down resistors (the shift registers are CMOS devices and would not work properly with open inputs), or were other aspects like debouncing in mind?

A 3.3meg resistor added between one key line and ground suggests that the capacitive pull-down idea did not work 100%.


Oberheim FVS-1 restoration project


A nice, but completely not working Oberheim Four Voice came to the workshop some time ago.

First of all, the power supply had to be repaired and converted to 230V mains voltage by using a toroid transformer.
It has been mounted to a small sheet of Al metal to avoid drilling additional holes into the Obie.

After repair and installation of the power supply PCB, the main part of the restoration might begin

(PSU photos to follow)

Every module on the front panel will undergo a thorough treatment: all potentiometers will be removed, opened, cleaned, lubricated and tested. PCBs will be cleaned, checked for cracked solder connections, switched will be cleaned and lubriatced. Electrolytic capacitors will be replaced, and as this model used a special type of film capacitors which tend to crack (sometimes called tropical fish caps due to their color stripes indicating their capacitance value), also the film capacitors will be replaced. Actually several have broken apart from their leads by the slightest touch, so they are no way reliable for continued service. Poor fish.


The polyphonic keyboard assigner

This module scans the 61 key keyboard and assigns the generated CV and gate signals to the 4 SEM modules according to the adjustments made on the front panel.

Actually, it should do so, upon arrival, no single gate pulse was seen, nor a correct CV.
A defective 723 caused most of the logic to be without supply. But once energized, a hand full of dead CMOS chips paved the way to a working assigner.

[Photo to follow]

The Programmer

A special module of the four voice is its programmer. It allows to set up various parameters and store them into an internal memory with 16 places, separately for each SEM.
It also features two ADS envelope generators for the SEM’s VCA and for modulating the VCF.

The parameters are:

  • frequency setting (offset to other sources) for both VCOs and the VCF
  • A-D-S for both envelope generators
  • Frequency of an internal LFO, individual to each SEM (called “vibrato”)
  • Amount of LFO to OSCs (1+2 simultaneously)
  • Amount of 2nd envelope to VCF frequency (called “modulation”)

The voltages are converted by a 6 bit A/D converter and then into a bit stream to be either stored into 2 1024 bit SRAMs or bypassed around the memory in manual setting. Due to this, testing and troubleshooting of most parts of the circuits is possible in real time.

Every of the 2 channel boards of the programmer features 4 proprietary envelope generator ICs which are said to be a predecessor of the famous CEM 3310. It is far away from being compatible though, and the custom Oberheim ICs also have an additional VCA integated to the envelope chip.

This photo shows the programmer stacked together, with the channel 3&4 board on top. Some CMOS ICs as well as several LM324 had to be replaced here. The cermet presets are kept in place, while the rarely used carbon track presets are to be replaced.

One potentiometer on the programmer’s front panel unfortunately had a severe crack in the resistive track, so I had to mix up parts of the original potentiometer with the track of a brand new 24mm Alpha brand pot.

A set of new T1 3/4 light bulbs completed the programmer.

The output mixer

The Four Voice has an active output mixer with individual volume and pan controls. Apart from cleaning the potentiometers, two of the used µA741 op amps were defective, one had zero output, the other a large offset.
I decided to use NE5534 with approproate compensation here, whereas all other op amps remain original for sound reasons.