Prophet 600 Refurb – Part 1

P600 as bought

Overview In June 2020 I rescued a rather forlorn and broken Prophet 600 for way too much money, but I decided it was worth the time and effort to get it back it to perfect working order. Although financially it does not make sense, as I will probably spend more on it than its currently worth, once I have fixed the P600, I plan to implement a number of upgrades.

• Gligli OS upgrade
• New power supply
• Stereo chorus and voice panning
• New Fatar keybed
• New walnut end cheeks

The Prophet 600 was announced at NAMM 1983 and shipped a month earlier in December 1982 with the first MIDI implementation ever (thanks Dave!). It was a reasonably successful product with 6000 (approx) sold until mid 1985, against the heavy competition from the DX7. The innovative use of digital envelope generators and a digital LFO pushed the capability of the original Z80 microprocessor, the slow envelopes and obviously stepped pots put customers off. The budget nature of the P600 was a bit too apparent!

CPU board

Part 1: Getting The P600 Working The first stage is to get P600 tested and working again with no modifications, although dusty the circuits look to be in good condition and better than I expected. I downloaded the Technical Service manual from July 1983, as I am sure this is going to be a constant companion for the next few weeks! My Prophet 600, serial number 1406, arrived at the workshop on 12th June and I logged these initial visible faults:

• Large number (11) of keys missing
• The keybed is in very poor condition, needs to be replaced
• Slightly damaged Mylar control panel
• No control knobs
• One pot missing
• One pot with spindle cut
• OS ROM missing, all other chips in place
• All cabling intact but disconnected
• Pitch and Mod wheels stiff
• Battery disconnected

The front panel was cleaned and found to be a very good condition with only one visible chip down to the metal which will be carefully infilled with black paint. The rear and underside of the casing have not survived as well and there are numerous areas of black paint missing. The white silk screen lettering has survived, so it needs a black repaint feathered into the original. The rubber feet are in place but the wooden end panels will be replaced. All the intact pots rotate ok and need a clean, and the toggle switches work fine.

Power On Whilst a set of OS (Version 8) and Diagnostic ROM’s arrive from Germany, I can test some basics; is the power correct and the microprocessor being clocked. There are no visible signs of power failure, such as blown caps or regulator heat damage. The first step is to power on the transformer at the correct 240V but disconnected from the CPU PCB. I then checked the AC output voltage rails which should be 18 and 36 VAC. Measurement shows all ok at 22 and 44 VAC with slight transformer whine.

Onto powering up the CPU and Voice PCB’s (they are hard wired together) and checking the analog and digital DC rails. I left the keyboard, bender and panel PCB’s disconnected at this stage. The power rails measured good (+4.97V, +15.07V, -4.96V,-14.95V) and there is a 4 MHz clock at pin 6 of the Z80 chip. I ordered new OS ROM and Diagnostic ROM chips from a German eBay seller and they arrived very quickly.

Initial Repairs Two new potentiometers were ordered from Wine Country in the US and the wooden sides measured, so that new American walnut sides could be made by MintCase. The piano hinge for the front panel is missing the six machine screws that attach it to the rear casing. I ordered a pack of ten M3 x 8mm black posidrive machine screws, and they were sometimes a loose fit requiring a M3 nut to be fitted as well. More progress in Part II to follow, when we try and boot up the P600.

OB-Xpander Replica – Part I

Overview The Oberheim OB-Xpander was a prototype analog synth created around 1982, and not to be confused with the later Oberheim Xpander which although similar is significantly different. There is very little information about the first Xpander, just a picture in Mark Vails Vintage Synth book (volume 1) and some text in the A-Z of Analog Synthesizers. From this limited information I have concluded that the OB-Xpander was based on the OB-8 and developed in the Summer/Autumn of 1982 (because the panel graphics reflect an early OB-8 with Page 2).

Features Here is a probable feature list:

• Part of the Oberheim System alongside OB-XA, DMX and DSX
• Four independent “multi-timbral” analog voices, using one OB-8 voice card
• Four independent LFO’s in the  modulation section
• Keyboard controls, Split/Layer, Unison have been removed
• Probably the same OB-8 CPU board with some minor modifications
• Four sets of CV/Gate inputs as well as the Oberheim Computer Interface
• Stereo and Mono Outputs with preset voice panning
• Cassette storage for patches is retained
• No MIDI, as this did not arrive at Oberheim until Spring of 1983
• Page 2 included, as the Modulation has the additional white silk screening

Modulation Section The Modulation section needs to be per voice with 4 separate LFO’s, to ensure each voice has different sounding patches. The OB-XA has two analog LFO’s for the Lower and Upper voice banks, whilst the OB-8 has two digital LFO’s with the extra sawtooth waveforms that are screen printed on the OB-Xpander panel. So could the OB-8 OS have been extended to generate 4 LFO’s in the OB-Xpander? The limitation is in the Z80 processor speed, but the keyboard scanning workload has been removed, CV/gates added and the envelopes are analog.

The OB-Xpander has a switch in the lower corner of the modulation section, which the OB-XA does not have and was used by the OB-8 as VOLUME MOD. However the labeling is shorter, so maybe this was just VOL.

Master Section This has been reduced down to one column of controls;

• MASTER VOLUME potentiometer
• AUTO switch, labelled as per OB-XA
• MASTER TUNE potentiometer

Control Section This remains as one column but has one change;

• PROGRAM VOLUME potentiometer sets Voice Volume
• PORTAMENTO potentiometer
• OSC2 DETUNE potentiometer and red LED

Oscillator Section The VCO controls are the same as the OB-8 and OB-XA and the panel lettering verifies the OB-Xpander is based on the OB-8 voice card, as it has the lettering underneath the Pulse and Saw switches for Triangle. This is how early OB-8’s were lettered before the Page 2 functions were added on the silk screen.

4 Voices on CPU board

Keyboard Section This has been renamed to three words which are too blurred to read, maybe PROGRAM VOICE SELECT. There are switches to select each voice for programming and patch save/recall. with a fifth switch (assumed) to call up a four layer program (one of 12 available, as in Double mode). The cassette Play and Check functions are retained with the corresponding red LED. The OB-8 VOICE board actually has 4 voice select signals from the CPU board (at A13, A15, A17, A19) but only two are used for Lower and Upper voicing across two PCB’s. This confirms that the OB-8 is the basis of the Xpander and that it would have used 4 voice boards, each with only one voice populated.

Programmer Section The patch selection remains the same as the OB-8 with the familiar row of A-D and 1-8 switches, with 120 patches and a WRITE/RECORD switch at the far right hand side, labelled as per the OB-XA.

Prototype Only The OB-Xpander never made it past prototype stage, probably due to being only 4 voices, and the limitations of the Z80 processor running 4 independent digital LFO’s. Oberheim reworked the idea in early 1984 as the Xpander, with 6 independent voices, lots more digital control, MIDI and CV/gate. The processing power was increased to cope with more features and the Xpander has stood the test of time and is one of the greatest analog synths ever made.

Case Dimensions Peter Forrest states this as; 90cm x 30cm x 12.7cm, the same depth and height as the Oberheim DSX and with a weight of 9Kg, which is half the weight of an OB-8. The Xpander case does not look deep enough for a set of OB-XA voice cards to fit in.

Licio Comisso OB-8 VOICE card

Replica It is possible to recreate the OB-Xpander? It would make a very nice sounding 4-voice multi-timbral synth. All the CEM chips are readily available (CEM and AS), and both the VOICE and CPU boards could be recreated from the schematics. Licio Comisso has already successfully cloned the OB-8 VOICE and CPU boards to recreate the OB-8, but when I contacted him he only had OB-XA boards available.

A rather wide 30″ case would need to be fabricated, with a silk screened front panel. Using smaller width switches might mean the case width could be useful reduced. The power supply would be mounted off the CPU board, and improved.

OB-8 OS Modification I initially considering changing the OB-8 Z80 source code to support the Xpander. The largest change is to increase the existing two digital LFO’s to four, which is no easy task as there is no source code and its quite a large operating system to reverse engineer (16 KB), however this has been done before. The rest of the changes required are smaller. I disassembled the source code but it is a lot of work and time to get the changes made.

In addition I need to find a way to retain the calibration capabilities which use the Bender Board and 1st octave of the keyboard, probably by using a set of miniature slide switches mounted internally. This means retaining the keyboard scanning.

Custom Gligli OS A possibly easier approach is to modify the P600 Gligli code to add the extra digital LFO’s, and at the same time removing the digital ADSR’s, cassette interface, and keyboard scanning. This would also give a strong MIDI capability, but the user interface and MIDI Sysex/cc needs changing to match the Xpander, and then there is all the Page 2 functionality to consider, and the loss of the Oberheim calibration software. The advantage is that the Gligli code is open source and written in C.

Conclusion Whilst an exact replica of the Xpander is possible, the limitation of 4 voices is a bit too limiting, given the amount of time and cost needed to build a replica. My preference is to build the OB-Xpander but as a desktop OB-8 with 8 voices, Page 2 and MIDI. This means no OS changes are needed and the CPU board can have MIDI built in. I will also remove the Noise circuit and add in two new filter modes (12dB HP and 6dB LP). In Part II we will look at the design of this new OB-Xpander.

Korg Wavestation Refurb

Overview I bought my Korg Wavestation new in 1990 as a grey import with no serial number for £1200, £400 of RRP. It was a big purchase for me back then, and my first digital synthesizer, which I used for many tracks in the early 1990’s. I loved the wave table sounds and the keyboard with after touch. I added a few sound cards, Drums and Performance 3. As my studio grew the Wavestation was rather side lined and then put in the garage for storage for 20 years, where it gathered dust and scratches.

In 2020 I dug the WS1 out to refurbish and possibly sell, but once plugged in and making sounds in the new Garden Studio I decided to keep it. I do have iWavestation on an iPad but it is not quite the same as the original in sound or play-ability. It does have the advantage of displaying the waveforms.

The Refurb My Wavestation needed a new back light, new front panel buttons, keys cleaning and some scratches on the screen removed. The scratches on the panel, created during its long stay in my garage would have to stay. The component values referred to in this post are for the original Wavestation, the component numbers and values are different in the AD rack version. The disassembly of the Wavestation is a complex and lengthy process, I photographed and documented every stage. Here is a visual guide and more information on the new LED replacement process.

New White on Black Display

New LED Display The large 64 x 240 LCD is best replaced with a modern LED display which is nearly plug compatible, it lasts longer and enables the noisy inverter to be removed. I bought my white on black display from BuyDisplay with the pin connector fitted. The display is connected to a 20 pin-header on the main PCB at CH13A, I made a new 20-pin ribbon cable with sockets to connect to the new display. I reused the old 2-wire LCD EL power cable that goes to CN11A (CN4 on early models) on the PSU board, with a minor change to the board (detailed below).

Here is a list of the changes needed:

• Pins 19, 20, 21, 22 should be removed or cut from the header on the LED display.
• Pin 19 needs to be connected to Pin 3 (+5V) using thin wire to ensure the right font is used.
• Add a 3K9 resistor between Pin 2 and Pin 4 to modify the contrast voltage.
• Connect the A and K terminals on the back of the display to the old LCD back light cable.
• Remove EL transformer (T2) from the PSU board.
• Remove C28, C29, R17 and R18 from the PSU board as they are not needed.
• Add a 100R resistor between +V pad of C28 to where Pin 4 of T2 was.
• Replace R17 with 6K8 and R16 with 3K9 resistors, to get the contrast correct.
• I replaced C30 on the PSU board with a modern MLCC 100nF capacitor, as preventative maintenance.

Once all the modifications are done and checked, the Wavestation can be reassembled, and powered on. The display works very well, although not exactly a black background it is very close, and it makes using the Wavestation a joy.

EX Upgrade The Wavestation of 1990 can be upgraded to EX capability by installing 4 additional wave ROM’s (total of 2MB) and the EX OS which is at version 3.19. The EX upgrade was more about drums and piano, but there are additional Prophet VS waves in there as well. A total of 119 waves were added and 8 more digital effects added, most of the new wave sequences are rhythmic.

OS ROM 3.19  The operating system is contained on two NEC D27C1000A-12 ROM chips (the Wavestation AD uses the -15) at IC18 and IC19, they together hold 256 Kbytes of data. They have a slightly different pin out to modern equivalents like the MBM27C1000A-15Z, which need pins 2 and 24  swapped during programming. I have used the original chip type to avoid complications. The OS binary is available from the Synth ROM Database.

Wave ROM The wave data is held in four 512 Kbyte Mask ROM’s (2 Fujitsu and 2 NEC) that have been encoded at the factor, with a total of 2MB. We need to use EPROM’s that we can burn with the wave data and I have used ST M27C4001-12F1 chips encode with the wave binaries available from the Synth ROM Database. Four 32-pin turned pin IC sockets were soldered onto the main board along with four 100nF 2.5 mm spaced ceramic capacitors. The four new ROM IC’s are located in the sockets as follows:

• IC2 – WS4P0
• IC3 – WS3P9
• IC4 – WS3P8
• IC5 – WS3P7

I bought the chips and had them programmed by buyicnow.com for under £50 including postage. This a a very easy and cheap way of getting the upgrade chips and sockets. The original Korg upgrade was £300 in 1992.

The Prophet VS Legacy The Wavestation is a major development of the Prophet VS from 1986 and most of it ROM waves were reused, the Wavestation added wave sequencing as well as many more waves than the VS. The 1990 Wavestation uses Prophet VS waves from 35 to 125, including bells, vocals and some traditional Piano and Bass waves.  The Wavestation documentation does not contain a description of these waves but with some detective work looking at the Poly Evolver waves which are also VS waves, its possible to add the wave descriptions, which are here Korg Wavestation – Prophet VS Wave Descriptions.

The Wavestation EX added in the Prophet VS waves 126 to 155, however the ROM memory in a VS stops at location 127, so these 30 waves are possibly from one of the six Prophet VS RAM cartridges which contain 32 waves. The VS126 to VS155 waves are visible in iWavestation but there are no descriptions.

Korg WSC-3S Cards

Patches The Wavestation comes with 3 banks of sounds, one in ROM and two in RAM. Korg produced quite a large library of patch cards as well as three PCM cards with new waves, and many professional patches were commercially released by companies in the 1990’s. A full list is here. I have a large library of sysex files stored on my PC and my personal favorite patches are loaded onto a MCR-03 RAM Card. I use the iWavestation to preview patches and then I load them up via sysex.

The Wavestation was commissioned into the studio on 8th April 2020 and replaces my S-50 in my active setup (as the W-30 has 240MB of S-50 sample banks on hard disk). The refurb has worked really well and the Wavestation now gets lots of use in creating new songs, the key is to try and avoid all the presets that have been overused in the 1990’s .