Welcome to RetroActiveSynth

Based in Dublin, servicing and repairing synthesisers within Ireland & the UK.

We are one of Ireland’s premier repair facilities for synthesisers of all makes and eras. We have unrivalled knowledge of Roland, Korg, Moog and many other keyboards and have 30 years experience in bringing these fascinating musical instruments back to mint condition.

In fact, we even remember when most of the vintage analog synths were new and state of the art, and our magazine archive even has prices, adverts and reviews for a great many.

Servicing of All Brands of
Synthesizers, Keyboards & Drum Machines

From the 60s Vox’s to today’s Virtual Analogs we have seen and serviced the full gamut of music technology. From the lowliest Casio keyboard to the mightiest Prophet synthesizer and everything in between, we have a great track record in fixing Ireland’s classic instruments.

So if your synth or drum machine is not working properly just contact us and in almost every case we can repair it. We understand that each repair job is unique and over the years – 30 years in fact – we have encountered and remedied a very wide range of issues related to these wonderful machines.

You’ll find that we are passionate about the restoration of vintage keyboards, including midi controllers and all kinds of digital pianos. Our long list of happy customers will testify to our expertise, so whether it’s a Minimoog, a Yamaha DX7 or a TR909 or indeed any of the vast spectrum of musical instruments which require an equally vast level of knowledge for instrument repairs, then you’re in the right place.

To contact us call John on Tel: 087 9762840 or drop us an email at …
synthnerd@gmail.com

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The Wave 2.3

We received this about a week ago, it has been dead for a long time and it was dead now.

But we didn’t even think of powering it up until the Backup Battery situation was checked out, it’s a NiCad and our caution was justified.

ppg-before-sml

The keyboard looks a bit uneven because it is, the Pratt-Reed bushes are totally shot, they were so bad that all of the old bushings broke up while being removed, this requires all of the “legs” that the bushings reside on to be cleaned with some scrubbing and a lot of Isopropanol to ensure no bits of old bushes are left behind, this would make the action uneven and exacerbate the key height differences. Rebushing a 5 Octave keyboard is surprisingly time consuming, especially when the keys are filthy dirty, a recent online poster to a Forum said it took him 9 hours to do his synth and he thought that he was just bad at it… Not at all, we do this a lot and it takes pretty close to that most of the time, at least if you do it right.

ppg-io-battery-sml

This picture is of the IO board (it is called that in PPG Speak but we would call it the CPU board) the battery had leaked but seemed to with hold all of its nastiness under the battery itself, there looked to be some track damage but actually none of the tracks had been eaten away, and the 74HC138 chip at the bottom left of the picture seemed ok, which was surprising as it was immediately in the firing line of any leaky fluids, Another reason to be glad we didn’t power it up.

We put in a big Lithium Ion battery to replace the original, with a diode to replace the 100 Ohm charging resistor that the original had, as you would do with a Korg Polysix or 61.

We went to the main four boards that plug into the Motherboard, there is much folklore on the web that these have troublesome connectors, and there may be some truth to this. On this particular PPG two of the boards had Gold plated DIN connectors, but the other two did not, so we metal polished both of these back to shiny and were happy with these.
We were wrong about the Gold plating, but more on that later.

According to some information picked up from the Facebook- Synthesizer Spares or Repair group we found out that a PPG is a Brain Dead synth until it has it’s backup RAM filled with useful Data, and this certainly seemed to be the case. All the basic functionality of a synth barring sound was evident, we had a fully working keyboard with MIDI out, pitch bend and Modulation included, all of the pots could be altered, the display and all the switches were working, evrything but a noise. We concluded that the IO (CPU) board was good to go. Getting presets into the synth proved to be as problematic as a couple of the synths from last month, every attempt failed miserably. We Scoped the tape interface IC’s and basically got no sign of life from the OpAmp that takes the cassette input, the TL084 quad chip was obviously unwell and had to be replaced. Subsequent testing showed tape signals appearing right up to the IC that feeds the signal into the IO board.

We used the Wav file available from www.ppg.synth.net as this was surely going to be a good one, we played it and it seemed fine but a little clicky at the very end, but went ahead and tried it. At least the synth seemed to be taking an interest in the Data even if it displayed a number ‘9’ error at the end, but it was only at the very end, and unlike some synths, there was enough of a Data load to fill most of the preset banks.

We started to get sounds of this thing which did sound PPG’ish and very nice but they were very intermittent, as in perfect sometimes but degenerated into clicks and thumps for most of the time. When we had the voices working, we had eight of them with no bad artefacts, so all good there. Fiddling with the Proz board (PPG speak, we would call it the Digital Oscillator board, maybe Proz is easier!) in its sockets would make it work or not, which suggested poor contacts, which we thought were Gold plated but in fact were not, dammit we were caught out again with Fools Gold for the third time in the last two months.

Spending a couple of hours cleaning these “Gold” contacts back to shiny Silver took a while, but the results were no better, as the symptoms seemed to be a loss of the waveform data stream from the Proz to the Voice cards. So in our insanity, we pulled out, polished and replaced all 24 of the DRAM chips on the Proz board (that is 24 chips with 16 pins each and both sides of each pin polished). This did not improve things one bit, but no harm to do, as we did clean and reseat all of them, so that is another thing not to worry about. We suspect, as we have for days, that the Motherboard socket connector has been strained and needs to be replaced, the slight bending tension on the Proz board at the B connector does seem to keep the nice noises happening for longer, but there is no way that this is a useable synth, we will have to put this one on hold until some spare parts arrive from far flung places.

Later on in this month, and while working around all of the various pieces of equipment that flood into the workshop, we got back and did a bit more on the PPG. There are some weird anomalies in this synths behaviour, for example, program 2 is a lot more reliable than program 38 for example, as in it is playable for a longer time, and while it is is playable, changing the waveform in the Wavetable on both the main Osc and the Sub seem to work fine. It doesn’t seem to be the Motherboard connectors, we have had them all out, checked for weaknesses, and carefully put back in. There are a lot of socketed Octal latch chips on the Proz board, 74LS377’s and we have those, so a one by one replacement of those is the next step, although we would be very (pleasantly) surprised if it were as simple as that!

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Korg DW8000

We got this one in recently, with a dead battery and complete loss of Presets which is not so unusual for a 30 year old synth, and the battery was certainly dead, but the strange thing about the DW8000 is that if the memory is scrambled the synth behaves as if the main CPU board is past saving, if you try to edit a parameter for test purposes and the original value was corrupted, then the synth will crash and reset to program 11, both the parameter value slider or the up and down buttons will do this.

After a new battery was installed in a holder, we tried to reinstate the original programs back into it by means of a Sysex transfer but this just didn’t work, we could see numbers incrementing on the display indicating that presets were being sent over but the results were rubbish. A bit of research showed that this particular 8K had the oldest of Firmware in it, and the possibility that that version didn’t handle Sysex properly (Version 07). We downloaded Wav files of the same factory presets and had a go with that, and even this didn’t work, dozens of attempts being made with differing levels from the computer but with no success. In the end, we dug out an old Cassette player and used the original factory tape that the owner got with the synth and this worked fine, 8 banks were counted on the Green display and the Red displays showed Good at the end.

Of course this synth had other issues apart from the usual battery problems, being a well gigged instrument this one had a beautiful thick line of rust under the keyboard where liquids had been spilled on the keyboard, and quietly ate away at the chassis when no one was looking. A nice drop of something fell through the Joystick and landed on Voice 1, eating through a resistor on the voice PCB. And of course most of the Tact switches are shot, in these mid eighties Korgs they take a lot of abuse along with use, as every function of the synth is handled by the minimum number of switches, 17 in a Poly 61 and 22 in a DW8000 (they have the same switch).

dw8000-rust-sml

The interesting thing is that a Korg synth of that time (A poly 800, DW6000 or DW8000) behaves as if their CPU boards are completely busted when the onboard battery backed RAM has been corrupted, it can take quite a bit of work to enable the RAM to take a clean new factory bank (or any other), the P800 is not too difficult, the DW6000 is a little more difficult, and the DW8000 is a bit of a pig. There is a routine on power up to initialise the RAM on a DW8000, hold down the 5 and 8 buttons (if they work) on switch on and this will reset the corrupted RAM chip. The synth still produces no sound but at least won’t crash endlessly if the Parameter value controls are used, and you can quickly set up a sound by raising the Oscillator levels (parameters 13 and 23) and then the filter frequency, and the two Envelope generator Sustains. This will give you a sound which you can write to memory to ensure that your new battery is working, and more importantly that there is nothing wrong with the RAM or its circuitry.

The irony of the fact that it took the original factory tape in an actual cassette player (albeit a good one) is not lost on us, the tremendous amount of hassle involved in trying to get the right levels from an allegedly working Wav file to work with this synth from a computer way exceeded the difficulty in replacing the battery in the first place. Our ongoing problems with Prophet 5 tape tranfers led us to recording a Wav file (which is widely said to work) onto cassette tape and to try this out in the future, the P5 is said (by Sequential themselves) to be sensitive to the speed of the tape recorder, and ours has pitch control, another parameter to enjoy !!!.

Actually this is what we did for the P5 , we played a carefully recorded Wav file from a cassette deck into the P5 and it worked first time, giving us 40 working and correct sounding presets (according to the end of the Owners Manual). It also confirmed after a couple of days of testing that our Veroboard memory replacement seems to work very well, and this synth is good to go!

In the grand scheme of things, most Wav files available for most synths on the Net do work, and the all time most difficult was the Prophet 5, and then very recently the DW8000, though we did find the file online to upgrade the DW8000 to the last version, V12 which may have cured the Sysex problems of Version 7, and we did burn and install this, but we didn’t test the scenario, this synth was done, and the next one was on the bench. We don’t have the luxury of exploring every possibilty for every synth that comes in, that is the job of the well equipped hobbyists that populate Forums and proudly proclaim their successes (and the absolute best of luck to those individuals, they are sometimes a useful source of very interesting knowledge), but they are not under such time and space constraints as we endure.

This DW8000 is good to go, the battery change was easy, the rest was not. Apart from the difficulties reloading the factory presets, we had to replace several switches, which meant that the keyboard had to be removed, as did the output Jack panel board because the two front panel boards are connected by a loom that runs behind a plastic pillar which requires removing the output board to get the panel boards out. We also used rust stopping chemicals on the base plate, the kind of stuff you use on cars, scraped the excess rust off, cleaned the area thoroughly and then applied a second coat of rust stopper. This should stop it getting any worse over time. We also replaced the 2 pin IEC connector with the more common 3 pin version, and obviously added a safety Ground, this required filing out the metal power connector bracket by about 1 mm on each side, which if you have the files is straightforward enough, but a little time consuming.

The Polysix yet again!

We get quite a lot of these, and their state of repair varies widely, the range of problems normally encompass the battery, the keyboard and the Tact switches, you will usually get at least two out of three of these. On this particular one, the battery problem had been sorted out many years ago, and while a coin cell wrapped in gaffa tape with wires to the board wouldn’t be our way of doing things, at least it works and there is no damage to the CPU board, R91 was also correctly replaced by a diode so this particular synth retains it’s presets.

This particular Polysix played six voices, and in tune, so it would seem that we had little to do, but about 20 keys of the keyboard didn’t work, and unfortunately there are three different possibiitiies as to why the keyboard won’t play correctly. dirty contacts is the logical first call, and the whole keyboard and contacts were cleaned thoroughly with IsoPropanol and while this made an improvement the results were not satisfactory.. while the keys made contact eventually, there was a noticeable delay between key strike and voices sounding on some keys, and this seemed to vary by how recently the contacts had been cleaned again, as in cleaning them right now gave you a good result, but an hour later, they were no better than they were originally. The top 7 keys of the keyboard could not be made to work at all, despite scrupulous cleaning, and the fitting of rectangular Polystyrene shims under each of them (the keys themselves have a hollow rectangular section which presses down on a circular rubber button, (who on earth thought that was a good engineering idea?) the shims at least provide a solid flat surface to press on the circular rubber buttons and in theory should provide a much better contact with more even pressure, and while this works on difficult keys a lot of the time, it is not infallible. We fitted a contact strip from a Poly 800 onto the top 7 keys, and this made them work perfectly, we don’t know if it just the newer contacts or a combination of the cleaning and the shims and the new rubber buttons, but it did take the new buttons to actually make the keys work reliably. The Poly 800 buttons seem to be made of thicker rubber than the originals and seem to make the keys a little firmer to press.

p6kbd-shims-installed

The above picture is of the shims installed in the keyboard below C3, which was horribly intermittent, and while it worked for a while, even a day later the performance deteriorated to the unuseable. The synth came in to have a CHD MIDI kit fitted so theoretically this was a moot point, but these days a lot of small studios will try to leverage the most out of one keyboard controller, and while the CHD doesn’t do MIDI out, the KiwiSix certainly does, and this does require a fully functioning keyboard.

As of tomorrow, we will have a shipment of Bob Wiegel’s famous contact pads for old Panasonic keyboards, the website address is www.sounddoctorin.com, we will soon see if these will be enough to resurrect a completely knackered keyboard assembly, one of our clients today told us that it did for his Monopoly, with the only problem he had being the extra cleaning of the gold plated contacts on the keyboard PCB for a few notes.

PolySix keyboards do not have Gold Plated contacts.

There I have said it, one of the Holy Grail’s of Internet synth folklore that the Polysix has gold plated contacts on the PCB, which in fairness we assumed to be correct because everyone said so and no one argued or tested this out. Having had so many difficulties with this one, we tried to make the Gold shiny, we have successfully shined Gold plated connectors on Minimoogs, Prophet 5’s and other synths that had them. Using good quality metal polish on the PCB the colour of the keyboard pads went from a dull Gold colour to a shiny Silver type colour, we saw this before on our Fluke Scope when we also assumed that the contacts were gold plated, but turned out not to be, with a major improvement in functionality thereafter. The Fluke also has rubber membrane type switches, so perhaps the Carbon button contacts are a cause, but it could also be from environmental issues and a synth that is more than 30 years old.

That being said, restoring the keyboard PCB to it’s original condition did not make the keyboard work much better, so it’s a kind of race between the sounddoctorin pads or a complete set of Poly 800 type contact strips.

And the winner is Bob Wiegel’s Magic Buttons, we didn’t muck about with just using them on dodgy keys, we replaced all 61 and got an instant good result. All keys working perfectly and with good response times and behaving as well as a 30 year old Panasonic keyboard can. If you have an old knackered out Panasonic keyboard (and you probably do, a huge number of synths used this mechanism in both non-velocity and velocity sensitive versions) then we can definitely recommend these, we will buy a lot more immediately.

bobs-magic-buttons

The above picture is of the new contact buttons on their carrier, and the six contact block from the synth already has had the new contacts applied, (hence the tweezers).

My Goodness, More Guiness

Actually on this synth (the Polysix) the keyboard was one side of the problem coin, the Tact switches were the other. Surprisingly enough, most of them worked, but a lot of them felt extra sticky with a very distinct click when pressed, which they shouldn’t. Removing the black screws from the front panel and releasing the programmer assembly revealed the horror of the innards, sticky black goo over all the metalwork, the usual dust on the PCB’s turned into a dark brown sludge and some kind of new lifeforms growing over the Arpeggiator Speed Pot and slide switch bodies, a lovely green patina over the legs of a lot of the switches, a picture says a thousand words, so…

switch-board-dirty

The brown staining doesn’t look too bad from the picture, but cleaning it off with cotton buds showed how much there was and how dirty the pcb got, we did the Kiwisix LED mods at this time because we had the switches out and taking out a few more components made cleaning the pcb a bit easier and more successful. The switch bodies themselves were brown and sticky, and to get to the point we replaced all 19, two new keytop carriers being necessary as they were broken, and one keytop (which was missing) which came from a TR808, but which did fit quite well in the front panel, and doesn’t look too much out of place.

dirty-metal

We tend to fix first and Photograph later, so it is not too common for us to have before pictures, but here is another example of the staining on the metalwork and pcb’s, it actually doesn’t look too bad from the Photos, but the reality is much worse than the pictures display, but in the end the complete replacement of all of the switches and the thorough cleaning of every part did fix this issue.

prog-section-complete-sml

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The Prophet 5, What a lovely Synth, when it works!

The number of Prophet 5’s coming into the workshop over the last few months has grown to three Rev 3.0’s, to say they come direct to the Bench would be untrue, they come into storage awaiting the time and the right moment to get some attention.

Over the last several months they have all gone into Triage, where they spend a day on the bench while we assess what is wrong and what it will take to put right, standard stuff for most synth repairs you would think (without the several month bit) but all of these have been broken for a long time and been in storage for a decade so we don’t feel too guilty about that.

The Rev 3.0 was the first of the P5’s to use the Curtis chipset and was a complete redesign of the synth compared to a Rev 2, but it did inherit a couple of (in hindsight) bogey factors from the Rev 2 era, firstly those ancient 2708 EPROM’s that require 3 different power supply rails, and those ancient and unreliable 1k X 1 6508 RAM chips that store the programs, that is 1 Kilobit for each chip and eight of those to make 1 kilobyte.. Two out of the three P5’s here have bad memory backup problems, the IC’s are almost unobtainium and very expensive if gettable, and old unreliable chips even if bought again have no guarantee of lasting very long.

p5-rev-3-voice-indicator-sml

The Sequential service manuals of the time are very good and well written and even informative, but with over thirty years under their belts, the old Prophets need a better system of voice assignment indication than “put your fingers on the VCO chip and if that voice goes out of tune, then that it the one being triggered”, That just isn’t good enough for a synth with multiple problems, especially oscillator tuning problems. The above picture shows our latest creation, a clip on voice indicator for Rev 3’s. It uses a 16 pin sprung IC test clip with a small stripboard with some high efficiency LED’s and is attached to (or over) IC 340. That chip is the latch that drives all the Gates and it made diagnosing a P5 with multiple voice problems considerably easier, and there were multiple problems on the first one to be finished.

The first one to go had to have…
The Keyboard Rebushed and totally cleaned, and all the contacts cleaned, realigned to make them all work.
All the the Pot’s taken out of the front panel boards, stripped, cleaned and relubricated and of course tested.

Everything was starting to look better when after another switch on, all Analogue functions of the CPU board were dead, no CV’s no Sample and Holds, no tuning up, nothing. After a couple of hours of careful Scoping it looked like IC 319 had died suddenly (the Prophet ailment yet again) and replacing it confirmed that it had. And then we get to the Voice card where there was one CEM3340 dead on osc1 on Voice 3 and very strange filter behaviour on Voices 2 and 3. A lot of Scoping and even some chip changes were done to discover that two of the CA3280’s were gone dealing with filter Polymod on Voices 2 and 3.

If not for the Voice indicator we built, then this would have been even more of a Headf**k than it was, and it was still quite one of those. The total chip count to sort this beast was 9, including two Curtis chips and two CA3280’s along with a handful of logic IC’s, as well as all of the power supply decoupling Tantalum bead capacitors being replaced with Electrolytics right at the very beginning.

Tantalum Bead Capacitors

There are long debates about the good and bad points about Tantalum capacitors, and there certainly are some good ones, but the bad point is that they can go dead short on power up of the synth, the exact reasons are unknown, some say that it is due to age and that Tants have a lifespan, but as it is solid state device (it has no liquid electrolyte in it like Electrolytics do, the clue is in the name), why should it suddenly expire. There is no denying that Tant’s do go short, we have replaced hundreds of measurably shorted ones in a huge variety of equipment.

The other side of the Tant is that it is used in the Audio path as well as decoupling the power rails on Prophets, and so far there has never been a problem with them in any machine when used in that mode, they have a limited amount of current and voltage fed through them due to the circuitry around them, and they seem to last forever. The sound quality of Tants in the audio path is definitely not Hi-Fi, tests that we did years ago with CD quality audio showed this, for Hi-Fi the Tants sounded awful, but probably their presence in a Prophet is part of the sound and if we ever found a dead one we would replace it with another Tant.

Our rule of thumb is that if one side of a Tant has a raw power rail from the power supply on one terminal and Ground at the other, then replace it with a low ESR electrolytic. Simples!
There are 19 of those in a Prophet 5 that we replace, 8 0n the CPU board, 7 in the power supply, 3 on the Voice card, and 1 on the left side of the front panel pcb, a mixture of 2u2 and 10 uFarad.

One other side of the Tant is that it has very low leakage, it doesn’t dribble its charge away with the gusto of most Electrolytics, there are a couple of Tants involved in the Battery backup system for the presets, they have never failed and have never been replaced, but if they were replaced by Electrolytics I would suspect that the lifespan of the new battery would be reduced considerably due to unnecessary capacitor leakage.

Many years ago we replaced the leaky Electrolytic capacitors in a Korg Polysix’s LFO and PWM sections with Tants because the calibration routines wouldn’t work with the original Caps, a few suitable Tants in there sorted that one right out, the issue being the slowest modulation frequencies, the leakage of old electrolytics just prevented the circuits from going down to 0.1 Hz or lower.

The Next Prophet

The next Prophet 5 back on the bench was another Rev 3 but with an entirely different set of problems, the keyboard was totally shot, and the Pots were very flakey, but that is not unusual for a 30+ year old Prophet that seems to have had minimum maintenance in that time apart from 17 bits of guitar string replacing the famous Pratt-Reed J-wires (thats a quarter of the keyboard).The J-wires themselves are getting scarcer to obtain and the price is going up accordingly, so I am experimenting a little with items obtainable fron the Jewellery world, Gold plated Copper wire, Solid Silver wire, obviously of the correct diameter and as it turns out, one of the standard jewellery wire sizes is the same as the J-wires in Pratt-Reed keyboards. I don’t know if they will be as good as the originals for a long period of time but the originals failed regularly and at least jewellery consumables will always be available, and relatively cheap compared to hunting down the originals, although there is a shipment of expensive original J-wires coming in (hopefully the last one).

The big fly in the ointment with this P5 is the battery backed up RAM, the chips are shot. If your create a few simple presets and store them (with a brand new battery installed) and then try to recall them a few minutes later, they will have changed noticably in a matter of minutes even with power on. If you switch the synth off and then back on again, the newly programmed presets will have changed some more. But this one got worse, Prophet chip death kicked in again, when DB6 ( a Data Bus Line) suddenly died across the entire synth, meaning that some buttons didn’t work, one in eight keys didn’t work, it wouldn’t tune and the ability to remember programmed presets was much worse than before. The logical thing to do first is to remove the relevent chips that are in sockets first, because if it is not one of those, then either tracks will have to be cut or IC’s cut out, neither of which are good things. It worked out that IC 307, one of the memory IC’s and responsible for DB6 in the RAM had failed, and after its removal the synth returned to a kind of normal where it would work fully in Edit mode, where all voices responded perfectly to all programmming functions, all keys worked and all switches did likewise. As two of the P5’s here seem to have bad 6508’s, we tried to design a slight upgrade to some later and more reliable family of RAM, the Hitachi 6116LP, they are 2k X 8 RAM chips and would in theory allow an extra bank of Presets, but we had to design a Daughterboard to be PiggyBacked on top of one of the EPROM’s, and while the result seemed good for about 20 minutes, there were soon some signs of memory corruption over time. The Battery was new, the new RAM chip was tested as good, there had to be an issue with the timing of the signals into the new RAM which while it is faster than the 6508’s. has different requirements to be reliable. A world wide Internet hunt for any help came to nothing, the result was, “we don’t know, good luck with that, when you sort it out tell us how you did it”.

rev3-memory-change

We did design a PCB for this, but etching it at home proved to be very problematic, so for prototyping purposes we did it on some wirewrap style Veroboard, just to see if it worked, which it doesn’t. There will be no problem redesigning the PCB if and when we can make this work, the issue is the amount of time that will be taken up in testing to ensure absolute reliability

Actually two weeks later, after some research on the timing cycles and Truth tables of the three enabling pins on the 6116 (the old 6508 only has two) and trying to work out which of the three to not actively use, as the P5 inherently can only control two of them. First of all we went for the OE\ line, and this was not the correct choice, a detailed analysis of the Truth tables showed that the OE\ line was only effectual in 2 out of 8 logical possibillities for memory access, whereas the Chip Select function, CS\ was valid for at least 4 and was important to the timing of the memory write cycles also. Anyhow to get to the point, we rewired the daughterboard to use CS\ instead of OE\ and so far this has tested out perfectly, everything we program into the Prophet gets remembered correctly and multiple power cycles have not changed this. Strictly speaking, the Write Enable should be asserted at least 20nS after the Chip Select, and we half expected to have to insert a couple of 74LS00 gates in the write line to ensure this, but as the memory busses seem to run at 1MHz which is quite a slow speed even for old RAM, this thankfully seems unnecessary.

It seems that this synth is good to go but for one thing, the 17 Guitar strings in the keyboard, we have scrubbed and cleaned them endlessly but they are still unreliable, and we can only get 10 replacements from the US, and those are earmarked for another P5, the J-Wire shortage is proving to be a difficult problem, and as all of these three P5’s are all Rev 3.0’s with no MIDI, they still need their keyboards to work properly.

Despite the fact that this synth took a lot of work, the owner claims that this is a special sounding synth, so much better sounding than all the others he has played, and while testing it showed its calibration to be well out of spec, we haven’t touched any of the trimmers or tried recalibrating it at all, who wants a perfectly calibrated P5?

We could fully recalibrate the synth back to factory specs, but the client might not appreciate it.

One more interesting thing about this particular Prophet 5, until the RAM died, this synth was as it originally left the factory, no replacement Curtis chips, no new Op-Amps and no new Multiplexers. This is the only P5 we have ever seen in which this has been the case, every other one we have seen has had a long history of gradual failures sorted out accross the 30 years.

The Third Prophet

The third Prophet 5, which was the first of the three to arrive, will be the last to go, although a lot of work has gone into it over the last year or so, but it’s list of problems has been more interesting and difficult to diagnose than most. The original problem was that the presets in the first Bank were getting corrupted in a worsening way, every few switch power ups it would get worse, and then over time the whole contents of the RAM had turned to garbage. We put in a new battery, and replaced the 74C02 chip in the backup circuit with a new 74HC02 (which we believe to be equivalent in practise and far more reliable in time). Trying to program in a few Presets in the first bank didn’t work, the RAM didn’t remember anything at all. So it was logical that the RAM had failed, wasn’t it?

However before we got to debugging this, Sudden Prophet Death kicked in big time, the first failure was the 5MHz CPU clock module, a completely dead Synth. The only place reasonably convenient to get one of these from was Vintage Planet, who had inconveniently decided to shut up shop permanently. So we designed and built a replacement circuit on Veroboard with a 74LS00 chip on board to handle the Crystal and buffer it to feed to the LS74 that is the next stage. This worked for a short time, the synth tuned its oscillators, the A440 tone was spot on, and the synth mostly worked in non-preset manual mode, although the LFO was dead and the Filter on Voice 2 was just wrong. Replacing the CEM3340 in the LFO circuit cured that , but the weird Filter behaviour took a lot more to Debug, a replacement filter chip made no difference, as did testing all the surrounding components after lifting one end of all the resistors to measure them, and replacing the socket, and the Op-Amp associated with the Filter.

After a lot of time, we replaced the Filter Envelope chip and this fixed it, but we were fooled by the fact that the Envelope looked good on a Scope on an AC coupling setting, it was just when we measured the Envelope output from the chip with a meter that we found it was -8V and moving up and down correctly, whereas the others moved about 0V, so logically the chips output stage had failed. So far so good we had a basically working synth with dodgy RAM so we called a temporary halt while we researched a better replacement.

By the way, we had long ago replaced all 19 Tantalum Bead caps in the synth to try and stave off Sudden Prophet Death Syndrome (SPDS!), but this P5 still had a bit more suiciding to do, U319 failed, a 74LS138, and this meant total shutdown of everything, no keyboard, no switches, no tuning nada. Replacing this restored once again most of the synths basic functionality but another 74LS138 failed soon after. The P5 now seems stable for the last few months, as in nothing else has died since, but the Ram problem still remains. Some testing with Prophet number 2 on this blog and number 3 showed weird behaviour on the memory write lines on the no.3 and this points to a failure of the 74LS138 that is U318, evrything else about it is fine, but it looks like it enables writes to the NV RAM when it shouldn’t and whatever the contents of the databus at the time are inadvertently written to the RAM chips which explains the permanent gibberish in the NVRAM. We should have an answer to this shortly, when we can get back to it, and will report the results.

The results are in, we replaced U318 and the results are the same, so we will have to build a new memory board for this one also, everything else in the synth seems to work fine in every respect other than any semblance of memory retention.

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Kawai SX210

Sorry for the delay in updating this, but it really has been extraordinarily busy over the last several months.

This is an interesting machine, and we have had two of them over the last few months, both of them purchased by the same person and both needing different kinds of work to make them perfect. Unlike the badly designed CPU systems of the SX400, the 210 has a delightfully simple and logical approach to fulfilling it’s task.

sx210-complete-sml

This synth has a lot of things in it’s favour, 8 Voices each with SSM2044 filters, a 3 phase chorus unit (both like a Korg Polysix) and 32 presets just like the Polysix. Each parameter has a button and the variable ones are instantly adjustable using the rotary encoder. The Black switch tops are for on/off functions and the Grey ones are for variable parameters. Before the days of LCD displays, this synth has six digits of 14 segment starburst LED display which allows you to name presets with real names, programmed using the keyboard to select the letters and/or numbers that you want.

But there is one small fly in the SX210’s ointment, and while it may seem trivial, it makes using the synth very difficult if the LED’s above each switch don’t work. The LED’s above the switches protrude above the buttons by a couple of millimeters, and this is enough to make them vulnerable to damage from either transit damage or from overly energetic button pressing. Like most synths of the era and even to this day, LED’s are driven in a grid by high speed interfaces from the main CPU, so losing one LED due to a pcb track break could stop 4 or 5 others from functioning, and the way that the SX210 is designed is to balance the current through the LED’s of the whole panel to avoid too much flicker and variations of brightness.

The PCB tracks on the front panel devoted to the button LED’s are very small and thin and easily breakable and this seems to be an Achilles heel of this synth, both SX210’s that came through here had problems with the LED’s and in both cases broken (and practically invisibly so) tracks were the reason.

One other big issue is the NiCad battery that the synth was originally fitted with, it can go bad and leak like in so many other synths of the time. Most of the remaining 210’s out there had this sorted out years ago, but this picture is of one that wasn’t.

sx210-bad-acid-sml

Everything in that area had to be cut out and replaced and in the final analysis, three pcb tracks had been eaten away and as this section drives the gates for the envelopes of the voices, dead voices were the result.

This is a great synth when sorted out and it’s only real Achilles heel is the lack of a MIDI interface for it, which is slightly ironic as it’s cassette interface in and out are run from the serial interface of the 8031 CPU chip which is certainly capable of handling MIDI. Someone out there, just write some new firmware to handle MIDI using the CPU’s internal abilities and we can sort out the hardware end. An SX210 with a good MIDI interface could be the next big thing in the synth world.

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