Back in the early 90′s I bought a pretty deluxe keyboard synth, the Ensoniq TS-12. This is a pretty large and heavy beast with a 76-key weighted keyboard. The synthesis is based on sample playback with a large ROM collection, a powerful digital effects engine for reverb, chorus, delay, etc., a built-in sequencer for recording, plus the ability to load additional samples off of 3.5″ floppy disks to onboard DRAM. (It shipped with 2MB, which I’ve since upgraded to 8.)  There was also an option which very few people took advantage of, which was to add a proprietary SCSI interface to the unit which would allow much faster loading of samples, with large libraries available using a CD-ROM drive, or a hard drive formatted and loaded up by one of Ensoniq’s other units at the time, the ASR-10, which was capable of creating and writing new sample patches. They shared essentially the same playback engine and effects chip. The TS-12′s built-in SCSI interface software, however, was purely read-only.

The TS-10 and 12 still do pretty well on the eBay vintage synth market. Ensoniq was, unfortunately, bought up by Creative Labs in the late 90’s for their PC soundcard business, and not much later, shut down altogether. Creative massively overexpanded during this period and couldn’t sustain their growth.  This is a bit of a tragedy, since both Ensoniq and E-mu (another classic sampler synth company bought by Creative that suffered the same fate) were top-notch in their field but are now no longer around to support their earlier products.

Recently while dorking around with my TS-12 and realizing that performing some of the factory expansions would be both cheap and easy (old 4MB SIMMs and 256KB static RAM chips cost next to nothing), I decided that I wanted to tackle the SCSI expansion as well.  Unfortunately, the SP-4 SCSI expansion boards were never produced in large supply and they’re evidently fairly rare to encounter for sale.  In poking around at some of the old completed eBay sales, by looking at pictures of the thing I realized that the board itself would be quite simple to reverse engineer, if only I could find the right controller chip.  Everything else was stone simple – a 7805 regulator, a pullup resistor, some terminator resistor networks, and a few aluminum electrolytic and ceramic caps.  I could even see where most of the traces went.

The first pic I found wasn’t great for reading the numbers off the chip, but I could just make out the logo… AMD?

An old ebay description pic of the SP-4

With a little more poking around using Google image search, I was able to find a better pic and could read off the numbers: The controller is indeed an old AMD 33C93A-JA16, a fairly large 44-pin PLCC, and a datasheet was then also readily turned up online.  Also, the better pics showed both sides of the board, allowing me to determine the paths of the vast majority of the traces and build a fresh schematic in DipTrace.  (I find DipTrace far easier to comprehend than Eagle, despite Eagle’s foothold in the hobbyist community.)  Another enthusiast from one of the internet forums had actually done exactly this same project several years back and shared his own schematic, which was quite nice, since it allowed me to verify that I had the netlist right.  No errors found relative to the other guy’s, so good job there.

Speeds for old SCSI-1 are glacially slow by today’s standards (a whopping 5Mhz!) and the electrical spec is single ended rather than differential, so there are no big worries about signal skew, controlled impedances, or anything like that.  Still, I was careful to optimize the layout as best I could with the shortest routings I could find, significant ground plane copper pours on both sides and between traces where possible, and minimization of unnecessary vias.  I kept the decoupling caps as close as possible to the terminator networks and the controller chip’s power input.

Before it’d be worth spending money to make boards, though, I needed to find a source for the obsolete chip.  An onshore company in California was perfectly happy to quote me prices for low volumes of the chip, but the price (well over $100 for a single chip, although with very steep discounts for chips 2->N) was too daunting for a hobby project.  It seemed like it was time to plumb the depths of the Chinese online chip vendors.  For no particular reason, I went with one called UTSource, which said they’d send one for a whopping 7 bucks, including shipping.  I was a little worried about fraud or getting counterfeit goods, but hey, for that little up front, there wasn’t much of anything to lose.  I went for it – and about a week a later, as promised, it arrived nicely packaged and protected, as one would hope, with good bubble wrap and sealed in a labeled anti-static bag.  Whoohoo! Assuming that the chip isn’t counterfeit and entirely nonfunctional, we might be in business…

With that hurdle out of the way, it was time to get a board made and collect the rest of the parts.  Digi-key supplied the connectors and other components, and OSHpark made a batch of 3 boards with their nice blue soldermask. (That deep blue is my favorite color.  So much nicer than puke green.)

Here’s the bare board as it arrived:  Something about the plated connectors just looks so pretty.

My reverse-engineered SP-4

I also ordered a nifty little mounting plate from They’re a little pricy but they did a great job. This piece is a requirement to mount the board within the synth’s case.


It was finally time to put the whole thing together: My workplace has a really nice “Maker Garage” with all sorts of nifty tools, including a cheap but functional reflow oven purchased off eBay from another Chinese company.  I could easily hand-solder the PLCC, but why? Squeegee down a little solder paste with a stencil from, turn on the oven, and about 5 minutes later I had a perfectly soldered PLCC onboard.  Next time, though, I think I’ll make my own stencils using the Garage’s laser cutter and some blank mylar.  Anyone know a good tool for converting Gerbers to some easily laserable vector format?

Hand solder the simple passive stuff, and voila:


(Edit: Yes, I know the soldering job looks lousy, but it’s actually better than that. I was in such a hurry to try the thing out that I shot the picture before cleaning off the flux that had spattered around the backside.)

After inserting into the TS-12 and attaching the ribbon cable to the main system board, it comes up perfectly as expected in the disk menu with a brand new “SCSI” option!  The fact that all the needed driver software for the board already exists in ROM on every TS system board makes this possible without having to find an original unit first: Big smiles all around.  Now to go find that ancient SCSI CD-ROM drive to try it out with!

I did find a few flaws in my design during the assembly of the prototype, which I can easily fix if I decide to make any more: I’m toying with the idea of putting up a kickstarter to make some more of these for any other vintage synth folks out there who may need one, given the lack of availability of the original article.

1) Switch to all surface mount parts for ease of assembly, and lower cost, minus the through-hole connectors and the heatsunk regulator: New board design is ready.

2) The electrolytic axial lead caps I chose to match the original pics were too big for the footprint I used in the layout.  Whoops!  They still work fine, but I’ve found more suitable SMT caps for any future runs.

3) In the new board layout, all pins connected to ground use thermal standoff traces.  Direct connection to the copper pour creates a huge thermal mass that has to be heated up for quite a while before the solder will make a clean connection to the board, especially with lead-free solder, so skinny traces allow the pads to heat up more easily while still getting a good electrical ground.  I’ll also spend more time cleaning off the flux on any future attempts, although this stuff is no-clean and doesn’t present a shorting risk.  I just don’t like the messy bottom side.  More surface mount components also helps reduce the need for that anyway.

3) Didn’t realize that my measurements for the panel were a few mm off: First, the posts that the panel bolts to are NOT laid out parallel to the horizontal plane of the board for some unfathomable reason, one is a few mm lower than the other.  As a result, my prototype sits just slightly diagonally within the case since my slots were drilled horizontal to the board. :-/  Think I want to definitely fix that one so it doesn’t look goofy.  At the same time, I can fix the bug in the panel design file that caused the slots to be routed 0.5mm too shallow, so I had to manually complete the cuts with a dremel, resulting in a slightly messy edge, as you can see above.  It’s invisible once it’s mounted in the unit, but I know it’s there. 🙂 (Edit: I ordered a modified version of the panel with the holes drilled in the right place.  Now it sits perfectly in the case!)

4) The label on the board reads “Digital Reproductions” while the panel says “Electronic Reproductions.”  I think I like the former better, but I should at least be consistent.

Next update after I get an actual CD-ROM to try out…

If there are any vintage Ensoniq enthusiasts out there who think they might want one of these for their very own, leave a comment!  I’m not looking to make a profit and I’m totally happy to make some more to help out the community.  The more takers I can find the cheaper I can make them.  Maybe someone else out there might be interested in one of these fine “Digital Reproductions” products?