Tonight I experienced my Roland S770 sampler in full color, and it was deeply satisfying.
First, a little story, and then I’ll try to give more of the technical details at the end:
Some of the old Roland samplers from the 80s have the luxurious option of a monitor output. On my sampler (the S770), there is both an RCA plug (for monochrome composite video) and a mysterious 8-pin DIN plug that supposedly would provide color video. Getting the monochrome RCA output to work is pretty easy, but I really wanted the color output. I already tried once to wire up a converter and failed miserably. Well, tonight was different.
After my initial failure, I decided to actually order the right parts and do a better job of wiring everything up. I bought an 8-pin DIN plug from Mouser (part 171-0288, to be exact) and picked up some different transistors. I set aside the breadboard and actually soldered everything together this time. The 8-pin DIN plug was a major pain, and the whole thing probably took me a couple hours. I’m very slow at soldering.
I plugged everything in and fired up the sampler with great anticipation:
I guess it was progress, but it was still pretty unusable. I felt a strange confusion of joy and disgust. After checking everything over a couple times and finding it all right, I honestly didn’t know what else to do. But at the same time, I had to admit that I was getting some sort of signal, so the theory behind the whole operation was pretty sound.
I went back and poked around on the circuit board, paying close attention to the wires that fed the horizontal sync. I replaced the transistor but found that the video display was still slanted. I began to lose hope. And with that, I also began to lose caution. At one point I disconnected the little yellow wire that fed my horizontal sync line to the monitor and was connecting and disconnecting it from its transistor. I’m sure that’s a very bad thing to do. At one point I accidentally brushed it against another leg of the transistor and the signal immediately snapped to attention. It was perfect! I was somewhat stunned.
I checked the schematics and found that my “fix” was definitely not in agreement with the official circuit. I didn’t really care though. I wired it up with the wire now touching the other leg of the transistor, plugged everything back in and quickly declared victory before it could stop working:
This has been a lot of work, but it’s really nice to have the color output. It adds something special to the whole Roland S770 experience. Here are a few more pictures, and then I’ll try to explain whatever technical details I can remember:
Technical Details from Someone Who Just Thoroughly Undermined His Electronics/Circuit Credibility:
The surest way I could find to hook up to Roland’s RGB output is with an old CGA monitor. If you’re in Europe, SCART is supposedly an option. However, I have no idea what SCART is and I’m not in Europe. Also, I happened to have an old CGA monitor laying around the house, so I went with that option. I’m using the super-old IBM 5153 monitor. Folks have reported success with various Commodore or Atari monitors also, but this IBM monitor is all I had. CGA monitors aren’t nearly as common as they once were, but I’m sure they can still be had for very reasonable prices if you watch eBay for a little while. I never see them at thrift stores anymore, sadly.
Information about the Roland sampler video format is scant but not impossible to find. In particular, this thread gave me some hope that it was possible to build an adapter to convert from the Roland output to a CGA input. In particular, they talked about a schematic. I studied the schematic long and hard, through it was a bit blurry. Then one day while I was flipping through my S770 owner’s manual, I found the exact same schematic in the back. So to help anyone out, here are some higher-resolution scans of that video schematic info:
As I mentioned, I bought the 8-pin DIN plug from Mouser. It feels a little cheap, but it did the trick and I didn’t initially see many alternatives for good 8-pin DIN plugs. The schematic drawing mentions a mysterious “DTC 11 AFE” transistor. I took the advice on that afore-mentioned thread and went with the more common BC550. That worked fine. For the cable, I just lopped off the ends of an old printer cable and used eight of the wires inside. Unlike CGA monitors, old printer cables are in ample supply at the thrift stores. I got the female D-Sub-9 connector for the CGA monitor to plug into at Radio Shack.
That thread mentioned not needing the 1k resistor for the vertical sync. I tested both with and without this resistor and found that it didn’t make a difference, so I left it in.
However, here was my change to the circuit that proved critical in getting it to work with my adapter. I have no idea if this is applicable to other monitors, but I know it was needed for my IBM 5153. Instead of connecting pin 4 on the DIN plug (the “H” line on the schematic) to the base of the transistor, I basically bypassed the transistor and just wired the straight into pin 8 on the CGA monitor’s D-sub connector. I’m not sure what I did there, but I’m not going to argue with a nice-looking color display on my monitor.
The only thing that isn’t working well is the horizontal offset on the screen. It’s just a little too far to the right, probably by about half an inch. Unfortunately, my monitor doesn’t let me adjust this. I’m not sure if there’s something I can do to the circuit or if I’m just stuck like this. However, it’s still very legible and I’m tired of working on the circuit instead of actually using the sampler, so I think I’ll just let this horizontal offset issue slide.
Anyway, here’s the final product. Please ignore those three op amps on the side. They were the beginnings of another project on that circuit board from years ago and I was too lazy to unsolder them. Despite several hours of soldering, it’s pretty messy, and it still looks pretty crappy. But I’ll throw it in a little case and forget about all that while I happily use the new color display on my Roland sampler.