I bought a Photon MIDI guitar system (serial number 2605) several years ago on EBay and mounted it out an 1974 Les Paul I had. The arched top of the Les Paul prevented the pickup PCB from aligning ideally with the strings so I removed it and the system sat unused until the past year or so.
I bought a 500 Variax by Line 6, a modeling guitar which provides a wide range of guitar sounds, which I thought would be a perfect compliment to the Photon. Now I could add MIDI to any number of different guitar sounds, including acoustic, 12 string or even a barely passable sitar guitar sound. And the top was flat so I thought it would work well.
I drilled the holes for the optical pickup and mounted it on the Variax and quickly discovered that the G string was not functioning. This started a long journey to repair the system and led to this discussion of the ins and outs of what I learned about the Photon system.
The first thing I discovered is that there is very little technical information out there on this. I found one guy (I'll respect his privacy and not name him or his Gibson contact) who said in an email:
"All trace of old Photon stuff has disappeared along the road of life. All I could find was an old photon and busted up cartridge. There is one person who might be able to help you. "M. D." at Gibson. Not sure how if he is still there, but he built the pickups for John McLaughlin's rig using the Photon on an acoustic. I think he also did a magnetic hex pickup conversion as well.
Hope this helps."
It didn't of course. I could not find any information on M. D. at Gibson or anywhere else though I didn't search too deeply to find him.
As an electrical engineer with a background in optics I figured it wouldn't be too hard to find a replacement for whatever part had gone bad. I quickly determined that the photodetector on the G string was bad. By measuring the forward voltage on another string I determined it had to be a phototransistor versus a photodiode, though later measurements seemed to indicate that for the three low strings, they did use a photodiode.
I located a source for side looking phototransistors (Optek Technology ) and purchased a few OP550A phototransistors. The OP550 comes in four flavors, A through D which are selected for on-state collector current with the A version spec'd at 2.55mA minimum. It has a 30º (one side) half power point or 60º overall. See the graph below reproduced from the spec sheet of the 550A.
