After two months away from it (lots of schoolwork, and then a bit of time to relax after the semester ended), I'm picking up this project again.
One of the things that was on my mind when I last worked on it was my desire to obtain a spectrophotometer to characterize the color filters in the filter wheels I have, as well as any filters I may use or make in the future. Just last week, while browsing at the provincial government surplus store, I noticed that they had six Turner 350 spectrophotometers for $25 each! One of them, which I bought, was a slightly newer version with a black faceplate and power switch on the front panel. Before I bought it, I first plugged it in and powered it on (which resulted in the lamp lighting up and the meter going to full scale, as expected) and then tried to test it by sticking a strip of colored plastic from a drink bottle in the sample holder (which had no result, not as expected).
I brought it back to Protospace on a Tuesday night (when we have our weekly open house) and guests April and Rachael very eagerly helped me disassemble it (being somewhat careful to protect the PMT from light), and then they and Protospace member Nick helped me attempt to diagnose it. Unfortunately, we didn't get very far. The main thing we noticed was that the needle would vibrate, and sometimes wander erratically down the scale, when we touched various parts inside, but it was never consistent. Probing the 1 Vpp output with an oscilloscope resulted in similarly inconsistent signals that sometimes looked like multiple frequencies superimposed.
Further investigation of the connections to the PMT revealed that each of the PMT's pins had a stub of another wire on it, suggesting it had been cut out of another piece of equipment (possibly another Turner 350) and installed in this one. Perhaps this individual machine has a problem that causes it to damage PMTs, or perhaps the PMT was damaged by light during the replacement or during our disassembly. (Edit: No. PMTs can only be damaged by excessive light when powered up.) Next I will attempt to reverse engineer the whole circuit and figure out which points would be useful to probe.
In other news, I intend to perform proof-of-concept testing soon with filters alternating under the user's direct control, with both colorsighted and colorblind subjects. With the mockup I did a couple of months ago, I did notice that things matching the color of the currently active filters looked brighter to me, which was the expected/desired effect. That counts as a successful prototype, right? I also need to get the color filter wheels turning under a microcontroller's control. Protospace member Derek mentioned something that does that in the context of discussing another member's project and I thought it might be useful for this, so I'll be looking into that as well.