Ok - I did some more tests. Unfortunately, the results aren't very conclusive...
First, Gary - I checked to see if there was any potential difference between the isolated ground of the Scanner amp PSU and earth ground. There was no difference.
However, as Bill mentioned earlier, since just about every device in the projector is grounded to it's case, which is bolted to the optical table, which is already at ground potential, I suppose it's not surprising that I didn't get any reading other than zero. (I didn't disconnect the scanner amps from the PSU, so my guess is that they were grounded through the scanners to the optical plate.)
Just for grins, I connected a ground wire from pin 25 to the ground on the scanner amp power supply. Then I removed the ground from the scanner input. No change in scanned images. (I looked at the test pattern, as well as the quadrature square wave pattern with first the X axis collapsed, and then the Y axis collapsed.)
Then I connected pin 25 back to the input connector on the scanner amp, and left the other ground wire from pin 25 connected to the power supply. Still no change.
Finally, I removed the ground wire going to the scanner amp power supply and left pin 25 connected to the scanner amp input. Still no change.
I should note that in my projector, the red, green and blue modulation signals are connected with the positive leads going to the R+, G+ and B+ pins, and the negative leads are connected to pin 25.
If I get *really* ambitious, I might yank those negative leads loose from pin 25 and connect them to the R-, G-, and B- pins, just to see if it makes any difference. But I don't see that happening any time soon. (I've got another big project in the works at the moment, and I still haven't done my taxes yet!
)
Adam