[N2IXK]

After a bit of testing, I think I have found the problem with the unit. It isn't the color wheel itself, but the optical sensor that measures the color wheel speed/position.

I took the entire color wheel unit out of the optical assembly and checked for obvious issues like tight bearings, etc. Finding nothing, I plugged everything back in. The projector would then operate normally, but (obviously) gave a monochrome picture. I used a scope to look at the signal from the color wheel sensor, and it looked reasonable (narrow positive going pulse of ~ 3Vpp at a frequency dependent on wheel speed). Feeding a standard NTSC video signal into the unit produced a 120Hz signal (2x the frame rate), just as I was expecting.

Finding nothing obviously wrong, I reinstalled the color wheel and immediately the unit was back to the old lockup and shutdown routine. I hooked the scope up to watch the wheel speed signal during the failure, and the amplitude of the pulse SLOWLY fell to ~2Vpp or so, when the wheel speed suddenly shot up (presumably as the wheel speed servo went out of lock), then the unit went into shutdown. Hmmm....

I disconnected the wheel speed sensor, and injected a substitute pulse from a function generator. As expected, the projector functioned, but with wild variations in the colors because the wheel was no longer synchronized to the video signal.

Armed with this information, I concluded that the optical sensor was dropping it's output level as the wheel assembly was heating up from the lamp right next to it. I pulled the wheel assembly out again, hooked everything back up, and tried heating the wheel speed sensor with a heat gun while the unit ran. The pulse amplitude dropped below the critical level, and the unit shut down.

So now I need to find a replacement for the little optical sensor. It is mounted on a tiny PCB, with a couple surface mount resistors and caps. It is a tiny 4-legged SMT device, presumably an infrared LED and a phototransistor in one package, which bounces light off the hub of the color wheel, and looks for the little black timing mark on the hub. No visible numbers or manufacturer logo on it, unless it is on the bottom side (will desolder it and check). May be time to start digging through catalogs and datasheets to find something similar. I have my doubts that calling Dell will produce any positive results, as they likely only sell the entire color wheel assembly as a unit at an obscene price.

UPDATE: After a bit of digging, I have identified the sensor as a Sharp GP2S27 series photointerrupter. The part was discontinued as of last year (and the suggested replacement is functional replacement only--different package), but DigiKey still has a few GP2S27s in stock. A few are on the way at < $2 each, with shipping costs to exceed the cost of the parts themselves...