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Replacing the EDU/SCC Phototransistor Posted by TonyJ [Email]  (#407)  [Profile/Gallery]
(more from TonyJ) on Mon, 13 May 2002 23:13:12
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The EDU and SCC displays in our 93 CSE had become unreadably dim during the day. The problem was intermettent at first, then became a hard failure. Guided by some previous posts on this list, I suspected the phototransistor in the SCC that controls backlight intensity. With an old analog clock ready as a possible donor, I removed the SCC and tested the phototransistor. It was indeed dead. I was unable to locate a visible wavelength phototransistor in any of my electronics catalogs, but there are many IR units available. Hoping that an IR unit would have enough sensitivity in the visible range to work or that sunlight has enough IR energy, I used a Radioshack 276-142 detector from an emitter/detector pair that I happenned to have on hand. They also have a detector-only with similar specs that would probably work. When replacing the phototransistor, use a grounded low-watt iron and be very careful. The replacement I used has a different case (more later) and only two leads (emitter and collector), but it fits nicely in 2 of the 3 the PC holes. The base isn't used (electrically) anyhow. The collector (short lead, flat on case) goes towards the outer edge of the PC board, while the emitter is closest to the flexible ribbon cable between the two PC boards.
Thus replaced, everything worked fine, except that at night, the display was much too bright and varied directly with the dashboard illumination setting. The case of the replacement phototransistor is all clear, while the original was black, except at the business end. So IR light was leaking from the lamp that illuminates the keys into the phototransistor. I solved this problem as follows: wrap the black plastic tube in aluminum foil, surround with shrink tubing and black tape. Wrap all but the end of the phototransistor in shrink. Add a plastic cap (I think from Pentel erasers) wrapped in foil and shrink and black tape over the end of the tube behind the phototransistor. I'm not sure how much of this IR shielding was overkill, but it now works perfectly under all ambient lighting conditions..
TonyJ
93 CSE Turbo 5-speed 177K, 94 Aero 5-speed 77K
posted by 172.168.16...
, Mon, 13 May 2002 23:13:12 <-- Viewing This Message, Tue, 14 May 2002 23:59:14, Tue, 14 May 2002 23:45:47
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