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 Non-responding phototransistor
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By: Greg (offline) on Monday, November 26 2007 @ 11:10 AM PST (Read 4983 times)  
Greg

I'm on board #5, been pretty much smooth sailing after the initail problems (which were all my fault). But I now have a new problem.

Quadrant 1 does not respond to light. The LEDs in quadrant 1 do respond to the adjacent quadrants "waves", but oddly, one "n" bank and one "p" bank (in opposite halfs of the quadrant) are quite dim.

I replaced the phototransistor, I triple-checked every solder joint (re-melted each one with the hot iron), and verified the values of all the resistors. No change.

Any ideas?

At this point, I'm about to replace the chip.

Greg


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By: Windell (offline) on Monday, November 26 2007 @ 11:44 AM PST  
Windell

When a chip goes, it almost always goes all the way; it's rare indeed for a chip like this to partially fail. Since the ripples from other quadrants do show up in quadrant 1, it means that the chip is indeed working correctly.

To tests for photosensitivity of a quadrant, test whether (1) its own LEDs respond to light applied to that quadrant, and (2) whether LEDs in other quadrants respond, as a ripple, to light applied to quadrant 1. If it really isn't there, there isn't much to go wrong. The most likely cause is that despite your double-checking, a bad solder joint, or perhaps a tiny solder bridge, remains. That should be clear since you've already replaced the phototransistor, and the old one was apparently okay.

So, touch up each of the relevant components and solder joints again: Especially those by R101,R102, R103, and those of the photosensor. Since the inputs from the neighbors are working, even the photosensor input to the chip is apparently working correctly.
(And, you *do* have the photosensor pointing the right way, right?)


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By: Greg (offline) on Tuesday, November 27 2007 @ 05:36 PM PST  
Greg

Excellent input! This helps narrow it down.

I pulled-out the multimeter and started probing. Turns-out, the trace between Q101 and R101 was open. I either nicked it with the side cutters trimming leads, or damaged it pulling the original (failed) phototransistor.

I could not actually see the void...no matter...installed a jumper and all is good!


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By: Windell (offline) on Tuesday, November 27 2007 @ 05:48 PM PST  
Windell

I'm glad that's working better.

There really shouldn't be any "holes" in the boards, they are 100% electrically tested before shipping, BUT, should one occur, they are hard to see!


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