February 2011 Archives

Update: Another round of this stuff... and sure enough, wrecks immediately.  Always fun when you think, "hm, I think I'll turn here" and your car decides, "hm, no you won't."  But hey, I managed to control it and not cause any extra problems.  Also, remember kids, the (!) light is the "you're driving awesome" light.

Just the other day, I was troubleshooting a relatively simple board.  A set of infrared LEDs and photodiodes, resistor network, some diodes and assorted resistors, a transistor, visible LED, and a single IC.  All in all, I would estimate the cost of the board at about $40, assuming you didn't want to mass-produce the PCBs and were silly enough to pay for them one-by-one.  The vendor, however (and they are quite reputable, too), wanted $120 for a replacement part.

The way the board had failed wasn't terribly uncommon--the output was essentially signaling that an IR beam was being broken at all times.  The manufacturer gives a relatively good troubleshooting guide for symptoms and their causes, and sure enough, this lined up with what the device was behaving like.  They also give a good set of schematics for the mainboard, but curiously enough, omit schematics for some of the subassemblies.  The troubleshooting guide stated something along the lines of, "If you see this condition, the sensor board is bad and should be replaced."  This was really fishy to me... no way would the entire board be bad.

I check all the photodiodes, and sure enough, they're all receiving the IR beam properly.  My next thought from tracing this very simplistic two-layer PCB was that either the IC or the transistor was dead.  The transistor seemed to be more related to driving the LED as a visual indicator, so I put my money on the IC.  Of course, without a parts list or schematic, determining what this IC was from the chip markings proved a little difficult.  After misreading one digit, I determined the part was an 8-channel Darlington sink array.  The logic diagram on the datasheet showed exactly what was going on: this was essentially being used as a giant NAND gate (really, more like a negative input logic OR, but who's counting?).  I place my order with an electronics supplier for a handful of this IC and the transistor and wait for them to show up.  Part cost for the IC?  $0.80/ea (for singles).  The transistor was even less.

Once the parts arrive, I go ahead and decide to swap the IC first.  After all, it's socketed, and sure beats desoldering the transistor on the board.  I go plug it into the unit for a quick test, and no LED lit.  Halfway there, so I break the beam, and the LED lights.  Yup.  Bad IC.  So, for $0.80 in parts, I fixed a board that the manufacturer wanted $120 for.  Yeah.  Totally makes sense why they wouldn't want to release schematics or parts list, huh?

In short, when electronics die, it's often not worth troubleshooting.  But if the device is simple and expensive enough, it's quite possible you may be able to beat the game and repair your device for far less than a replacement.  In this case, there was absolutely no actual hard work required: just diagramming the circuit used mentally and swapping a socketed IC.  Just look before you pitch something and start over.