Last night I ran an after class session where we assembled the kits. 4 of 5 worked fine.
The first thing we did was to verify the content of the kits and everything checked out. Then we all went step by step through the instructions (fantastic by the way). Everything was great until we got to testing the 5v USB pads on our first unit. Pad 1 to pad 4 read -0.3v which is obviously bad (yes the meter was oriented correctly). I popped the batteries out and picked up the unit to triple check that we had no solder bridges or shorts and the ship was so hot it stuck to my thumb. I was not quite expecting this so the unit ended up getting flung across the lab as I recoiled, causing the unit to get pulled along with it before tearing free.
All 4 other units passed their checks and assembly was finished before we went back to the first failure. I could not find any obvious issues so we started probing some things. Now I will admit that this was my first experience trying to troubleshoot a circuit, and our professor will not be available until tomorrow to help us, so there may be some obvious things that I have missed. Here was my procedure.
- Continuity checks on pins based on the high res pictures of an unpopulated board. Confirmed there are no shorts. I'm assuming this is only a 2 sided board.
- Compared all resistors vs a working unit. All resistors checked out.
- Used a bench supply to charge the capacitors and check that they held a charge. Caps worked fine.
- 1-4 seconds saw a 1-5 degree C rise.
- at 5 seconds the chip shot up to 150C before we cut the power.
We were not brave enough to try a known good chip in the failed board, nor were we willing to risk the suspect chip in a working board.
Ian