Tuesday, April 12, 2011

Circuit Repair at Mike's

The other day I mentioned that the dome and sound automation circuit had been running for about ten minutes (eight, actually), and then stopped, accompanied by the smell of burning electronics. I diagnosed the problem as having been caused by the 7805 voltage regulator not doing its job. I measured 12 volts in, and 12 volts out, rather than the expected 5 volts. Given that this feeds the 5 volt PIC chip, it's impressive that it lasted as long as it did with 12 volts feeding it.

Tonight I went over to Mike's for a second opinion, and help with the repair. As I did, he put the 7805 on a multimeter, and saw the same readings. Mike suspects that the soldering of the current-leveling capacitors to the 7805 may have cooked it during installation.

I had a spare 7805 and PIC chip, so he helped solder the replacement 7805 in place, and swapped the PIC chips in the socket. Using a butane soldering iron as a torch helps the shrink tubing do its thing.

After verifying that this regulator is indeed converting 12 volts to 5, we tested the circuit for over 10 minutes without trouble. I'm pretty sure that this should work.

Other than final wiring cleanup, the board is pretty much done.

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