Many thanks to Danny M. and Tim F. for taking an interest in fixing my latest busted Cricut!
This was the first one yet with problems I couldn't figure out. The relay clicked on and off with the power button, but nothing else happened. Using the voltmeter I was able to figure out that the CPU was getting no power, and that there was something funky going on with the 5v regulator: on the working one, pins 2 and 4 read 5 volts; but on the broken one, pin 2 was at 18 volts, and pin 4 at 0. This was pretty much where I got stumped. I saw lots of ugly looking solder joints, but nothing obviously fried or broken.
Fortunately, I also have another Cricut Create that DOES work, and so I brought both to the hackerspace last night for Microcontroller Monday.
Tim F. helped out by looking up the datasheet for the regulator, and showed me the illustration for a "typical application" in the datasheet. Using the meter I discovered that there was resistance and/or no connection between the output and feedback pins when they should have been directly connected through a resonator.
Around this time we also realized that while my older (working) Cricut had a 2-layer board, the newer (busted) one was a 3 (or more) layer board and much harder to trace. Fortunately when they changed the design, pretty much nothing else changed. Same board size, same components in mostly the same arrangement. This meant that the proper connections could be traced on the old/working board, and compared to the busted one.
Danny stepped up to the plate here and almost immediately found a broken solder joint on one of the resonator pins. We reflowed the two solder joints, plus a few others (the quality of the solder joints on both boards is pretty atrocious, no idea what kind of monkeys are assembling these things....) And THOUGHT it was fixed.
However, it still wouldn't power up. Hmm!
As it turns out, the broken solder joint also managed to disconnect both of the inner layer traces leading from the resonator to the regulator. After more continuity testing Danny added two small jumper wires to repair the busted traces, and Voila, the machine now powers up!
I was all set to send the machine home with him to play with, but when I tried to actually cut something, discovered the cutter solenoid does not move. The carriage and paper scrolls around properly, but the pen doesn't move up and down. (YET!)