Our CVS blood pressure meter (a relabeled Microlife unit) ran its pump for a few seconds this morning, gave up, and spat out Err 3, which translates into “Inflation of the cuff takes too long”. Not surprising, as the motor wasn’t running.
The AA alkaline cell quartet has plenty of mojo and no corrosion, but the motor doesn’t even turn over. The display is fine and the pressure release valve clicks, so it’s not completely dead.
This unit is sufficiently old to have the compelling advantage of transferring data through a USB (mini-B) connection, rather than a Bluetooth link through some sketchy Internet cloudy Android app, so it’s worth at least a look inside. Four screws and some internal snaps along the sides hold the case together; it’s a surprisingly easy teardown.
The business side of the PCB looks good:
The various wires and solder joints for the “high current” parts look OK, although the wires likely don’t go all the way through the PCB:
Q4 and Q5 look like they switch the compressor pump motor and pressure-release valve. D3 and D4 should tamp down the inductive energy, but they look like they’re in series with the outputs. Yes, the Valve wires are both black.
The motor has a foam vibration isolation wrap, which is a nice touch. Although you can’t see them well, all its wires & solder joints look like they’re in good shape:
The hose sticking out toward you plugs into the black right-angle fitting in the lower right corner of the picture. It’d help to have smaller fingers than mine, but I managed to get the hose off and on the fitting with only minor muttering.
Seeing nothing obviously wrong, I installed the same batteries, poked the switch to start a measurement, and the motor ran fine. Of course, the measurement failed because the cuff & pressure sensor weren’t connected.
Connect the hose, plug in the cuff & wrap it around my arm, poke the button, and everything works fine.
Reassemble everything and it still works fine.
I still think there’s a bad wire or solder joint in there somewhere, so this delightful “repair” can’t possibly last very long …
The Smell of Molten Projects in the Morning 
That sort of mysterious “works after I put it back together but didn’t really do anything” behavior is a welcome result. But you’re right… when that happens to me (about once every two years), confidence is shaken.
It’s hard to fix if it’s working…
Obviously, it needs more fixing!