Joining a dead-again subwoofer on my failed repair hall of shame is my Philips Norelco Multigroom MG7790. It had been halting partway through a haircut session, stopping the motor and pulsing its amber LED. My first guess was a failing battery, but the battery looked OK. I then thought the problem was a clogged cutting blade module. I cleaned it up and it tested fine through one session, but then it died again on a second session with a still-clean cutting blade module. I had misdiagnosed the failure a second time and again out of ideas.

Taking it apart, I confirmed battery voltage was not the issue, measuring at 4.08V. I could spin the motor freely by hand, so it's not a stalled motor. I know the button works, because pressing it would start the amber LED pulsing instead of the motor turning as expected. So... what's left? There's not a whole lot to this device!

Examining the circuit board, I didn't see any obviously damaged components. And there were quite a few of them, more than I had originally expected from a device that turns a motor on and off.

Here's the back side of the circuit board, completely empty. Seeing it was a single-layer design, I briefly hoped I could try to understand how this circuit works, but I didn't get very far.

The good news is that, as a single-layer board, I could light it from behind to get clear look at most of the traces. I haven't built any kind of lighting setup for this, so it's still just my cell phone's LED flashlight same as my quick side-light experiment. Here's a crude mosaic of several different shots, taken with the LED behind different parts of the circuit board. One thing is clear: there are a lot of test points on this board.

The bad news is I have no idea what most of these components are. Some of them are labeled like resistors, but some others are engraved with only a few difficult-to-search characters, only slightly better than the remaining components which have no markings at all.

And everything is just so tiny. For a sense of scale, here's a 1:1 pixel crop of the lower left corner of the previous picture. This was a well-used hair trimmer, and my teardown is plagued with little bits of hair getting everywhere. That small black cylinder in the middle of this closeup is one such hair fragment, conveniently providing a sense of scale of these components. These tiny parts are beyond the reach of my current skill level.

While somewhat frustrating, I have to be OK with not understanding everything as I play with retired electronics. Especially since I don't have any of the technical documentation proprietary to the company. In keeping with this mindset, I'm going to take apart a few inkjet printers.