Braun Type 5417 Hair Trimmer Repair

I keep my facial hair like I keep the lawn – I let it grow naturally (on its own), then trim it down every week or so and clean up the edges every other time. I do so with an electric lawnmower; not battery-powered, because I don’t mind the inconvenience of a power cord tether and I don’t want to have to replace another rechargeable battery every few years. Wait, I started this post to talk about my hair trimmer…

The Braun 5417 “cruZer6 beard&head” hair trimmer has been running well for me over the past 7+ years I think I have had it. The NiMH battery stopped holding a charge, unfortunately. Like anything I own that has stopped working correctly, I opened it up to investigate, knowing full-well that I was probably just going to replace the batteries (I have done this with countless electric toothbrushes over the years). I was impressed with the build quality, and the gasket design to keep the electronics ingress-protected; that attention to detail is likely why it has lasted so long on its own. The gasket is a molded-rubber concave profile in the lid that provides a very satisfying fit, and is secured with six ramped securing tabs that are just perfectly aligned; some high quality plastics engineering went into this electronics enclosure. There is even an expanded-PTFE breather port to equalize pressure differentials due to heat dissipation!

I cut the tabs off the old batteries and removed them, then sanded the ends of some EBL 1100-mAh low-self-discharge NiMH AAA cells and soldered them in place. I hit the power button, and… it just blinked the red LED for 5 seconds.Hmmm… maybe it just needs to be charged. Nope! After repeated charging cycles until the internal timer cutoff, it still did the same thing. I desoldered the batteries, discharged them, and put them back in to charge again; afterwards, the battery voltage confirmed that the trimmer’s circuitry is charging the batteries just fine, it is just refusing to operate!

The mistake I made was just throwing out the original batteries, because now I have no idea what is wrong and cannot test if it still operates correctly with them re-installed. Most likely, the internal battery health detection circuitry is tuned to the original batteries’ performance, and the new batteries’ low internal resistance and longer charge-profile is being rejected by the firmware. But… what if it is actually something more nefarious?

Was my modification being defeated, maybe?  I was suspicious of two small ceramic, non-conductive (potentially ferrite) beads inserted into the plastic molding (under the motor wires seen in the picture), but I’m still not sure what they are for. One theory might be a kill-switch in the microcontroller programming somehow related to disassembly; if that’s the case, then I am impressed with the level of evil effort they put into keeping the life-cycle of this product down! If not, then I honestly have no clue what they are supposed to be there for; one contacts a single ceramic capacitor (C2, maybe it is supposed to damage that component on removal?) and the other is centered on three test pads (TP17, TP18, and TP20, maybe that acts as an air-gap resonator between two pads and is coupled to the third pad for removal detection?). I want to know, but I also want to put this back together and give myself a haircut.

Whatever the case, the trimmer is in good condition since I keep it clean and lubricated, so I just located the motor drive transistor, and ran some 32-AWG magnetwire leads to a slide switch that I mounted to the outside. Now I just use a different button to operate it, and I can move on with my shorn life. :P

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