Repair Norelco 950RX Razor (NiCd to Lithium battery conversion)

Updating a 40 year old electric shaver

Note: Click on (most) pictures to embiggen.

The Norelco 950RX is a rechargable battery operated razor from the late 1980s. It uses two sub-C NiCd batteries, and charges directly from 120VAC. When new, it was good for about 1 shave a day for three or more weeks before it needed a recharge.

The problem with mine was that the NiCd batteries were losing capacity, and it was a pain to charge when travelling, since (a) it needed it's own proprietary electric cord, and (b) it was only rated to charge on 120VAC, so when travelling out of North America, I needed to carry a voltage converter to drop 208/220 to 120V.

In addition to the two NiCd batteries and charging circuitry, there is also a cute little LCD battery charge level indicator, and a green LED that illuminates to indicate "charging" or "is now fully charged". I thought about trying to re-use the LCD battery indicator, but it looked like that was harder than it was worth. There's also a low battery beeper, which I decided was an unnecessary complication.


The old batteries and charge circuitry, with (spoiler) the schematic of the replacment electronics.

The Plan

I decided to replace all the original electronics with a 18500 battery, which puts out between 4.2 and 3.5 volts (from fully charged to empty). That means I'd need a regulator to drop the output of the battery to the 2.5v that the NiCd batteries put out.

Interestingly, the NiCd batteries are about 600mAh, and an 18500 cell is about 2000mAh, so I should have three times the battery life with the Li-ion cell.

The hardware parts needed are:

  • 18500 Lithium Ion battery; about $12 from any random eBay vendor.
  • Li-Ion battery charger; about $1 from any random Chinese eBay vendor.
  • Low drop-out regulator; see discussion below
  • Battery charge level indicator; about $3 from a Chinese eBay vendor.

Building the Real Hardware

I couldn't find a switching regulator that would drop from 3.5 to 2.5v, but I did find a linear regulator that would work. This uses a TPS7A4700 regulator and costs about $8 from a Chinese vendor via eBay.

The LED battery level indicator will draw a few mA, so I don't want to leave it connected all the time. That means, that in addition to the "power" switch on the front that turns off the motor, I need another switch to disconnect the battery charge level indicator. This switch is "hidden" inside the hole in the bottom of the razor; I did this to prevent it from being accidentally turned on when packed in my luggage (which has been known to happen).


Tube squeezer that I used to crimp a piece of copper sheet.

Because the linear regulator would generate some heat when the razor is in use, I decided to add a small heat sink to it. I used the above tube squeezer to corrugate a piece of copper sheet, which I then soldered to the heat sink surface on the regulator.

I attached the hardware to the case using Goop, which I think is the world's best general purpose glue.


The unit with the replacment guts. The charger is the board at the left. The blue power switch disconnects the battery from everything. In the center is the LED bar graph display of battery charge level. Right of that, you can see the edge of the regulator board with it's corrugated copper heat sink. Far side of the case is a slide switch that controls the enable pin on the regulator. There's a piece of paperclip that connects the switch to the original slider on the front of the razor.

The purple rings that the wires are wrapped around are ferrite cores; those are noise reduction filters that were in the razor from the factory. The diode across the motor is also a factory original snubber.


The finished unit showing the LED battery charge indicator.

FAQ

Q. Will you make one for me?

A. No, I'm retired and so not interested in a job.

Disclaimer/Warning

This is just my "build log". I don't claim that building this is safe or recommended.

Soldering irons are dangerous, be careful. Oh, and don't eat the solder.

William Dudley
June 20, 2020

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