Logitech K750 ‘repair’

tl;dr: Thought keyboard was broken, only needed battery replacement.

k750Since the Logitech K750 was first released, I was a fan. But the $100 price tag put me off. It’s most noticeable feature is the fact that it has a bar of solar panels along the top of the board which are used to charge the built in battery. This should mean you never have to replace the battery. A dream come true to me, who in general is disposable battery averse. Besides that it’s a standard wireless keyboard that uses Logitech’s unifying receiver, I additionally like the style and design of the keyboard.

So joyous was I to find it on a half-price special at some stage, promptly ordering it and enjoying it’s use. Two years later, on returning from a short trip, the keyboard no longer worked. Not registering keypresses, nor activating any of it’s notification lights. The battery is not supposed to be replaced, but is relatively easily accessible, so I popped it out and measured the voltage.

ml2032The battery used is a rechargeable coin-cell battery, similar to what is used on motherboards, an ML2032 3.0V battery. My battery measured 2.9V, so I assumed it was still fine, and something else had gone wrong. The keyboard had a 3-year warranty I was still within, so after the retailer rejected me, I contacted Logitech who were kind enough to send me a replacement that continues to work perfectly.

This was over a year ago; in-between a lot has happened, but Logitech didn’t ask me to return the old keyboard, and I couldn’t bring myself to throw it out. I figured I’d salvage the solar panels or something. I found it the other day and decided to take another look. Although the battery comes out relatively easily, the keyboard’s faceplate is glued into place. So to disassemble it you have to pry that off, making reassembly difficult.

After that there are a number of screws to remove, and then the base separates, giving you access to the circuit-board. On the front are a few test-points. With my old battery plugged in, V_BAT showed 2.86 V and V_MAIN showed a steady 2.0 V. I tested some other components but couldn’t find anything notably wrong with the board. Solar panels provided a charge.

So I popped my battery out of my new keyboard, and lo and behold it worked. I measured the voltage and it reflects marginally over 3.03 V. Apparently a 0.15 V drop on this battery is enough to stop the keyboard working. I measured the V_MAIN on the test points, and it showed an match 2.0V, like with the ‘bad’ battery. This also meant my disassembly was completely unnecessary. So with it in pieces, I ordered a replacement battery.

Now I have two working K750 keyboards, although one is slightly disfigured.

2 thoughts on “Logitech K750 ‘repair’

  1. Gareth, thanks for sharing your experience. I have a similar issue with my K750, in that it stopped working completely, but a new ML2032 3.0V battery didn’t solve the problem.

    Having already taken it apart to check the on/off switch (it was fine), and inspired by your article, I measured V_BAT at 2.96 V for the old battery and 3.00 V for the new one. I wondered if such a small difference in voltage – even smaller than what you had — could really make it stop working? To investigate, I hooked up a bench power supply & found that it worked absolutely fine with a voltage of 3.00 V displayed on the power supply (which gave a V_BAT measurement of 2.99 V, i.e., less than V_BAT for the new battery) and, in fact, it worked all the way down to 1.50 V (V_BAT measured at 1.49 V).

    So it doesn’t seem to be simply a question of sensitivity to a small drop in voltage as the battery ages, at least not in my case. But I’m otherwise baffled. Any further thoughts?

    • Hey FC. That’s really interesting. I’m afraid I don’t have any immediate ideas as to why it worked with your bench supply, but not on the new battery. Normally I’d suspect it was something to do with the amount of current the battery can supply, but in this scenario, the current draws must be minuscule. Doubly confusing is that off the power supply it continued to work well below expected voltages.

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