Sharp PC1211

Sharp PC1211

This one was iconic back in it's time. It's still a nice small BASIC pocket computer, but the initial problem I had with it was the batteries. It uses 1.35V mercury cells originally, and they aren't available any more as mercury is very naughty.

Most modern button cells are alkaline and generate 1.5V, so putting four of those in this machine would result in 6V, which is quite a bit higher than the 5.4V that the mercury cells would have generated. It's probably going to cause a failure at such a voltage. Fortunately, if you add a diode in series with the batteries you can get 5.4V from 6V as 0.6V is the standard diode forward voltage drop. I also have a service manual for this machine and looking at the circuit diagram it looks like there's no centre tap taken from the batteries, so the lower voltage should work.
I soldered a diode in to the machine, which was easy as there's plenty of room available:

The new diode is the one to the right of the diode with the yellow stripe on it. It's a bog standard 1N4148 I had in stock. After fitting some LR44 batteries, the PC1211 started up:

Jolly good news. I'll see how happy it in in the long term.
The printer is a different story and for now hasn't sprung into life. There's an internal battery pack which looks remarkably well considering it's age, so I'll try recharging that for a while. If that fails, then I'll swap the batteries for new ones.