Yes, folks, there’s plenty of room left in even your tiny “stock” case to include re-chargeable BBU INSIDE THE COMPUTER. This makes it immune even to pulling the plug! My 16K machine has the BBU circuit, exactly as above, “under the hood.” I must say that using AA cells makes things rather tight, and it required some case re-fitting. I got a little wiser when I customized my 64K TS1000; by using what’s commonly called “2/3-AA” cells (rated 225 mAhr) and everything fits fine. Backup protection is only up to about 25 minutes, but this is plenty of time to find a candle and SAVE to tape if the lights go out.
Finding these mini-AA cells could be a bit of a problem, as they’re primarily an industrial component. We still have-a few sets of these left (they’re the cells we used in Baby BBU) if some of you are interested, and can get more if many of you want them. Cost is $16.00 ppd. per set (7 cells), wired in a pack and ready to go.
But where to put this stuff? Remove the bottom case-half from your machine. You’ll see a big open space under the keyboard – plenty of room if it weren’t for the regulator heat-sink. So what we’ll do is turn the heat sink around so it faces the other way and leaves us PLENTY of room. Unbolt the sink, and flatten it out with a hammer (it has a slight bend which is really un-necessary). We’ll then re-install it “upside-down”, but first you’ll have to cut out a small notch near the regulator hole (with a hacksaw or “nibbler,” finish with a file) so that the reversed sink will clear the grounding strap (and won’t short any circuit traces on the “Issue 2” -ZX81 board).
Now is a good time to make the subassemblies; the perf board and the battery pack. Build up the circuit including the resistors, transistor, and diodes on a 1″x1″ piece of perf-board. You can omit R4 and Z1 from the PC board, and just solder them in series with flying leads off the ends, and the assembly covered with “shrink tubing” or tape; connect this slightly lumpy wire between the switch and LED2 (if you’re using LED2 at all). Provide flying leads to connect to the other stuff (+ to battery, circuit input and output, LED1). Mounting the battery and board is easy with “sticky-foam.” Mount the LEDs in whatever holders you like in the upper right corner of the top case half (above the edge-connector cutout). Where you put the switch depends on your tastes and switch size, I suggest a miniature slide switch mounted on the rear apron of the upper case-half. This might present difficulty with some plug-ons, but even with my Memotech stuff there is enough clearance to reach in with a metal ruler to flip the switch. (You don’t want to make it TOO accessible!) You can file the switch post down if need be, or you may modify the channel selector switch to act as “power on-off.”
Capacitor C1 should be 1000 uFd. or greater at 16V. It should be no larger than about 5/8″ diameter. It can be super-glued to the case either next to the battery (if it’s not too fat) or at the rear of the case between the center post and the modulator.
Finally, you’ll have to cut the main 9V trace on the computer board between the power jack and the rest of the circuit (see diagram) to allow you to put the BBU circuit in-line. A total of 3 wires go to the board at the power jack; “from supply,” “to computer,” (the other side of the cut trace) & “ground.”
Now, it’s just a matter of hooking everything up. Check and double-check your connections. ARE ALL POLARITIES CORRECT?! The diagram should help, run your lines as shown but dress them along the corners for a neat appearance, re-install the board, connect the three wires to the computer power jack, and plug in the power supply for the “smoke test.” LED1 should come on, and LED2 should glow when you flip the ON switch. Computer with built-in back-up is now complete. Give it a final test, if you were careful and used good parts you now have a machine with features not found on anything within 10 times the price. (By some strange synchronicity we are having our first electrical storm of the season as I am writing this. I ain’t worried! And now you won’t be, either.)
