Kitchen Sync

As has already been said in the pages of SYNC and elsewhere, Clive Sinclair deserves some sort of medal for introducing a much improved version of an already popular computer and selling the new version at a lower price. You don’t find deals like that often these days, on either side of the Atlantic.

After you decide that the ZX81 is the computer for you, your only remaining decision is whether to save 550 and buy the kit version, or whether to go the fast and easy route and buy a “plug-in-and- run” ZX81.

Should you build your own ZX81? If you’ve never built anything electronic before, no. If you haven’t used a soldering iron (a small, 25-watt or less, narrow-tipped iron with rosin-core solder) before, no. If your patience is not one of the things for which your friends admire you, no.

But if you’ve built kits before, Heathkit or otherwise, if you can solder with confidence, and if you have a couple of spare hours on hand —yes! You can have a good time, save enough money to make it all worthwhile, experience the warm inner glow that comes from using something you assembled yourself, and be reasonably sure that your ZX81 will work the first time you turn it on.

The engineering of the computer is topnotch, as is the packaging. The board is silk-screened and solder-masked, and poses no problem if you have a soldering iron with a small tip and some fine-gauge solder. The instructions for assembly. however, leave a bit to be desired if you like (or need) to have things spelled out step-by-step for you.

Getting It Together

Our kit came carefully packed in a plastic foam container, not just thrown in a box. The components were packed in little bags although they weren’t grouped in any order that paralleled the assembly instructions. We were told by a friend, however, that his kit was packaged in a large cardboard box rather than plastic foam like ours. (We’ll get a third opinion when our other two kits arrive – there are three of us, and one ZX81 can’t satisfy all of us at once..)

The assembly instructions aren’t Heathkit-style “step-by-step”; they might as well read, “Put the components in the proper locations and solder them in.” If you read the instructions carefully and work slowly. this won’t be an impediment. But, if you can’t wait, and rush ahead, making assumptions along the way, you’ll blow it, like we did. Since the board uses plated-through holes (the inside walls of the holes are metal-plated just like the traces), it can be frustratingly difficult to remove misplaced components after you’ve soldered them in.

The instruction sheet starts off by recommending that you read all of the instructions carefully. A good idea, but “memorize carefully” is what it should read. The arrangement of the assembly instructions and the “warnings and hints” is such that you must remember the “warning and hints” as you go through the assembly instructions. If the “warnings and hints” had been mixed in with the assembly instructions, we would have made three fewer mistakes than the three mistakes we made. We didn’t have anyone to caution us, though; now that we’ve told you, you shouldn’t have any trouble.

There are many extra “unused” holes on the board (it is common on plated-through boards to use holes to connect traces from one side of the board to the other), and the board is so small and tightly packed that it is easy to place a lead in the wrong hole if you are not careful. A picture of a completed board showing proper component layout would have been a great help. We’ve included such a photograph with this article. (Figure 1).

The parts list includes along with the component name (R3, etc.) and the component value a description of the marking on the components. This is handy for people who may have not memorized the EIA resistor color code, and in identifying some oddly-marked parts (like some of the capacitors that are marked in nanofarads (1000 pf), an uncommon unit in the United States).

Our kit had the correct number of each part, but our friend’s kit was missing one memory chip and a resistor, and had two extra transistors and couple of spare resistors of different values. Be sure to check your parts against the parts list before you start building-Sinclair will gladly replace shorted parts, and it’s disappointing to have to stop assembly in the middle to the kit to wait for a part to come in the mail.

The instructions suggest that the resistors are pre-cut and bent, a time-saver to the kit builder. However, both the 470 ohm and most of the 1K ohm resistors come full-length. This, as we discovered ten solder connections too late, is no accident. The pre-cut resistors are installed flat on the board, while the full-length ones are mounted on-end, sort of standing up. While a warning to this effect is indeed contained in the instructions, this warning is not where we were looking when we installed them: hence, we put many of these resistors in wrong and had to make a trip to Radio Shack. Also note that some parts (like R30, a 10-ohm resistor) are used in the U.S. version of the kit, but not the U.K. Be sure you’ve read the instructions carefully to decide which parts are or aren’t used. Figure 2 will help you in proper component placement.

Also included are two “resistor packs.” These packs have a right end and a wrong end, and must be installed the right way. The instructions state that the common end is marked by a white dot-ours had no dots, white or otherwise, but instead a black square towards the middle but off to one side. No big problem, really — the end closer to this black square is the common end. If the instructions had reminded us. “Install the resistor packs, remembering to place the marked end near the “C” silk- screened on the circuit board,” we probably wouldn’t have soldered them (both!) in backwards. Desoldering a seven-pin part from a plated-though board, we discovered. is very close to impossible without destroying the part or the board. We had to wait a week for the replacements to come from Sinclair to pay for our error.

We discovered a spare part at the end of our assembly, which we soon decided was the ceramic filter (it looks like a three-legged capacitor). It had been left out of the “install all of these parts” list. The kit won’t work without the filter, so you should (as we did) put it in, even though you’re not explicitly told to do so.

The installation of the modulator posed a problem we couldn’t solve ourselves. According to the instructions, the two leads go in the holes marked Fr/UK1 and UK2. Common sense told us that this may be wrong, since we knew that neither “Fr” nor “UK” have television systems exactly like “US.” We, of course, wanted to use the holes marked “USA.” but there are three such holes and only two leads. We then decided to look at the schematic – wrong move. Apparently there are several versions of the ZX81 kit, and the schematic appears to be an interesting combination of all of them. (Printing one schematic and one instruction set for the world is probably one of the ways Clive manages to keep prices so low…)

Thoroughly confused, we decided to call Sinclair for assistance. They informed us that there are two variations of the kit being delivered to the US – a UHF kit with a two-lead modulator using the Fr/UK1 and UK2 holes, and a VHF kit with a three-lead modulator that uses the “USA” holes. Problem solved. According to our friend, later kits like his are packaged with a supplementary instruction sheet that makes all this clear.

The keyboard for the ZX81 is a self- contained unit, unlike the ZX80’s, which used the bottom of the printed circuit board and a stick-on overlay. A thin film with conductors “printed” on it serves to connect the keyboard unit to the computer; this film slips into a connector you install on the printed circuit board. It is a good idea to clean the ends of the keyboard connector film before you plug it into its connector. Light rubbing with a pencil eraser will ensure a good connection. Slipping the keyboard straps into the connector proved to be a little tricky, but an even pressure and some verbal urging does the trick.

We checked the voltage produced by the power supply before we plugged the ICs into their sockets, just in case something else had gone wrong in the assembly: we couldn’t wait another week for replacement ICs to come in the mail. If you have access to a voltmeter, you can check the power supply by plugging the AC adapter into the ZX81’s power jack, then measuring the voltage across pins 2 and 3 (middle and right-hand) of the voltage regulator (which is attached to the metal heat sink). If you find five volts there, everything’s ok so far. If you don’t happen to have a volt meter, try using the “burnt thumb” method- apply power to the board and place your thumb on the plastic case of the regulator. If your thumb starts smoking, the regulator is using more current than the designers planned, and you probably have a short somewhere. Check your soldering, and look for solder bridges across traces.

Be sure that you unplug the kit before installing the ICs, unless you want to zap a couple chips.

The Moment of Truth

After all our troubles, the machine worked the first time we plugged it in. We put the case together (it’s held together by screws instead of these funny plastic things used on the ZX80), plugged it in. and entered our first program. After playing around with our new toy for a while, we became ZX81 converts (and thus our two more kits on the way). Our only remaining question was why Sinclair put the vents on the bottom of the case. Must have been so the heat could fall out.

From start to finish the whole building process took about five hours, including a little backtracking. Considering the $50 savings offered by the kit version, you earn about $10 an hour, have a good time (barring disaster), and end up with a great computer for the lowest price around.

All things considered, the quality of the kit is excellent; Sinclair didn’t cut corners anywhere. Everything Sinclair has ever made has been ingenious, well-built, and surprisingly inexpensive, and the ZX81 is not exception. Sinclair remains hard to beat for engineering excellence-but tonight, in Benton Harbor, Heathkit sleeps easy.

Authors P.S.: Since writing the review. we have received both a UHF and a VHF kit from Sinclair. There are some significant differences between the two which will be covered in a future column.