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5. TESTING
The completed board should now be checked very thoroughly for stray blobs of solder, dry joints, leads
not trimmed, etc. Also make doubly sure that all components are in the right place and the right way
round, and that the "stand up" resistors are not touching anything else. If everything seems in order,
the board may be tested before you put it in the case. Rest the completed assembly on an insulating
surface (e g these instructions) making sure there are no wire offcuts or similar trapped underneath.
The keyboard's "tails" may now be plugged carefully into their connectors: the one with 5 stripes goes
into KB1, the one with 8 stripes goes into KB2 These "tails" are quite fragile, so handle them gently.
The keyboard itself should sit (the right way up) just in front of the circuit board- Do not remove the
backing paper from the keyboard at this point.
You may now connect the computer to the T.V. and power supply and try it o u t - s e e the main
instruction manual for details
Once you are sure the computer is working correctly, put it in its case - see section 7. Do not strain the
keyboard connections unnecessarily by using it uncased.
6. FAULTFINDING
Experience with the ZX80 has revealed that the majority of faults on kits are due to bad soldering. If your
computer does not work, switch it off and CHECK IT AGAIN. If you find a bad joint or short, shame on
you i You should have checked more closely the first time. If you are sure the fault is in the circuitry, try
these tests--
(a) If the computer does not work at all, leave it on for a couple of minutes and feel the regulator - it
should be getting warm. If not, check the power supply, and that the plug is in the right socket (the one
nearest the keyboard). Otherwise, look at the connection to the T.V. and make sure it is tuned in
properly-try between channels 33 and 39 UHF.
(b) If the computer works and then goes off, and the regulator gets very hot, it isn't bolted to the
heatsink properly.
(c) If the cursor appears on the screen, but the keyboard will not enter, check firstly that the keyboard
"tails" are properly in their connectors, and not twisted in anyway Also make certain that the diodes
and the keyboard connectors are all the right way round.
(d) If the screen goes clear but there is not a cursor, try disconnecting the power supply and waiting a
few seconds before trying again.
(e) If horizontal black and white stripes pass through the picture, suspect the power supply. If you are
using your own supply, it may need to be better smoothed (if the computer is otherwise working) or of
a slightly higher output. See the power supply specification (section 9).
7 CASE ASSEMBLY
(i) Take the case top - the part with the raised "Sinclair" logo and "ZX81" printed on it - and feed
the "tails" of the keyboard through the slot at the top right hand corner of the keyboard recess. Do not
remove the backing paper form the keyboard yet, just locate it in the recess: see fig. 9. Hold the
keyboard temporarily in place with a rubber band or a little sticky tape.
(ii) Hold the circuit board as in fig. 10 with the keyboard connectors next to the slot with the "tails"
poking through, Plug the "tails" into their respective connectors as shown in the diagram, and turn the
board over so that the components face into the case top behind the keyboard.
NOTE. Special attention must be made to ensure that the correct length of screw is used in the correct
hole. The short screws are yellow in colour, the long screws are black in colour. Fig, 11 shows where
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