Sinclair ZX81 Computer Kit
Assembly Instructions
U.S. Version for UHF Channel 33

Introduction Important - Work Area - Tools - Precautions against Static Electricity
ZX81 Kit Components List Component Identification
Circuit Board Assembly Resistors (Flat Mounted) - Diodes - Resistors (Vertical Mounted) - Resistor Packs - Capacitors - Transistors - IC Sockets - Regulator - Jacks - UHF TV Modulator - ICs - Keyboard - Case
Drawings Silkscreen Layout Schematic

Introduction

Dear Kit Builder,

Congratulations on your purchase of a ZX81 computer kit. We are sure you will get a great deal of satisfaction in using a computer that you yourself built.

These instructions are largely a rehash of the ZX81 instructions supplied by Sinclair Research Ltd. Although Sinclair's instructions are complete, we feel the information is not given in the best format and the best sequence in which it is required when actually assembling your kit. So, mostly what we have done is present the same information in a linear fashion, pointing out what you need to know in the order you need to know it.

If you have never soldered before, or built an electronics kit before, it would be best to seek the aid of someone with experience to help guide you. If you have even a moderate amount of experience, you should be able to assemble your kit in about three hours of easy work by just carefully following these instructions.

Happy kit building.

 

Important

Read through the instructions before you start assembly. If anything seems unclear or difficult, contact us for advice before going ahead.

Workspace
& Time
You will need a clean, dry and well lit workspace in which to assemble your kit. If possible, try to find somewhere where the parts can stay undisturbed in case you do not finish the kit all at once. it is a good idea in any case to split the work up - say assemble the circuit board one evening, then test it and put the case together the next evening.
Precautions
against
static
electricity
There are not many integrated circuits (IC's) in the kit, but they are all fairly expensive items and most of them are susceptible to damage from static electricity. There is no cause for worry if a few precautions are taken:
  • Use the sockets supplied with the kit - never solder the IC's direct to the board - and keep the IC's in their protective packing until you are ready to plug them in.
  • Never insert or remove the IC's or do any soldering with power applied to the computer.
  • Use a soldering iron with a properly grounded tip.
  • Carpets and clothing of man-made fibers, and synthetic soles on shoes, are prone to building up a static charge. Ground yourself by touching a large object, preferably metallic, prior to touching the IC's. If you do get a shock, try changing your clothes.
  • In general static electricity only becomes a problem when the air is dry, such as in the winter with hot air heating systems. A room humidifier will almost certainly cut down on static under these situations.
Tools
These are required tools
[_] A light electric soldering iron, say 15 to 25 watts with a fine tip.
[_] Fine gauge solder with resin flux core; NOT acid flux.
[_] A pair of sharp sidecutters.
[_] A Philips head screwdriver with a No. 1 point.
[_] A medium size ordinary screwdriver.
These items are optional, but useful
[_] A magnifying glass for examining solder joints and looking for short circuits.
[_] Some desoldering braid or other solder removing tool. Better still, take care that you put the components in right the first time - removing them can be very difficult.
[_] A piece of foam is useful to stop components falling out when you turn the board over to solder them.

ZX81 Kit Components List

Resistors
Check No. Value Markings / Comments
[_] R1 10K Brown Black Orange
[_] R2 680 .
[_] R3 . NOT USED
[_] R4 18K Brown Gray Orange
[_] R5 330 Orange Orange Brown
[_] R6 470 Yellow Purple Brown
[_] R7 470 " "
[_] R8 470 " "
[_] R9 470 " "
[_] R10 470 " "
[_] R11 470 " "
[_] R12 470 " "
[_] R13 470 " "
[_] R14 470 " "
[_] R15 220K Red Red Yellow
[_] R16 1K Brown Black Red
[_] R17 1K " "
[_] R18 1K " "
[_] R19 1K " "
[_] R20 1K " "
[_] R21 1K " "
[_] R22 1K " "
[_] R23 1K " "
[_] R24 1K " "
[_] R25 1K " "
[_] R26 1K " "
[_] R27 1K " "
[_] R28 680 Blue Gray Brown
[_] R29 1M Brown Black Green The band may be yellow
[_] R30 ------------ Not used
[_] R31 ------------ Not used
[_] R32 ------------ Not used
[_] R33 4K7 Red Purple Red
[_] R34 220 Red Red Brown

 

Resistor Packs
[_] RP1 8 * 10K 10K 9 leads
[_] RP2 Not used
[_] RP3 5 * 10K 10K 6 leads

 

Capacitors
[_] C1 47pF 47 Ceramic disc
[_] C2 47nF 473 Z Ceramic disc
[_] C3 22uF 22u Electrolytic 16V
[_] C4 47nF 473 Z Ceramic disc
[_] C5 1uF 1u Electrolytic 5V
[_] C6 100pF 100, 101, n10 Ceramic disc
[_] C7 47pF 47 Ceramic disc
[_] C8 47nF 473 Z Ceramic disc
[_] C9 47nF 473 Z Ceramic disc
[_] C10 10nF 10n, 103 Ceramic disc
[_] C11 47nF 473 Z Ceramic disc
[_] C12 47pF 47 Ceramic disc

 

Semiconductors
[_] IC1 Sinclair Logic IC 40 pins
[_] IC2 2364 ROM 24 pins
[_] IC3 Z80A or D780C1 40 pins
[_] IC4a & IC4b PD2114LC RAM 18 pins
[_] *IC4 * Some kits may have the following single IC instead of ICa & IC b. MK4118 24 pins

 

Regulator, Transistors, Diodes, Ceramic Filter
[_] REG 7805 5 Volt Regulator
[_] TR1 ZTX 313 Transistor
[_] TR2 ZTX 313 Transistor
[_] D1-D8 1N4448 Diode Color: Yellow, yellow, yellow, gray
or Yellow, brown, yellow, gray
or 1S44 with 2 Yellow bands
[_] X1 CDA 6.5MC 3 lead ceramic filter

Some diodes may have their number printed on them instead.

* NOTE: D9 is not used.

Other Components
Check Qty Description of Item
[_] 1 Modulator type UM1233
[_] 1 Modulator trim (black card)
[_] 3 3.5mm jack sockets for power, ear and mic.
[_] 2 40-pin IC sockets
[_] 2 either 2 24-pin IC sockets
[_] 1 KB1 5-pin keyboard connector
[_] 1 KB2 8-pin keyboard connector
[_] 1 Ready made flat keyboard
[_] 1 Aluminum heatsink
[_] 1 4BA nut, bolt and washer for fixing regulator and heatsink
[_] 1 or 2 or 1 24-pin socket, and 2 18-pin sockets
[_] 1 Printed circuit board
[_] 2 Case halves
[_] 4 Rubber feet
[_] 3 Black (long) Philips head screws
[_] 4 Yellow (short) Philips head screws
[_] 1 Sinclair or Timex power supply. 9 Volts, 600 ma. min

3. Component Identification

Before you start assembly, check the components against the component list and make sure you know what each part is. We have tried to cover all different markings of the components, but variations are possible.

Note in particular that the computer's memory may be supplied either as two 18 pin IC's (IC4a & Icb) or as one 24 pin device (IC4), and that assembly is necessarily different for each version.

4. Circuit Board Assembly

Resistors - There are 17 resistors that install vertically. These include:

TABLE 1. Vertically Mounted Resistors

R7-R14 470 ohms ( 8 resistors)

R18-R26 1K ohms (9 resistors)

All the other resistors install horizontally and have their leads bent so that they fir into holes 0.5" apart.

NOTE: Resistors are not polarized components; it makes no difference which direction they are inserted into the printed circuit board.

We have found from experience, that the easiest way to build these kits is to first install some of the parts that lie flat, including the horizontally mounted resistors and diodes. This is because after inserting the components, the board can be turned over and still lie flat as you are soldering. Keeping this in mind, assemble your kit in the following sequence. Notice that we will save the vertically inserted resistors listed above in TABLE 1, until we have finished inserting the horizontal ones.

Resistors (Flat Mounted)
Check No. Value Markings / Comments
[_] R1 10K Brown Black Orange
[_] R2 680 .
[_] R3 . NOT USED
[_] R4 18K Brown Gray Orange
[_] R5 330 Orange Orange Brown
[_] R6 470 Yellow Purple Brown

Resistors R7-R14 mount vertically and will be put in later.

Resistors (Vertically Mounted)
Check No. Value Markings / Comments
[_] R15 220K Red Red Yellow
[_] R16 1K Brown Black Red
[_] R17 1K " "

Resistors R18-R26 mount vertically and will be put in later.

Diodes

There are 8 diodes, D1 through D8. The diodes get inserted on 0.3" centers. The cathode ends have a band on them, or in some cases where the diodes have more than one band, the cathode is the side with the widest band. Insert the diodes into the board with the cathode ends corresponding to the flat bar symbols inked on the PC Board.

[_] D1 1N4448 Color: Yellow, yellow, yellow, gray

or Yellow, brown, yellow, gray

or 1S44 2 Yellow bands

Some diodes may have their number printed on them instead.

* NOTE: D9 is not used.

Didoes    
[_] D1 1N448 or as above
[_] D2 " "
[_] D3 " "
[_] D4 " "
[_] D5 " "
[_] D6 " "
[_] D7 " "
[_] D8 " "
  D9 not used

Resistors (Vertical Mounted)

Now that we have inserted all the low profile flush mounted components, let us go back and finish up the resistors.

There are four oblong boxes, (or rectangles), labeled R7-R10, R11-R14, R18-R22 and R32-R26. These all contain a row of resistors standing "on end" as in fig. 8. Take care when mounting these: use only the holes corresponding to the smaller box.

Four 470 ohm resistors R7-R10, get inserted vertically into an area marked on the PC board with a rectangle. See Fig. 8.

Resistors (Verically Mounted)
Check No. Value Markings / Comments
[_] R7 470 Yellow Purple Brown
[_] R8 470 " "
[_] R9 470 " "
[_] R10 470 " "

Four 470 ohm resistors R11-R14, get inserted vertically into an area marked on the PC board with a rectangle. See Fig.8.

Resistors (Vertically Mounted)
Check No. Value Markings / Comments
[_] R11 470 Yellow Purple Brown
[_] R12 470 " "
[_] R13 470 " "
[_] R14 470 " "

Five 1K resistors R18-R22 get inserted vertically into an area marked on the PC board with a rectangle. See Fig. 8

Resistors
Check No. Value Markings / Comments
[_] R18 1K " "
[_] R19 1K " "
[_] R20 1K " "
[_] R21 1K " "
[_] R22 1K " "

Four 1K resistors R23-R26, get inserted vertically into an area marked on the PC board with a rectangle. See Fig.8

Resistors
Check No. Value Markings / Comments
[_] R23 1K Brown Black Red
[_] R24 1K " "
[_] R25 1K " "
[_] R26 1K " "

The resistor packs (prefix RP) have a "common" end marked with a white dot. This should go at the end marked with a "C" on the board.

Resistor Packs
[_] RP1 8 * 10K 10K 9 leads
[_] RP2 Not used
[_] RP3 5 * 10K 10K 6 leads

Capacitors

The electrolytic capacitors (C3 & C5) will have a + or - symbol printed on them, and the + wire is usually longer. (See fig.3 and fig.7.).

Capacitors
[_] C1 47pF 47 Ceramic disc
[_] C2 47nF 473 Z Ceramic disc
[_] C3 22uF 22u Electrolytic 16V
[_] C4 47nF 473 Z Ceramic disc
[_] C5 1uF 1u Electrolytic 5V
[_] C6 100pF 100, 101, n10 Ceramic disc
[_] C7 47pF 47 Ceramic disc
[_] C8 47nF 473 Z Ceramic disc
[_] C9 47nF 473 Z Ceramic disc
[_] C10 10nF 10n, 103 Ceramic disc
[_] C11 47nF 473 Z Ceramic disc
[_] C12 47pF 47 Ceramic disc

The transistors (prefix TR) go in the board as shown by the picture printed at their positions - i.e. with their rounded corners facing the edge connector.

Transistors
[_] TR1 ZTX 313 Transistor
[_] TR2 ZTX 313 Transistor
[_] X1 CDA 6.5MC 3 lead ceramic filter

The ceramic filter is not polarized and may be inserted either way around.

Ceramic Filter
[_] X1 CDA 6.5MC 3 lead ceramic filter

IC Sockets

The IC's have one end identified by a notch, and/or dimple next to pin 1. (see fig.1). Note that all the IC's face the same way on the board, i.e. with their notches toward the edge connector. Although the I.C. sockets do not need to go any particular way round, you may like to put the beveled corner at the notch end of the I.C. position as a reminder, since the semicircle printed on the board will be covered by the socket in some cases.

There are 5 IC sockets that have been packaged along with the IC/s in a special conductive foam. The foam protects the socket and IC leads form getting bent, and the conductive property of the foam keeps static electricity from destroying the IC's.

When you are ready to insert a socket into the printed circuit board, remove the socket from the foam carefully so as not to bend its leads.

CAUTION - Visually check that none of the leads are bent. If you find a bent lead, gently straighten it with a pair of needle nose pliers. Remember, the lead are brittle and can not be bent very much, before they will break. Do not insert a socket into the PC board unless all its pins are first seen to be straight. Then carefully and gently align the socket with its marking on the PC board and insert it.

Visually check before you do any soldering, that all the leads come through the holes of PC board properly. If you start soldering and then notice you missed putting all the leads of the socket through the board it will be a lot of trouble to remove the socket.

It can be a bit tricky trying to keep an IC socket flush to the PC board when you turn it over for soldering. One method that works well is to use a small piece of masking tape to temporarily tape the socket to the board until it is soldered. Whatever method you use it is recommended that you first solder must one or two pins on the socket and then visually check that the socket is flush to the board before proceeding with the other pins. If it is not flush, reheat the one or two pins and make it flush before soldering the other pins.

Sockets for IC
Important - Solder the sockets onto the PC board
DO NOT SOLDER THE IC's
[_] socket for IC1 Sinclair Logic IC 40 pins
[_] socket for IC2 2364 ROM 24 pins
[_] socket for IC3 Z80A or D780C1 40 pins
[_] sockets for IC4a & IC4b PD2114LC RAM 18 pins
[_] socket for *IC4 * Some kits may have the following single IC instead of ICa & IC b. MK4118 24 pins

The regulator (REG) and heatsink need to go in a particular way round - just follow fig. 4.

*NOTE: If you are an experienced kit builder and happen to have some heatsink thermal joint compound, it wouldn't hurt to put some between the regulator and the aluminum heat sink. This only becomes important if you add a number of peripherals to your computer, and this regulator is called upon to provide power for them.

The regulator (REG) and heatsink
[_] REG 7805 5 Volt Regulator
[_] Aluminum heatsink
[_] *Optional heat sink thermal joint compound,(not supplies).
[_] Screw 4BA
[_] Washer 4BA
[_]  

The jack sockets need to have their business ends ( i.e. where the plug goes in) facing outwards, away form the components. This should be obvious by inspection of the board and case.

3.5 mm Jacks
[_] J1 3.5mm Jack Power
[_] J2 3.5mm Jack Ear
[_] J3 3.5mm Jack Mic

The modulator also needs to have its business end facing outwards, away form the components. This should be obvious by inspection of the board and case.

Put the modulator's wire through the holes marked "Fr/UK1" and "UK2". Put each lead through the hole it is nearest to : do not cross them over. Do not try to bend the thick pins on the modulator: hold it in place by hand while soldering. The black card trim is a push fit over the aerial socket.

[_] UHF Channel 33 TV Modulator
[_] Modulator Trim (Black Cardboard)

VERY VERY IMPORTANT!

READ THIS BEFORE INSTALLING KEYBOARD CONNECTORS KB1 & KB2

Two different brand of connectors have been used by Sinclair and there is an important difference between them.

BRAND 1 CONNECTORS - These are the original Sinclair connectors. If you align the connector within the printed rectangle on the PC board things will work out properly. See figure 12b.

BRAND 2 CONNECTORS - If you align these connectors according to the printed rectangles on the PC board you will be putting them in BACKWARDS! Figure 12c show how to properly install the connectors if you have them in your kit.

CORRECT INSTALLATION of KB1 & KB2

With either brand of connector, proper installation of the connectors can be accomplished if you make sure the following holds.

[_] KB1 - Connector KB1 must have its contact springs away form the nearest PC Board edge.

[_] KB2 - Connector KB2 must have its contact springs nearest to the nearest PC Board edge.

Figure 12a. Component side, bare ISSUE 1 ZX81 P.C. Board

Figure 12b. Properly installed Brand 1 connectors

Figure 12c. Properly installed Brand 2 connectors

The IC's have one end identified by a notch and/or dimple next to pin 1. (See fig.1). note that all the IC's face the same way on the board, i.e. with their notches toward the edge connector.

Integrated Circuits
[_] IC1 Sinclair Logic IC 40 pins
[_] IC2 2364 ROM 24 pins
[_] IC3 Z80A or D780C1 40 pins
[_] IC4a & IC4b PD2114LC RAM 18 pins
[_] *IC4 * Some kits may have the following single IC instead of ICa & IC b. MK4118 24 pins

*NOTE: IC2 has only 24 pins but it is being inserted into a 28 pin IC socket. Notice the printed marking on the printed circuit board. The IC should be inserted accordingly. Refer t fig.11.

Keyboard & Case

Refer to Sinclair Assembly Instructions Page 4. Sections 7I and 7ii.

[_] Ready made flat membrane keyboard

[_] 2 Case halves

NOTE: The following self-tapping screws require a Philips-head screw driver.

[_] 2 Yellow short screws For mounting PCB

[_] 2 Yellow short screws For Case Halves

[_] 3 Black long screws For Case Halves

[_] 4 Adhesive Rubber feet

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Sinclair UHF KITS Card Enclosure for 60hz U.S.A. video

This package contains an extra component necessary for the assembly of this kit. It is R30, a 0.5 in. jumper wire. Insert this component at the position marked R30 on the PCB. Be very careful in the placement of this jumper. There are three holes to the left of the box marked R30. Insert the jumper into the middle one of these. This jumper selects 60hz video as opposed to the normal U.K. 50hz.

Insert the UHF modulator following the instructions given in the Assembly Instructions leaflet. Be sure to insert the two wire leads into the holes marked "Fr/UK1" and "Fr/UK2". These are in effect the UHF locations for the modulator. (Disregard the USA openings on the PCB.)

To obtain video display tune TV between channels 30-40.

PC Board Silk Screen

Schematic Diagram