The Lil Old ZX Spectrum 48k Service Manual

SECTION 4.1 - FAULT DIAGNOSIS AND REPAIR

LIST OF CONTENTS

• Introduction
• Test Equipment
• Modification History
• Modifications - Issue 1 Board
• Modifications - Issue 2 Board
• 32k Extension Memory - (16k-48k)
• Hitachi vs NEC ROM

INTRODUCTION

Test Equipment

Section 4 is intended as a guide to fault diagnosis and repair of the ZX SPECTRUM. It is assumed that users have a resonable knowledge of electronic servicing, theory and standard fault-finding techniques and have access to the test equipment and tools required to carry out the task. The table below contains a list of the minimum recommended test equipment and materials.

Engineers who are already familiar with the Sinclair ZX81 will find some similarities in the ZX SPECTRUM. The Spectrum, however, is a more sophisticated device with colour and sound circuitry.

The ZX SPECTRUM is supplied with either 16k or 48k of on-board RAM. The 16k version is wired so that it can be expanded to 48k by the addition of a number of appropriate plug-in ICs. For details of the memory upgrade, see later on here.

Modification History

Two major modifications have been implemented on the Spectrum boards:

(a) A modified layout of board (ie Issue 1, Issue 2 and Issue 3)

Issue 1

This layout allowed for the 16k RAM to be mounted direct on the board, with the 32k extension consisting of a subsidiary board mounting onto special DIL connectors. Approximately 26,000 units were manufactured.

Issue 2

This layout allows for all 48k of RAM to be mounted direct on the board; sockets, ready for a memory update, are fitted on 16k versions. A small number of Issue 2 units were manufactured using disc capacitors for 47uF and 100uF values, but subsequently axials were introduced.

Issue 3

This layout incorporates advanced circuitry for the colour tuning, removing the need for potentiometers and trimmers and the associated adjustment. It also allows for OKI ICs to be used in the 32k extension memory. The heatsink is also redesigned and is relocated at the back of the case.

(b) The Ferranti ULA, type 5C102, plus spider addition replaced by ULA type 5C112.

5C102

This ULA has a timing fault which was cured by connecting a 74LS00 IC mounted on a minature board and spider. This ULA was fitted to approximately 40,000 units.

5C112

This improved ULA has no spider modification, but has either a diode or resistor or transistor connected to it. The details are:

Diode/resistor mod

4k7 resistor from +12V to pin 30 on IC2 and diode (IN4148) from pin 30 of IC2 to pin 33 of IC1.

Transistor mod

TR6 (ZTX313) connected as follows:

Base to IC2, pin 30.

Emitter to IC1, pin 33.

Collector to IC2, pin 11.

6C001

This ULA alters the timing of the colour burst signal, and improves the performance of the Spectrum with certain television sets (eg Hitachi, Grundig). It also causes the screen picture to be shifted by one character width to the left.

NOTE: The values of certain components vary depending on which board and which ULA is used as follows:

Modifications - Issue 1 Board

A number of modifications were introduced, and these should be checked for and, if necessary, introduced retrospectively:

• 100pF capacitor between RAS and ground - necessary only when using ULA 5C102.
• 470pF capacitor between IC2, pin 28 and ground - required only when IC2 and RAM ICs are all of NEC manufacture.
• 1k resistor between RAS and 23V and 1k resistor between CAS and 12V, only when RAM ICs are of National manufacture. R57 (330R) must be removed, and the 470pF capacitor is not required.
• 47k resistor between pin 13 of LM1889 and ground - required only if the difference between white and yellow colours is inadequate.
• If disc capacitors are used for C41 and C49 (47nF) they must be replaced by axial components.
• Capacitor C46 (1uF electrolytic) should be replaced by a high temperature component.
• This modification is required for effecient operation of certain machine code software and should be implemented on all units. It has been implemented in manufacture for all ULAs 5C112-2 and later.

D14 replaced by C67 (100pF)

R24 changed from 3k3 to 1k

R27 changed from 680R to 470R

R73 (1k) added between IC1/32 and +5V

• A significant cause of problems has been found to be failure of the internal power supply transistor TR4. The circuit may be improved by changing the value of R60 from 100R to 270R and connecting a 4.7uF electrolytic (preferably radial type) with positive end to TR5 emitter (positive end of C34) and negative end to TR5 base (to R58).

Modifications - Issue 2 Board

A number of modifications have also been introduced for the Issue 2 board. These should be checked and, if necessary, introduced retrospectively.

• These components should be changed if colour quality needs upgrading:

R48 changed from 4k7 to 2k2

R49 changed from 18k to 8k2

R50 changed form 8k2 to 4k7

R72 changed from 47k (or 18k) to 10k

C65 changed from 100uF to 22uF

• Early Issue 2 boards were manufactured using a considerable number of 47uF and 100uF disc capacitors. C41 (47nF) and C43 (100nF) should be replaced with axial components to prevent undue risk of short circuits.
• To prevent lifting of keyboard template, a thin strip (6mm wide) of double-sided tape was introduced to the top edge of the template. (Previously, only the other 3 edges had tape). This modification should be introduced whenever template lifting is found to be a problem.
• See Modifications - Issue 1 Board, the last two.

Modifications - Issue 3B Board

• Although functionally similar to the Issue 3 board, the layout differs. The DC converter design has been improved, providing greater power supply reliability. Component changes are as follows:

C77 (100nF) added

C49 changed from 47nF to 560pF

R60 changed from 270R to 68R

TR4 can be either ZTX650 or TIPP31

32k Extension Memory - (16k to 48k)

Board Issue 1:

The extension memory is supplied as a plug-in module which is inserted into sockets near the edge connector. The ICs used are either TI 4532-3 or TI 4532-4, and all ICs in a unit must be of the same type (ie all must be -3 or all must be -4).

Board Issue 2:

The extra memory is obtained by plugging in 4 logic ICs and 8 memory ICs (see here). The ICs used are either TI 4532-3 or TI 4532-4, and all ICs must again be of the same type. It should be noted that the RAM select signal is 0V for the -3 IC and +5V for the -4. A link on the board provides the required select level.

Board Issue 3:

This allows OKI ICs to be used in place of TI 4532. Again, all ICs must be of the same designation. Appropriate links (2) must be fitted in the grid located on the board between the edge connector and the `mic` jack socket. Again the extra memory is obtained by plugging in 4 logic ICs and eight memory ICs.

Hitachi vs NEC ROM

During manufacture, wiring links are selected and inserted in the board according to whether a Hitachi or NEC ROM is used. These links are marked `H` and `N` as appropriate and are located as follows:

• Board Issue 1 - between the ROM and the heatsink
• Board Issue 2 - beneath the raised portion of the heatsink
• Board Issue 3 - adjacent to the loudspeaker

Clearly, if the ROM is changed for one of a different make, then these links must be changed as well.