The Lil Old ZX Spectrum 48k Service Manual




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.

Oscilloscope with

Oscilloscope probe (x10)

Rise Time: 0.02 us/cm
Variable power supply unit 0 to 30V dc
Mono cassette recorder With RECORD and PLAYBACK facilities
Mains extension lead `Safebloc` type
Multimeter General purpose
Colour Television Open Market
ZX Printer Sinclair
ZXTP test tape  
Blank tape Open Market
Double-sided adhesive tape 12mm and 6mm wide, Tesafix 959

(BDF TESA) or 3M equivalent

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.


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.


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.


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:

Board Issue No

ULA Type







R47 220R 1k 1k
R49 8k2 10k 10k
R56 220R 470R 1k
R63 220R 470R 1k

Modifications - Issue 1 Board

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

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

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.

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

Modifications - Issue 3B Board

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:

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

back to Section 3
return to Service Manual Index
continue to Section 4.2