Sinclair ZX Spectrum Service Manual
Spectrum For Everyone https://spectrumforeveryone.com/
21
4.1.2 Modification History
Two major modifications have been implemented on the Spectrum boards:
(a) A modified layout of board (i.e. 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
miniature 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 (e.g. 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:
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