Sinclair ZX Spectrum Service Manual
Spectrum For Everyone https://spectrumforeveryone.com/
22
4.2 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 efficient operation of certain machine code software and should
be implemented on all units. It has been implemented in manufacture for all ULAs 5C112E-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).
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