Sinclair ZX Spectrum Service Manual
Spectrum For Everyone https://spectrumforeveryone.com/
49
Substitute components as follows:
9.2.1 Circuit Operation
The modification is confined to adding/changing protection components in the rectification circuit to
improve reliability. Circuit function does not change.
9.3 ISSUE 3B BOARD - (ALREADY COVERED IN EXISTING MANUAL)
9.4 ISSUE 4 BOARD - NONE ISSUED
9.5 ISSUE 4A BOARD
This differs from the issue 3B follows:
• R32 deleted
• Two spare gates from IC24 have been used to improve ULA timing. It is important that ULA 6C001-
7 or later issues (the issue number is designated by the number after the hyphen) are used in this
circuit. The gates are fitted in series with the RASL output from the ULA and the input to the
display RAM multiplexers.
• Provision is made for fitting interference suppression coils and a modulator channel switch, as
required on VHF units.
9.6 ISSUE 4B BOARD
The Issue 4B board is the same as that for the Issue 4A with the addition of the dc-dc converter
modification. The board layout was changed at this stage as shown here. Due to lack of board space, C61
(22uF decoupler) was omitted. It is reinstated on subsequent board issues. The circuit diagram is shown
here.
9.7 ISSUE 5 BOARD
1000 of these boards were supplied. They are restricted to the UK market only. A single ULA, IC27,
(Mullard ZX 84/01) replaces the six 74LS chips. The circuit diagram is shown here. An inverter chip IC28
has been added and is used in the decoupling of the RAS/CAS function. Circuit functions do not change.
Some discrete components were added/changed as reference to the parts list show. The board layout is
shown here.
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