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ZX Spectrum +2A/2B, +3/3B

29 bytes removed, 18:34, 7 May 2019
+2B and +3B
The +2A/+3 circuit contains a design flaw in the audio output circuitry which causes serious distortion to AY-3-8912 music. A new design was developed in 1988 with redesigned audio circuits and manufactured as the +2B and +3B. Unlike the +2A/+3, Amstrad did not create one common motherboard for these computers. Instead the +2B board is stepped on the right hand side to allow two boards to be panellized side by side and use a much smaller area (hence cheaper to produce) than two +2A/+3 boards.
The identification of models is regularly confused contested because Amstrad sold large numbers of computers with a Z70833 motherboard (+2B) in cases that were moulded for bearing the +2Adesgination. The change to shipping +2B motherboards Z70833 boards instead of +2A Z70830 boards appears to have happened fairly early on because most +2A cases encountered contain a +2B motherboardZ70833. Z70830 motherboards populated as a +2A are consequently quite rare.
The +2B motherboard is the Z70833 ISSUE 1 or 2 ©1988. There is also an ISSUE 4 ©1990 which has a different FM modulator circuit for the audio (no example of an ISSUE 3 board has surfaced so it's possible which suggests none were ever produced/sold).
The +3B motherboard is the Z70835 ISSUE 1 ©1988. Unlike the Z70830 it has no provision for connecting a datacorder but the connections for an external floppy controller remain on the expansion port. Unlike the +2A and +2B all +3 cases are the same. i.e there is no +3B case moulding.
==Timings and Contention==

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