The Schematics:
These schematic diagrams are based on a painstaking analysis of the Amstrad +2A/+3 service manual and Amstrad +2B/+3B service manual along with following circuit traces around the boards to correct where the original schematics had errors. The schematics have been re-drawn using the gschem schematic editor from the open source gEDA project.
The ZX Spectrum +2A/+3 Motherboards (PDF file).
Re-draw Revision 20, updated 2020-01-23.
This schematic describes the circuit of the +2A/+3 motherboard (Z70830). Lists of the components omitted for either model are included. For details of the factory fitted modifications required to correct errors on the ISSUE 1 board see the Amstrad Factory Mods page.
The ZX Spectrum +2B ISSUE 1 Motherboard (PDF file).
Re-draw Revision 17, updated 2020-01-23.
This schematic describes the circuit of a +2B with a Z70833 ISSUE 1 Motherboard. Some boards have factory fitted modifications making them eqivalent to the ISSUE 2 motherboard.
The ZX Spectrum +2B ISSUE 2 Motherboard (PDF file).
Re-draw Revision 8, updated 2020-01-23.
This schematic describes the circuit of a +2B with a Z70833 ISSUE 2 Motherboard. Some boards have the capacitor C53 fitted and some have a capacitor soldered to the underside of the board or CPU. See Amstrad Factory Mods.
The ZX Spectrum +3B Motherboard (PDF file).
Re-draw Revision 11, updated 2020-01-23.
This schematic describes the circuit of a +3B with a Z70835 ISSUE 1 Motherboard.
Notes:
No guarantees can be given for the accuracy of these schematics. The original service manuals contained errors and further errors may have been introduced in the transcription process. Though every effort was taken to confirm the schematics' accuracy errors are discovered from time to time. Whenever an error is corrected the issue of the schematic is incremented and the date changed.
The +2B and +3B schematics are more likely to contain errors as I don't own any of these board and so consequently can't trace out the circuits. I also currently have NO details about the completely new sound modulator circuit fitted to the ISSUE 4 motherboards.
If you discover any errors in either schematic please let me know.
Thanks to everyone who has assisted by opening their Spectrums to provide photographs of their motherboards and trace tracks around the board
Physical differences and circuit factoids:
The Z70830 motherboard was designed in such a way that it could be assembled as either a +3 or a +2A depending on which component positions were populated. The schematic details which components are omitted from the circuit on each model.
The first version of the +2A/+3 motherboard (Z70830 ISSUE1) has some small errors which were corrected by factory modifications and corrected in the layout of the Z70830 ISSUE2 board. Another very minor change between these two versions was changing the silkscreen text for the LED cable header from "TO LED PCB" to simply "LED" with the addition of plus and minus indicators. This remains the same on all other boards despite the LED header never being used on the +2B.
Another thing which is present on the +2B boards but never used is the points to the immediate left of the power socket where the floppy drive power cable attaches on the +3 and +3B. An additional curiosity is that on the Z70833 ISSUE1 and ISSUE2 the silkscreen rectangle around these pins found on the +3 and +3B boards is omitted, but it returns on the ISSUE4. The voltage labels are present on all boards.
An oddity in the circuit of the +2A/+3 is the resistor R76. This resistor connects the active high reset signal (generated by IC15e and present on the upper pad 28 of the expansion connector) via a 10kΩ series resistance to the input of IC15f. In the +3 this resistor appears to have no purpose as the RESET signal can never override the output of IC18a.
If the board is assembled as a +2A however; IC18 is deleted since it is only required for the floppy drive interface. There is therefore no logic output connected directly to the input of IC15f, so it is pulled up or down to the output of IC15e. The purpose of R76 then appears to be simply to prevent the input of IC15e from being left floating when the Z70830 board was assembled as a +2A.
Amstrad removed R76 from the schematic for the +2B; however the circuit traces and component holes remained on the Z70833 motherboard layout for both ISSUE1 and ISSUE2 revisions (they were removed on the ISSUE4 board. None of the boards have any silkscreen markings at this position).
IC15f is entirely unconnected on the Z70833 ISSUE 1 board; the removal of R76 leaves the input floating.
R76 is present on the +3B motherboard (the Z70835 ISSUE1). This may be because when the necessary upgrades were applied to the Z70830 ISSUE2 board layout, the fact that R76 was no longer needed was overlooked (all provisions for fitting an internal cassette recorder were removed, so the Z70835 would never be assembled without all its ICs).
On some Z70833 ISSUE1 boards R45 is omitted and the inverted clock signal created by IC15a (present at CKEXT on the edge connector) is inverted again by IC15f and routed to the CPU clock input. This is achieved with factory fitted patch wires. This modification also includes a 330pF capacitor between the input of IC15a and 0v.
This change to the clock was integrated into the Z70833 ISSUE2 PCB. The clock signal output from pin 87 of the gate array (IC1) is no longer connected to the R45 component position which is still present but is shorted out by a PCB trace on the top side of the board. No R45 component is fitted. (The R45 position is removed completely on the ISSUE 4 motherboard). The capacitor mentioned above is given the designation C53, but is not fitted to all ISSUE2 boards.
A 100pF capacitor is fitted between the 5v rail and the D6 pin of the CPU on Z70833 ISSUE2 boards (Only seems to be fitted to ISSUE2 boards, not confirmed to be on every board). Some boards have this attached to the CPU socket pins on the solder side, others it is soldered to the CPU legs directly.
The Z70833 ISSUE4 board adds a 100pF capacitor labelled C250 to the right hand side of IC14. This is similar to the mod on the ISSUE2, but is connected between 5v and the D0 pin