ZX Spectrum 128k and grey +2 modification for D40/D80 cooperation

D40 and D80 are excellent floppy disk drives made by Didaktik. They were widespread in Czechoslovakia in past, but they are incompatible with 128k machines. There were few modifications of 128k machines to cooperate with them and here is simpliest and best one. It is based on Zilogator's idea, where PAG is pulled down during D40/D80 operations and 48k ROM is forced with switch.

Part list is very simple:
1 x slider switch
2 x 680R resistor
1 x 1N4148 diode
some wires.

Modification is simple - cut the PAG signal between IC29 and IC28 (IC7 and IC15 in +2) and put 680R resistor there. Then connect diode between A5 and weakened PAG signal (cathode goes to A5). After this, cut the ROM signal between IC31 and IC5 (IC6 and IC8 in +2). Then put mid pole of slider switch to IC31 (IC6) side of cutted track, left pole goes to IC5 (IC8) side of cutted track and right pole of slider switch goes to VCC through another 680R resistor. And that's it. When you want to connect D40 / D80, move slider switch to right side. When you want normal operation of your beloved 128, leave slider switch on left side.

Pictures:

Schematic.

Factor6's modified 128k (F6, thx for pic).