Double Density Modification
Details of this modification
were kindly supplied by Leslie Ayling, Australia.
(ex-Disk Smith Electronics Ltd. sales and service)
Modification to allow a double density adaptor to be fitted
(Note: I think this modification was only necessary for the X-4010 expansion unit. The X-4020 unit had jumpers fitted for this purpose - TMS)
Locate the FD1771 FDC chip. Cut the traces leading from pins 25,26. Join both pins, and solder a 10k resistor between the joined pins and a convenient source of +5v. This will disable the internal data separator so that the double density separator (on the double density adapter) can operate correctly.
Pin 25 is the /XTDS signal. Normally it is tied to ground indicating to the 1771 that an external data separator circuit is in use. Tying it through a 10k resistor to +5v tells the 1771 to use its (inferior) internal separator circuitry. Pin 26 is an input for an external clock signal when an external separator is used. When the internal separator is in use it should be tied high.