The pass-through input on the Spectrum is a 9-pin connector. The best way to use it is in conjunction with something like an Indivision, or one of the older "flicker fixer" boards, such as the Microway Flickerfixer or the A2320. Then you can use a 15-to-9 pin cable between the output of the flickerfixer and the input on the Spectrum. All of your screenmodes will then display correctly on any modern monitor, and you won't need to worry about a 15KHz compatible monitor. Alternatively you can use one of the C= silver 23-to-15 pin adapters, then a 15-to-9 cable. Of course you'll need a 15KHz compatible monitor for displaying older screenmodes, with this option.
