Modified FM1100 as a Local Oscillator source for an HF transceiver....

This *very* simple mod allows a spare A9 of K1 band FM1100 to be used as a synthesised local oscillator source for an HF transceiver - either because its 'homebrew' and not very stable(!), or because its a commercial crystal controlled HF transceiver that is kicking around (there are plenty of these from 20 years ago - Pye SSB200, Motorola Micom - that sort of thing).

The idea is to remove the Tx PA, and feed the VCO instead into a divide by 10 chip, such as an old Plessey SP8660. So a VCO running at 144.100MHz comes out at 14.410, which gives 3.710 MHz for a 10.7MHz IF. With a 100 channel head and Par 111 and 112 set to '0', this can give 500Hz step sizes - My drift was more than this, so its not too course. Using 'Tx' keying gives another 100 channels. With the SP8660, an upper limit of 180MHz input frequency means that 7MHz is going to be all that can be managed in this case, but that's all I wanted anyway.



Circuit Diagram of divider and emitter follower:



Things couldn't be much simpler (well, except that you could take 5v from an existing pcb, but that's an extra wire). The emitter follower seemed to be needed when it was used to drive a Motorola Micom, so guess the divider output drive is fairly high impedance.

In the photo, the front end/VCO board was hacked about so that the new circuitry could be added there, but this was unnecessary work really - it would be easier to leave the front end totally unmodified and simply add the new pcb where the Tx PA used to be. I'll do a layout for this in due course.

This sounds very good in use, so have a go....