Satellite Passband Frequency Tracking on Older Radios

Created 11 September 2001

Updated 23 October 2001 - Corrected errors in FT290/790 Mod schematic.

The good news is that you can pick up single band all mode transceivers such as Yaesu FT-290 Mk1's and FT-790 Mk1's for about GBP100 (US$140) each these days. If you want to use them on linear transponder satellites, the bad news is that they have no facilities for analogue transponder frequency tracking.

For analogue transponder satellite use, it's very handy to be able to track the uplink and downlink passbands simultaneously so that when you tune the uplink, the downlink follows.

So with my modification hat on I examined the FT-290 Mk1 and FT-790 Mk1 schematics, switched on the 'scope, and discovered that they use simple three wire (Common, Up/Down, Clock) rotary shaft encoders for their tuning dials. Here's the truth table:

Clock Up/Down Function
Transition +5V to 0V 0V Up
Transition +5V to 0V +5V Down

Initially it also appeared that the rest state for both clock and up/down was pulled down to 0V with a weak (330k) pulldown. The common of the rotary encoders is at +5V. So I modified both radios in order that (a) the Ground, Clock and Up/Down microprocessor connections are available via a 2.5mm stereo socket and (b) inserted a 3.3k resistor into the each of the Clock & Up/Down lines so that their states may be overridden with either a 'wire-or' or a 'wire-and':

Where to place the 3.3k resistors: right on the connector referred to in schematics. FT-290 Mk1 & FT-790 Mk1 are similar, but both wire colour codes and pin numbers differ!

Attempt #1

I then set to with the following simple cable with two diodes and two 2.5mm stereo jack plugs:

By changing the FT-290's frequency with the dial, the FT-790 also changes but as the 290 goes up, the 790 goes up, the 290 goes down. Because of the diodes, changing the 790's frequency with the dial does not affect the FT-290.

Position to mount the stereo jack plugs on the radios

Attempt #2

OK, attempt #1 was quite successful but suffered from four problems:

o    The rotary encoders don't always return to their weak pull down rest state, so the independent dial will sometimes appear to stick.

o    After a number of twiddles, either due to contact bounce or otherwise the two dials would get out of step.

o    There's no ability to switch to true (as opposed to inverting) transponder modes.

o    Unless you unplug and swap the cable, you can't change the role of the controlling transceiver dial.

So I built something a little more complicated to replace the cable:

Current draw is under 10mA. The firmware (including source) is here. The timing's been optimised for the FT-290 Mk1 and FT-790 Mk1, but you could use it for other radios with similar three wire rotary shaft encoders.

This is one of those scenarios where KISS didn't work better!


Mail Howard, G6LVB