Radio Modifications for EME
All of these modifications are completely reversable with no permanent marks made to either radio. Once done, there is no way to accidently transmit into your preamps (provided you power your sequencer from the same source as your masthead relays)
The 847 has a separate 2 meter keying line to the PA, which makes it a straightforward radio to modify, as everything is done at DC voltage levels.
This is simplicity itself. All it takes is cutting one line on J4003 (the 2 meter PA key line) and inserting the circuit as shown. Run the wires to/from the sequencer out the vents on the back panel of the radio. While my schematic doesn't show them, I'd strongly suggest some .1 uf bypass capacitors on these two lines.
While still simple to do, the 746 uses common RF stages in the PA for HF/50/144, so there is no separate 2 meter key line. After some experimentation with switching DC voltages on the lower level stages (which didn't completely cut off RF), I decided to switch the low level RF itself with a small reed relay. This works exactly as expected with no measurable drop in RF power out. Fortunately, Icom made this easy to do by providing an easily removed jumper in the RF input circuitry on the PA board, so no PCB traces need be cut.
Referring to the schematic that comes with the radio, you can see there is an "L8" line, that provides approx. 13 volts only when 2 meters is selected. This provides the means to insure that our modification only affects operation on 2 meters - operation is exactly the same as it was before the modification on all other bands (it doesn't matter if your sequencer is powered up or not on these other bands).
Still referring to the schematic, we simply remove jumper W3 and place the reed relay's normally open contacts across its pads, solder some small wires to various points on the PA board, and run the wires to/from the sequencer out the vents on the back panel of the radio. I'd also strongly suggest some .1 uf bypass capacitors on these two lines. I've experienced no problems with the relay overheating, even after hours of EME operation ( I used a 5 volt relay I had on hand, hence the 100 ohm resistor in series with +12. Depending on what voltage relay you use, you may not need this.)
This is another easy radio to modify for use with a sequencer. Everything can be done at DC voltage levels.
Simply cut the green wire running between CN9 and CN15 on the PA board (it's easiest to cut the wire close to CN15, as there is plenty of room to mount the required components) and install the circuitry shown. I installed two phone jacks on the back panel - simply open up the ventilation slots with a drill bit, in order to mount the jacks. If needed, everything can be removed with very little evidence it was ever there. This mod only affects 2 meters.