Trivia time: What's the one item that various analog HF rigs of the late 70s/early 80s (Kenwood, Yaesu, Drake and Cubic to name but a few) had in common?

Answer: a 5MHz VFO.

Some tuned low-to-high, others tuned high-to-low - depending on the mixing scheme used. It just so happens that a Kenwood R-820 tunes the same way a Yaesu FT-901 does. This got me to wondering if it was possible to slave the two (a la TS-820S/R-820) and run them as a set of twins.

When connected as such, Kenwood (and Drake's 7-line, FWIW) made provisions to have one of the pieces serve as the HFO source-of-truth. That way there's no discrepancy in the reference oscillators and the transceiver will accurately track the receiver tuning. We can't do so-called Full Transceive with different manufacturer's equipment and must rely solely on the VFO output of one to control tuning of the other. Fortunately, Kenwood made provisions for this via a "Transceive Selector" switch on the R-820's rear panel.

There are still a few technical issues involved in getting the two to work together, even though the VFO inputs and outputs are compatible:

  • The R-820 requires +12v applied to a pin on its Remote connector for muting purposes.
  • The R-820 requires +9v applied to a pin on its Remote connector while in Transceive Control mode to allow the internal VFO to generate a signal.
  • The FT-901 will output +6v on its "Ext 6V" line when a front-panel button ('EXT') is depressed in conjunction with one of the Memory Function buttons. This is used to control an external VFO (the FV-901DM) but the levels are wrong for the Kenwood receiver.
  • Any proposed modifications to either piece of gear would have to be absolutely transparent and in no way affect the functionality of any of either line's matching accessories. In other reconfiguration of the Kenwood's Remote jack, or butchering of circuit boards. Additions are permitted.
  • Tuning errors must be corrected using the RIT function of each piece of equipment. This is similar to the method used by the FR-101/FL-101 to synchronize their HFOs, as no override provision existed in that series of gear.

The solutions to these problems and a number of others involved with configuring the lash-up will be detailed in this thread as I prove them out. Schematics, photos and mod information will be provided. At the moment I've gotten the R-820 to control the '901, the '901 to mute the receiver upon going into Transmit. CW Sidetone from transceiver to receiver is provided, as is an anti-trip signal for VOX operation.

I had on hand one of the remaining few new Hirose 1300 series OEM Remote male connectors, which was the secret to making this project work. From a high-level standpoint: As delivered from Kenwood, a number of pins on the receiver's Remote connector aren't used for anything. I leveraged this and built a level converter for the VFO switching logic into the housing of the male connector. Extra lines were run to the receiver's connector pins; these supply or route additional switching signals for the VFO - and one pair connects a resistor across a set of control lines that facilitate the level converter operation. All the required magic is accomplished in the custom Transceive interconnect cable I built.

If one were to plug a stock TS-820S/R-820 Transceive cable into the modified receiver absolutely nothing out of the ordinary will happen. As pointed out earlier, the pins in question aren't used for anything in Kenwood's implementation.

The two pieces of gear (along with matching accessories) are currently in my station lineup, doing their thing. Something else I've given some thought to is how to interface the FV-901DM into all this, and I think I have a way - albeit one which will require some switching and external power. Control of transceiver by receiver, control of transceiver by external VFO and control of receiver by external VFO are all possible - as is the FV-901DM's Scan function (which leverages AGC levels). As it stands, I have just enough extra connector pins and cable conductors to run the needed signals...