Single Keyline Solution

A Solution to the Single Keyline, Multiband Transceivers

What is a Single Keyline Multiband Transceiver (SKMT)? Well, as many of you may remember prior to the SKMT’s such as the Yaesu FT-736R, each band of this and other multiband rigs had a separate ‘keyline’ for each band that was used to key auxiliary equipment such as an amplifier, preamp, etc… Today we have a new breed of multiband transceiver that has an extended HF and coverage on into the VHF/UHF range. For whatever reason, the Imperial Wizards decided (possibly economic considerations) that a single keyline would be sufficient on these transceivers that could employ up to 5 or 6 different external amplifiers (???) and/or peripheral devices, thus an “SKMT”!

If used as is, chances are the first time 5 or 6 employed devices are keyed (providing all the devices such as amplifiers are turned on, or placed in the ready state) at the same time, the keying transistor in the transceiver is going to have its current limit exceeded and cease to function forever more! And, of course, all of the amplifiers (if these are the peripherals) would have to be modified so that they employed diode steering in the keying circuitry. Even in the FT-736, I chose to go the alternate route with four amplifiers and had them RF activated even with the availability of four separate keylines. This could be one solution to today’s SKMT.

However, many of us like positive control of our auxiliary equipment and that is where I took the bull by the horns. The auxiliary plug on these transceivers offer a limited current, +12V source pin and generally a “send” pin, which is a relay or open collector (NPN) transistor that when activating the PTT will put this pin to ground. Using the combination of these two pins I designed a circuit that would control two relays (two double pole triple throw [DP3T] devices). I protected the source with diodes across the solenoids of each relay and paralleled the source with a resistor and LED to indicate when the circuit was active.

The three circuits of the first relay had their common contacts go to RCA phono jacks that would place the keyline of aux equipment at ground when the PTT was activated. On the N/C contacts I placed toggle switches with a grounded ‘common’ so that the individual band keyline could be selected.

The second relay had all three N/C contacts supplied with an external +12V source. Three toggle switches were then wired in series with the common contacts of that relay. The common of the toggle switches were then wired to the center contact of another set of three RCA phono jacks and paralleled with a resistor and LED to indicate power on when selected by one of the three switches. This second set of phono jacks were for powering individual preamps. The philosophy being that when the PTT of the transceiver was activated the two relays would be activated; the first set of three phono jacks would be available to ground a key line as selected by that keylines corresponding toggle switch. The second set of phono jacks would loose the +12V external source to the individual preamp that might be toggle switch selected. In the ‘receive’ or (unkeyed PTT) condition any of the three preamps could be selected or unselected at will using the three ‘preamp’ toggle switches.

In summary the overall circuitry provides three ‘keylines’ operable from one original (existing) keyline on the transceiver. Also, separate power lines are made available for the powering of preamps and the preamps are so protected by dropping the power to them upon activation of the transceivers PTT circuitry. This also allows for the preamp to be used or disabled at the operators’ discretion. If interested, a schematic will be made available upon request. w6oal at

Dave W6OAL –

Olde Antenna Laboratory 41541 Dublin Drive Parker, CO 80138