Argonaut 509 conversion for 30 meters

From Ham Radio Magazine, page 49, October 1984


As declining HF propagation renders the l0-meter band less of a dependable mainstay for the QRP operator, the new allocation at 10 MHz is coming into its own for this mode of activity. The operating restrictions imposed upon 30 meters actually help rather than hinder QRP operation.

The Ten-Tec Argonaut 509 may easily be modified to cover 30 meters with the same performance found on its five bands. All that is required for the modification is five minutes of time, four components, and a jumper wire.

Ten-Tec uses a 9-MHz IF with appropriate VFO frequencies. On 10 meters, the VFO ranges from 19 to 21 MHz. If we use the difference mixer product, rather than the sum, as in the original design, output occurs on 10 MHz rather than 28 MHz. Since the 509's transmitter stages are broadband amplifiers, the only changes required for 10 MHz transmit are to re-resonate the appropriate bandpass filter, composed of T7 and capacitors C17 and C18 on the 80262 front-end board (see fig. 1). Shunt C17 and C18 each with an additional 91 pF of capacitance. Depending upon component tolerances, retweaking T7 cores may be necessary for adequate output. The addition of the two 91-pF capacitors completes transmitter modification.

The receiver will function with much reduced performance without modification by turning the receive preselector completely counterclockwise, as mentioned in the 509 manual for WWV reception. To improve 10 MHz performance the receiver front end needs additional capacitance.

C1, on the main schematic, is switched across T1 to resonate at the desired band. For our purposes, C1needs to be shunted with another capacitor, again of 91 pF, as seen in fig. 2. T2, on the other end of the RF amplifier front end, must also be resonated at 10 MHz. Shorting C7 with a piece of tinned bus wire and paralleling variable C8 with an additional 47 pF completes our modification.

The modification is simple and quick. Lifting one end of each added component easily restores l0-meter operation when the sunspot activity Increases.

Modification of the newer Model 515 is identical. The rig that began it all, the 505, is not broadbanded on transmit.

Raymond Henry, AA4LL