Galaxy Saturn Turbo for 10m
A Galaxy Saturn Turbo (a.k.a. RCI-2990) pairs well with an Astatic D-104 microphone. The heatsink cools power supply pass transistors and the RF amplifier. The power supply is a known weak point in this radio, so don’t want to overstress the pass transistors with excessive transmit power. To improve cooling, an idea is to run the fans at 9 volts when not transmitting, and use a MOSFET to short the resistors to provide full 13.5 volts when transmitting. A 555 timer provides the delay after transmit, to allow for cooling 15 seconds additional after the transmission ends.
These tips apply in general for AM / SSB transmitters. Don’t crank up the ALC control for SSB. Pushing the SSB transmitter into the non-linear range splatters the band for tens of kHz with decrease in communications usefulness. Set SSB transmit power with two-tone test input to the rated 100 watts, which is comfortably linear and clean (and doesn’t overstress the power supply).
The AM Limiter and carrier should be set to allow clean modulation with an oscilloscope. Maximum transmit carrier 25 Watts, minimum 1/2 Watt. Monitoring transmit quality: on AM, use a germanium diode with a pickup wire in the air into a microphone input of a Radio Shack SW/PA radio. This gives very clean broadband monitoring with headphones.
For SSB, one needs an oscilloscope with sufficient analog bandwidth for 30 MHz. You can get a sense if something is amiss by monitoring the adjacent channel with an AM CB, where the receiving CB has no antenna. If ones hears spits and splatter, the transmitter SSB ALC is clearly set too high.
For FM, don’t touch the transmitter deviation control unless a spectrum analyzer or FM deviation meter is available. The spectrum analyzer is used with Bessel function table (or an FM deviation meter) to precisely set the maximum transmit FM deviation. For FM CB radio and 10m FM simplex, the maximum FM deviation is 2.0 kHz. This was determined by regulators considering the standard 10 kHz channel spacing. In contrast, in some parts of the world, 10m (29 MHz) FM repeaters may use 5 kHz maximum deviation despite the 10 kHz channel spacing. This practice may have evolved from the earlier days of 10m FM repeaters repurposed from the commercial low-VHF FM business band, where 5 kHz deviation and 25+ kHz channel spacing was common. We recommend setting the FM deviation to 2.0 kHz for widest compatibility with FM simplex operation. When the FM deviation is set too high, the FM signal splatters into adjacent channels and causes unintended muting of squelch on receivers “talkoff” set for 2.0 kHz deviation because the wider deviation signal is interpreted as noise by the squelch circuit.