Since my HF gear consists of tube equipment, it was irresistible to buy a used Galaxy Saturn Turbo (a.k.a. RCI-2990) with Astatic D-104 microphone for a very good price. Even if it wasn’t for me, I could clearly profit 2x+ after cleanup. It was the usual CB shop butcher job on some hacked on fans for the heatsink.
Heatsink fan fix
The heatsink cools power supply pass transistors and the RF amplifier. The power supply is a known weak point in this radio, so we don’t want to overstress the pass transistors. The 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.
Even some ham operators crank up the ALC control for SSB–a terrible idea because pushing the radio into non-linear regime just splatters the band for tens of kHz with no increase in communications usefulness. I set this to the rated 100 Watts, which is comfortably linear and clean (and doesn’t overstress the power supply).
AM Limiter and carrier
This had been cranked wide open (thankfully, not clipped) so I backed it down, not to 100% clean, but to about 130% modulation with loud input, so that the uncompressed human voice clips a little, while allowing a much higher average modulation percentage. One could implement an asymmetry circuit to allow cleaner >100% positive modulation in the future. Maximum transmit carrier 25 Watts, minimum 1/2 Watt. This would be a tempting radio to do a high-level modulation conversion on, where you’d have a 100 Watt audio amplifier added externally. Someday perhaps.
I need to get the service manual–even though there are generic tuneup procedures, I don’t like to resort to that when I can get the schematic and OEM tuneup procedure to do it optimally. I would also like to see where to do a capacitor changeout if needed to make the audio more optimized for flat frequency response on TX and RX.
Well, that’s what approximately 100% of users buy this radio for. I just listen of course, and converse with licensed hams on 10m. It’s really nice having a radio I don’t have to tune up. Just turn on 10m and go. It’s too bad there’s no “chipswitch” like the HR-2510 where you can cover 12m and 10m with that radio….
monitoring transmit quality
On AM, I just use a germanium diode with a pickup wire in the air into a microphone input of my Radio Shack SW/PA radio. This gives very clean broadband monitoring with headphones. For SSB, one needs an oscilloscope with sufficient analog bandwidth for 30MHz, which I don’t have. You can get a sense if something is amiss by monitoring the adjacent channel with an AM CB–if you hear spits and splatter, the ALC is clearly set too high (reduced power is needed).
All in all, I get a lot of enjoyment out of this radio, and may just hang onto it till the sunspots come back up for worldwide propagation.