I did some troubleshooting of a motorboating problem with the 20m direct conversion RX we've been working on. We moved the RX and TX circuit boards to separate cabinets in get more room around the boards and to cut possible interference between the RX and TX wiring, especially for the antenna tuner and oscillator variable capacitor wiring. We also install and modification from the TX QST article (Dec 1978). The mod, shown in the diagram below, cuts the connection that supplies power to the RX preamp Q3. The supply lead is routed to the transmitter "RX MUTE" switch, which supplies 12V to the preamp when the RX operates and disconnects the supply when the RX MUTE is active.
The circuit had severe motorboating, which went away when I removed the RX MUTE mod. I think we can run without the RX MUTE mod, so I'm going to leave it without the mod for now. Later we can add an RF choke and bypass capacitance to the RX MUTE line that would probably fix the motoboating.
The RX has a lot of hum and noise, but I suspect that's due to my setup: I don't have a real antenna or even a decent ground in for my workbench.
A bigger problem is that the VFO drifts too much to follow CW signals. There are two capacitors in the oscillator tank that I will replace with NPO caps, to see if that cures the problem.
Dec. 23: diagnose oscillator drift. Replace C19 and C20 with NPO caps.
- Drift is less severe
- Lowest frequency is about 14.2 MHz, need to fix
Dec. 23: noticed that oscillator produces about 500 mV RMS, but according to LTSPICE model and QST schematic, amplitude should be 2 V RMS. Changed out the oscillator transistor Q4 for another MPF102
- Amplitude increased slightly to 600 mV RMS.
- Disconnected oscillator from mixer by removing C22. Oscillator amplitude increased to 800 mV RMS.
- Check 1N914 diode D1 forward voltage.
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