EMRFD Message Archive 3847
Message Date From Subject 3847 2009-12-01 11:42:17 w7loz Post script on the 20m vxo transmitter from EMRFD After making more measurements on the transmitter, I concluded that the low power output was probably due to a wimpy 2N3866 driver rather than a problem with the FET final amp. The maximum drive being developed was considerably less than the 300 mw specified on the schematic.
Rather than replace the driver transistor, I made the following changes: On the emitter of the 2N3866, I reduced the 12 ohm resistor that's in series with the 0.1 uf bipass cap to around 5.5 ohms by tacking a 10 ohm resistor in parallel with it. I also raised the value of the 51 ohm resistor associated with the FET gate to 100 ohms. (Actually, at one point I removed this resistor altogether. The amplifier seemed to remain stable, but I reinstalled the 100 ohm resistor there for insurance.)
These changes, in combination with having previously lowered R1 to 6.8 ohms brought the power output up to a full 4 watts with 12.0 volts input from the power supply. I don't see any indication of instability in the circuit, and I believe the waveform at the gate of the IRF-510 actually improved a bit.
Appropriately, my first contact with the rig was with W1AW. My receiver consisted of twin 20 meter converters (KK7B's design) on the input of the 40m binaural I-Q receiver. The latter I've modified to accept two antennas for diversity reception. The modified I-Q receiver isn't the best on a crowded band, but it sure does pull in signals when they're afflicted with QSB, which was the case with W1AW yesterday.