It's been said many times that there's nothing like the sound of a direct conversion receiver, and a couple of weeks using the $20 Bill has certainly affirmed that for me, the direct-to-audio experience with a competent DC rig is indeed something special.
But, I didn't get into this hobby just to listen, so it didn't take long (maybe two days) before I started noodling and doodling a transmitter board to install inside the rig.
Being true to the theme of the original rig, I used all common and inexpensive transistors: 2N3904, 2N3906 and a pair of BD-139s in push-pull to deliver roughly 5 Watts when using a 13.8V supply. Nothing fancy and nothing being pushed very hard makes for a predictable and stable QRP transmitter.
"Splicing" it in to the rig was simple: The VFO from the receiver is "hard wired" to the input of the transmitter, meaning that it is always connected to both circuits rather than switched between RX and TX. At the other end, I connected a relay at the junction of the mixer and bandpass filter to switch the filter/antenna circuit between the two modes. Similarly, a relay is inserted between the audio driver and power amps, allowing switching between receiver audio and transmitter sidetone.
A few hundred Hz of VFO shift is needed when transmitting, else people will reply at your zero-beat frequency. To accomplish this, one set of relay contacts switches a gimmick capacitor in parallel with the main tuning capacitor. The gimmick is simply a pair of insulated wires twisted together, the number of twists was determined experimentally.
To use the rig as a transceiver, you simply tune stations to the lower side of zero beat, flip the T/R switch and pump the key. It doesn't get much easier.
Since building the transmitter board, I've made several QSOs and, other than my signal being weak, the reports are that the tone is pure and free of annoying clicks and/or chirp - everything I look for in a CW transmitter.
Simple rigs like this are a blast to operate. The appliance guys don't know what they're missing!
But, I didn't get into this hobby just to listen, so it didn't take long (maybe two days) before I started noodling and doodling a transmitter board to install inside the rig.
Being true to the theme of the original rig, I used all common and inexpensive transistors: 2N3904, 2N3906 and a pair of BD-139s in push-pull to deliver roughly 5 Watts when using a 13.8V supply. Nothing fancy and nothing being pushed very hard makes for a predictable and stable QRP transmitter.
"Splicing" it in to the rig was simple: The VFO from the receiver is "hard wired" to the input of the transmitter, meaning that it is always connected to both circuits rather than switched between RX and TX. At the other end, I connected a relay at the junction of the mixer and bandpass filter to switch the filter/antenna circuit between the two modes. Similarly, a relay is inserted between the audio driver and power amps, allowing switching between receiver audio and transmitter sidetone.
A few hundred Hz of VFO shift is needed when transmitting, else people will reply at your zero-beat frequency. To accomplish this, one set of relay contacts switches a gimmick capacitor in parallel with the main tuning capacitor. The gimmick is simply a pair of insulated wires twisted together, the number of twists was determined experimentally.
To use the rig as a transceiver, you simply tune stations to the lower side of zero beat, flip the T/R switch and pump the key. It doesn't get much easier.
Since building the transmitter board, I've made several QSOs and, other than my signal being weak, the reports are that the tone is pure and free of annoying clicks and/or chirp - everything I look for in a CW transmitter.
Simple rigs like this are a blast to operate. The appliance guys don't know what they're missing!
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