Toroid Coil In Hf Radio
Photo A. Flavorig! 73 Amateur Radio • November, 1989
by Michael Jay Geier KB1UM
sistor and its associated components), and replacing it with separate oscillator and mixer stages. With FETs and simple toroid coils, it's as easy as pie!
Description of the Transceiver
Flavorig is an 80-meter, Novice band 5 watt CW transceiver, built around a Radio Shack Fiavoradio™, which sells for $6.95. It includes an RIT circuit, sidetone, and minikey, and it operates on 10-14 volts, making it ideal for portable use. As much of the original radio as possible is used. In fact, several parts are REMOVED, with a few being used later, in other circuits. You can buy nearly all the parts at Radio Shack, and the few they don't sell can be ordered from Digi-Key Corporation or Radiokit (see addresses at the end of the article). Initial tune-up requires only another HF rig, an antenna, and a dummy load.
The Flavoradio's single-transistor front end is replaced with two FETs, Q1 and Q7 (see Figure 1). Ql isanRFamp, whose input is the incoming signal, tuned by LI, mixed with the local oscillator signal provided by Q7. LI, by the way, is the original AM band antenna coil, tuned to the 80 meter band by pulling it part way out on its ferrite rod. The oscillator, used for both receive and transmit, is coupied to the receiver only via proximity, a technique which has several advantages, not the least of which is simplicity!
The IF, detector, and audio stages stay pretty much intact, with only minor changes to increase overall gain and selectivity. The BFO is provided by Q8, which generates a very stable signal by using a ceramic resonator similar to a crystal. The BFO requires no tuning or adjustments.
The transmitter is, of course, not native to the radio. The oscillator signal is shifted to the transmit frequency when the transmit/receive switch is set to transmit. Q9 buffers the signal, and it is coupled via L2 to the Q10 driver stage. L2 is the radio's original oscillator coil, now used as an interstage transformer. Q10 generates enough current to drive the gate of the Qll final transistor, which provides between 1.5 and 5 watts of output on a 5QQ load. The filter which follows it cleans up the waveform, ensuring spectral purity.
The rig is keyed by providing power to the buffer, driver, and final stages. The oscillator runs continuously. Sidetone is provided by Ul, a CMOS 555 timer, which is coupled into the audio amp. In transmit, the receiver is disabled via D4, which cuts off Ql.
Let's Get Started
If you're like me, right now you're thinking, "Oh, no, I hafta wind coils." Well, only two, and they're extremely simple. One has
15 turns of wire, and the oilier has 30 turns with a tap after the eighth turn. No transformers, bifilar, trifilar, etc. I don't like that stuff either!
After you get the radio and other parts, the first order of business is to remove some parts from the radio. Pop the case open and remove the screws holding the board. Now, refer to the schematic included in the instruction booklet, and unsolder and remove the following parts: CI, Rl, R2, Ql, R3, C2, and C3.
Next, remove the oscillator coil, which is the can with the red core. Be sure to unsolder the tabs holding the metal can, as well as the coil pins. The can may come off separately, but that's OK: you can reassemble it. as long as you haven't damaged the delicate coil assembly. Do it carefully, because you'll use it later, in the transmitter! Now, remove R6, R7, and C5.
Take one of the toroids and about four or five feet of #26 enameled wire. Wind 8 turns through the doughnut, then pull out about one inch of wire and twist another inch back on it, to make a tap. Now, wind the other 22 turns. Run your soldering iron up and down the tap with a little solder. Spread the turns so that they go about two-thirds around the toroid. Finally, put some nail polish on the windings to keep them from unraveling,
You've just wound your oscillator coil. That wasn't so bad, was it? While you're at it, you might as well wind the other coil. This one's even easier. Simply wind 15 turns of the same type wire through the other toroid,
Coil Winding Data
L1: Radio's original antenna coil. Antenna coupling link is 3 turns, any kind of wire, wound over length of L1.
L3: 30 turns of #26 enameled wire, tapped a1 8 turns, on T50-2 toroid.
L5: 15 turns of #26 enameled wire on T50-2toroid.
spread the windings two-thirds around, then apply the nail polish.
Now, it's time to start building. First, install a 47kQ resistor in place of the R6 you removed. Now, stand the oscillator coil on end, and glue it to the board approximately where the old one was, with the glue applied to ihe area of the coil which has no windings. Run the three wires through pre-existing holes in the board. If there's any slack in the wires, hold them to the board with nail polish, because any movement will cause a frequency change.
If you do some planning, you'll find that you can use most of the traces on the board for the new front end circuit, with a few cuts and jumper wires. Exactly how you lay the circuit out is up to you; it isn't critical. I used the existing holes and traces for the two FETs and the resistors, and soldered the small capacitors and the 6.1 volt zener diode to the foil side of the board, making cuts in the foil as needed.
I recommend mounting the trimcaps on the component side, just below the oscillator coil, to make tune-up easier. It's a tight fit, but it can be done if you mount the oscillator coil up as far as you can. As with any RF circuit, keep component leads as short as possible.
When you've got the oscillator done, look again at the original schematic. Make a foil cut between C9 and VR1 (the volume control), and add a lOkQ resistor across the cut. Put a 0.01 mF cap from VRl's wiper (the center pin) to ground- Put another 0.01 mF cap between the base and collector of Q3. That completes the changes to the board.
You'll need to add a small board to the radio, to hold the BFO and transmitter circuits. I haven't included a PC board design, as most of us don't bother to make one for a single-quantity project. I used simple point-to-point perfboard construction, and glued the new board to the bottom edge of the radio board. Because I like my projects to be compact, I even used the curved space formerly occupied by the speaker magnet!
Wire the BFO and transmitter circuits and attach wires to the appropriate points on the radio. When connecting the transmit/receive switch, keep the wires short, and plan to mount the switch near the IF can with the black core. L2 is the OSC1 coil (with the red core) that you removed earlier. Looking at the coil with the pins down and the tapped side facing you, the ground connection is to the middle pin, and the 0.01 mF cap goes to the pin on the left. Turning the coil around to the untapped side, the FET connects to the pin on the right.
You must heatsink the IRF51! output transistor. Grease it with heatsink compound before attaching the heat sink. If you plan long keydown periods, or extended use at 13.8 volts, consider using a larger heatsink, as the transistor gets quite warm with the one specified.
You must shield the top pan of the board, where the VFO and tuning cap sit. because hand capacitance will make it difficult to tune stations if these parts are unshielded. Copper-clad PC board makes a good shielding material. Just be sure to connect it to the rig's ground (the black wire coming from the battery terminals).
Continued on page SS
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