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A CB transverter for the 80-metre amateur band Last month, we described the circuit of the CB Transverter For 80M and show how to build the PC boards. In Pt.2 this month, we give the final wiring details and describe the test and alignment procedure. PART 2 – By LEON WILLIAMS, VK2DOB The prototype was housed in a plastic instrument case with aluminium front and rear panels. This case measures 250 x 170 x 75mm and is called a Jaybox (from Jaycar). Other cases could be used but make sure that the rear panel at least is made of alu­minium to provide heatsinking for the two FETs. The PC boards are mounted inside the case on a 2mm-thick aluminium plate measuring 155 x 240mm. This baseplate is secured to plastic standoffs in the base using four No.4 x 12mm 28  Silicon Chip self-tappers. When the plate has been secured, drill clearance holes for the long screws that pass through from the base to hold the top of the case in place. The PC boards are mounted on the baseplate using No.4 x 12mm self-tappers and 6mm-long brass spacers. The location of each board is shown in Fig.8 and can be seen from the photo­ graphs. Before mounting the boards however, it is necessary to mark out the rear panel. To do this, first sit the mixer board on 6mm spacers, push the shield against the rear panel and mark out the holes for the SO239 input connector with a pencil or scriber. Do the same thing for the PA board to find the position for the FET mounting holes. The positions of the SO239 antenna connector and the power supply binding posts should also be marked at this point. This done, remove the rear panel from the case and drill all the holes. Ensure that the holes for the FETs are smooth and free from any burrs that could puncture the insulating washers. The front panel has only two holes, one for the Rx/Tx switch and one for the variable capacitor shaft. The position for this can be found by sitting the PLL board on 6mm spacers and making a mark on the rear of the front panel around the shaft with a pencil. This done, the baseplate can be drilled to take the self tappers that secure the PC boards. Fig.8. this wiring diagram shows the location of each PC board in the case. These boards are all mounted on an aluminium plate using No.4 x 12mm self-tappers and 6mm-long brass spacers. Note that leads that carry RF signals are run using minia­ture 50-ohm coax, while the rest of the wiring consists of medi­um-duty hook-up wire. The in-line fuseholder is wired between the positive binding post and the +13.8V pin on the power amplifier board. December 1995  29 This close-up view shows the mounting details of the two IRF510 power FETs (on the PA board). Note that these must be electrically isolated from the rear panel using TO-220 mounting kits, as shown in Fig.10. When all the holes are drilled, fit the front and rear panels in the base of the case, then mount the mixer board and secure the SO239 input socket using four 3mm x 6mm-long screws and nuts. The front of the board is secured to the base using two self-tappers and 6mm spacers. The PA board is secured to the baseplate using four self-tappers and 6mm spacers. Once this board is in position, secure the two FETs to the rear panel as shown in Fig.10. Smear all mating surfaces with heatsink compound before bolting the assem­ blies together and use a multimeter to confirm that the metal tab of each device has been correctly isolated from the rear panel. The PLL board can now be secured to the baseplate. The variable capacitor shaft extends through the matching front panel hole and is fitted with a large plastic knob. This done, mount the front panel switch, the antenna socket and the power supply binding posts. All that remains now is to complete the wiring as shown in Fig.8. Note that leads that carry RF signals are run using minia­ture 50-ohm coax, while the rest of the wiring consists of medi­umduty hook-up wire. The in-line fuse holder is wired between the positive binding post and the +13.8V pin on the power amplifier board. Testing Fig.9: here are the full-size etching patterns (top & bottom) for the power amplifier PC board. 30  Silicon Chip The transverter needs a 13.8V DC supply capable of supply­ing at least 2A. The completed unit is tested as follows: (1). Connect the CB radio to the input socket using a coax patch lead and connect a dummy load capable of dissipating 12W to the antenna socket. The rear panel of the transverter carries the antenna socket, two power supply binding posts and the input socket. The latter is connected to the antenna socket on the CB transceiver via a coax patch lead. Power for the unit can be derived from any suitable 13.8V DC source capable of supplying 2A. (2). Place the Rx/Tx switch in the Rx position and turn trimpots VR1 and VR2 fully anti-clockwise. Apply power and check that 13.8V is present on all three boards. If the fuse blows, there is obviously a fault that needs to be fixed. If the relays operate, check the wiring to the Rx/Tx switch. If the switch wiring appears to be OK, check the RF detector circuit for errors. (3). If everything is correct, check the output voltages of the regulators on the PLL board. These should be close to +5V from REG1 and +8.5V from REG2. (4). Connect the lead from a fre- quency counter to pin 10 of IC2 and check that the 10MHz oscillator is working. Now move VC1 from minimum to maximum and check that there is a change in frequency. The frequency at pin 14 of IC3 should be about 185kHz, depending on the position on VC1. (5). Connect the frequency counter to the output pins of the PLL board and note the frequency. Centre VC1 and, using a suitable tool, adjust the slug in L5 until the counter reads 23.705MHz (this should remain steady, even when L5 is moved a little either way). If the correct frequency Fig.10: the two FETs are secured to the rear panel as shown here. Smear all mating surfaces with heatsink compound before bolting the assem­blies together. cannot be obtained, decrease the turns on L5 to raise the frequency or increase the turns to lower the frequency. If the PLL will still not lock, check that the frequency at pin 3 of IC3 is about 185kHz. (6). Connect a voltmeter across the 100µF capacitor in the low pass filter (located on the board near VC1). Adjust L5 until a reading of 2.5V is obtained. When the correct position for the slug has been found, it should be locked in the former using a small piece of elastic. This should be placed between the slug and the former as it is screwed in. (7). Check that VC1 can vary the output frequency of the PLL board by at least ±5kHz. If the range is too small, change the connection from the 60pF pin to the 160pF pin on VC1. (8) Remove the dummy load and connect a 3.6MHz signal source to the antenna socket. Power up the CB radio and set it to LSB, with the RF gain control at maximum. Select channel 30 and adjust the fine tune control on the transverter until the signal can be heard from the CB. (9). Adjust the slugs in T1, T2 and T3 for maximum signal, as indicated on the CB radio’s S-meter. If you have the facilities, you can adjust the bandpass filter for a flat response across the band; if not, peaking them at the centre of the band will be adequate. If you cannot hear a signal, check that there December 1995  31 32  Silicon Chip Fig.11: this is the full-size etching pattern for the mixer PC board. Make sure that all groundplanes are correctly aligned before etching the boards. Fig.12: the full-size etching pattern for the PLL PC board. Check all PC boards carefully before installing the parts. Operating You must hold an amateur radio licence to use this trans­ verter. Basically, it’s simply a matter of applying power (13.8V DC), connecting the CB to the input connector via a coax patch lead, and connecting a 3.5MHz antenna to the antenna sock­et. The only time you need to touch the transverter is to adjust the fine tune control. This control can be calibrated if required. The front panel can be marked at the knob pointer posi­tions when the VCO frequency is 23.700MHz, 23.705MHz and 23.710MHz. The 23.705MHz position represents the 10kHz spot (eg, 3.610MHz), while 23.710MHz represents the -5kHz spot (eg, 3.605MHz) and 23.700MHz represents the +5kHz spot (eg, 3.615MHz). Due to the characteristics of the crystal oscillator, it will be found that the fine tune scale is not linear. This means that there is more control on one side of the 10kHz spot than on the other. This is a small price to pay for the advantages that it provides. With these calibration marks, it is a simple matter to find any frequency at a resolution of 5kHz in the band. Note that because both the CB radio and the transverter use a PLL which is crystal locked, the frequency stability of the system is very good. Finally, if you find that the sound of the relays operating during long overs between sentences is annoying, place the Rx/ Tx switch in the Tx position while you speak to override the automat­ic switching system. Of course, you must remember to switch back to the Rx position when you finish speaking, so SC that you can receive. Fig.13: this full-size artwork can be used as a drilling template for the two holes on the front panel. is +6.2V at pin 8 of IC1 and that the VCO signal is present at pin 6. If these check OK, look for problems with the transformers and the relays. (10). Swap the signal source for a power meter or a dummy load with an oscilloscope connected across it. Install a multimeter set to measure at least 2A in the positive supply lead. Operate the Rx/Tx switch so that the relays operate continuously and note the current drawn. Adjust trimpot VR2 slowly clockwise until the current reading is 400mA higher than the previous value – this should be about 750mA. Return the switch to the Rx position and remove the multimeter from the power supply lead. (11). Push the CB PTT button and speak into the microphone. The relays should operate and release about a second after the speech stops. (12). Switch the CB to AM and operate the PTT switch. Adjust drive control VR1 until a reading is indicated on the power meter, or on an oscilloscope if using a dummy load. (13). Adjust T5 and T6 for maximum power, as indicated on the power meter. Now move the channel selector slowly from 20 through to 40 and note the power output. If there is a peak at any point, it can be balanced out by adjusting T5 and T6 until the power output is even across the band. (14). Switch the CB back to LSB and whistle into the microphone. While monitoring the waveform on an oscilloscope, advance the drive control (VR1) until the waveform starts to compress, then back VR1 off slightly. The power meter or oscilloscope should show a power reading of at least 12 watts PEP. If you do not have these facilities, listen to yourself on another receiver or have a friend listen nearby. Advance the drive control until the signal distorts and then back it off a little. The transverter does not have an ALC (automatic loudness control) circuit, so it is important to set the drive control so that the PA is not overdriven. December 1995  33