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AMATEUR RADIO By GARRY CRATT, VK2YBX How to convert a CB radio to the 28MHz amateur band It's easy to convert a CB radio to the 28MHz amateur band. Just build and fit this simple oscillator circuit for 40 channel operation on AM or SSB. One of the main problems confronting the amateur operator when it comes to multi-band operation is the cost of multiple transceivers to cover every band. In particular, transceivers for the 10-metre (28MHz) band are expensive. But there is another way convert a low-cost 2 7MHz CB transceiver to do the job. While this task may appear to be quite daunting, it's really fairly straightforward. All you have to do is fit the simple oscillator circuit described here and perform a few circuit tweaks. The converted transceiver can be used for novice operation on 10-metres or for driving a transverter (ie, a frequency converter) for operation on the VHF or UHF bands. A 40-channel CB transceiver capable of SSB operation is an ideal candidate for conversion to 28MHz operation. These transceivers were imported in vast quantities so their prices are much lower than equivalent amateur-band models. CLARIFIER AND AMISSB OFFSETS 5.12MHz 10.24MHz -101--1.-~ VCO = 16.27MHz (CH 1) TO 16.71MHz (CH 40) CHANNEL SELECT PLUS SSB OFFSETS +N = 91 (CH 1) TO 135 (CH 40) DOWNMIX INPUTS = 0.91MHz (CH 1) TO 1.35MHz (CH 40) 10.695MHz AM 10.6975MHz LSB 10.6925MHz USB Tc, AM/SSB OFFSETS Fig.1: the Uniden AX 144 uses phase locked loop (PLL) frequency synthesis to set the output frequency. To change the band of operation, the tripler is disabled and a signal from an external oscillator injected into the PLL mixer. 84 SILICON CHIP As well, they were made to meet stringent DOTC regulations, thus guaranteeing reasonable quality. For this project, we chose to convert a Uniden AX 144 SSB transceiver which retails for around $329. However, many other late-model SSB transceivers employ similar circuitry and could be modified just as easily. PLL tuning Fig.1 shows the block diagram of the transceiver. As can be seen, the unit uses phase locked loop (PLL) frequency synthesis to select a precise 40-channel segment in the 27MHz band (26 .965MHz to 27.405MHz). What we have to do is change this to a 40-channel segment of the 28MHz band. The first thing to note is that the frequency bandwidth of t he transceiver is fixed at 440kHz (ie, 27 .405MHz to 26.965MHz). This means that it's not possible to cover the full 28MHz band. A good compromise, for either 10-metre or transverter operation, is to convert the unit to operate from 28.285MHz to 28.725MHz. OK, so we've now selected the frequency segment in which we wish to operate. We now have to decide how to convert the transceiver to this higher frequency segment. What we have to do is increase the VCO frequency, as set by the PLL, by a fixed amount on each channel. Because of the way in which the PLL and the VCO operate, the easiest way to do this is to change the frequency that's normally fed RB 100{) r---......---....--'¥1~-0+BV REG. 3xBA244 OR SIM. D1 R1 3.3k CS 10pF -----111--oOUTPUT .,. B L1,L2,L3 : 12T 25 B&S ENAMELLED COPPER WIRE CLOSE WOUND ON FERRITE SLUG EOC VIEWED FROM BELOW Fig.2: the external oscillator circuit is a Colpitts type with diode switching to vary the output frequency. Either AM, LSB or USB operation can be selected by switching the corresponding input to + 8V. from the P11 to the P11 mixer via the tripler. As Fig.1 shows, the P11 generates a 5.12MHz signal by dividing the 10.240MHz reference oscillator frequency by two. This 5.12MHz signal is then externally tripled to 15.36MHz and mixed with the VCO frequency to give a control frequency for the P11. This control frequency is simply the difference between the tripler and VCO output frequencies. In normal CB mode, the VCO frequency ranges from 16.27MHz (Ch.1) to 16.71MHz (Ch.40) and so the control frequency ranges from 0.91MHz to 1.35MHz. Note that the VCO output is also fed to both the transmit and receive mixers. To change the band of operation we simply disable the 5.12MHz tripler and inject a suitable (higher) mixing frequency into the P11 mixer from an external oscillator. This oscillator must incorporate some method of frequency compensation to allow for USB and 1SB operation, as well as AM operation. Oscillator circuit Fig.2 shows the circuit of our oscillator. It is a Colpitts type employing diode switching to vary the amount of inductance in series with the crystal (Xl). The nominal frequency is set by crystal Xl but is modified according to whether 11 , 12 or 13 are switched in. The mode switching function is achieved by conveniently utilising the 8-volt output from the mode switch on the front panel of the transceiver. Thus, for 1SB operation, the mode switch connects the 1SB input of the oscillator to + 8V. This forward biases D1 and connects 11C1 to the crystal. The AM and USB modes are selected in exactly the same fashion. Crystal frequency So much for the basic oscillator circuit. We now have to calculate the crystal frequency (Xr) required to give us the desired frequency band. To do that, we use the following equation: Xr = 15.36MHz + (N x 0.44)MHz where N equals the number of 440kHz band segments above the original CB band. In this case, we have to move the frequency band up by 1.32MHz (ie, N = 3). Thus: Xr = 15.36 + 1.32 = 16.68MHz. It is necessary to add 2.5kHz to this figure to allow for USB operation. This is because the series inductors in the circuit only allow the oscillator frequency to be reduced. Thus, the final crystal frequency becomes 16.6825MHz. Construction The circuit is built on a small printed circuit board coded 06210881 and measuring 53 x 35mm. Fig.3 shows how to install the parts. You can install the parts in any order you like although the three coils are best left until last. Keep all component leads as short as possible and push the transistor all the way down onto the board before soldering its leads. The three coils are all wound using 12 turns of 25 B&S enamelled copper wire on an F14 ferrite slug. This gives an inductance of 5-l0µH, depending on the position of the ferrite slug. Clean and tin the enammelled copper wire leads before soldering the coils to the PCB. It is a good idea to check the oscillator for correct operation before fitting it to the transceiver. use-~,.,....,,_ .._, AM----:rrs:;-·,._"'_,,"-"'l " ..,, "-..,.r~= LSB +BV REG. Fig.3: keep the leads as short as possible when installing the parts on the PCB. Fig.4: you can use this full-size artwork to make your own printed circuit board. OCT0BER1988 85 PARTS LIST 1 PCB, code 06210881, 53 x 35mm 1 16.6825MHz crystal, 30pF parallel cut, Hy-Q code GE05E, OC49 holder 1 BC548 NPN transistor 3 BA244 or similar switching diodes 3 F14 ferrite slugs Capacitors 1 1 1 1 1 1 1 .01 µF ceramic 330pF ceramic 1 OOpF ceramic 82pF ceramic 39pF ceramic 18pF ceramic 1 OpF ceramic Resistors (0.25W, 5%) 2 X 22k0, 3 X 3.3k0, 1 1 x 1 kO, 1 x 1000 This view shows bow the oscillator board was mounted in the Uniden AX 144 transceiver. The AM, LSB and USB inputs are connected to the mode switch pins on the PCB at the front of the chassis (top right). L12 470uH R93 22k C96 ... .047r This is easily done by checking the output with an oscilloscope, frequency counter or HF receiver. Of course, you will have to strap the AM select input temporarily to the positive supply so that the circuit will oscillate continuously. If everything checks out OK, adjust L2 for an output frequency of 16.680MHz. The LSB and USB modes should then be selected and 86 SILICON CHIP Fig.5: to disable the tripler, you simply lift one end of C73 (marked with a cross). The output from the external oscillator is then fed into C73 and L14 adjusted to give PLL lock on the new band. Ll and L3 adjusted for output frequencies of 16.6775MHz and 16.6825MHz respectively. These adjustments will interact slightly but don't worry too much at this stage - we'll adjust the oscillator more accurately later on. Fitting the oscillator The location of the oscillator board is not critical, although care X 2.2k0, Miscellaneous Hookup wire, 25 B&S enamelled copper wire for coils. should be taken to ensure that it is clear of the speaker when the transceiver is reassembled. It can be secured by soldering a piece of heavy guage tinned copper wire from the earth foil to an adjacent metal can on the main transceiver PCB. The + 8V regulated DC rail is available on pin 11 of the PLL chip. The next step is to locate the 5.12MHz output from the PLL and disconnect it from the tripler circuit. Fig.5 shows the location on the AX-144 transceiver. In this case, the 5.12MHz output appears on pin 10 of the PLL (IC2) and is fed to the tripler circuit (113) via C72 and thence to the PLL mixer (TR19) via C73. To disconnect the tripled PLL signal, you simply lift one end of C73 (marked with a cross on Fig.3). The output from the external oscillator is then coupled to the PLL mixer (TR19) via C73. Installation can now be completed by wiring the "mode select" inputs on the oscillator to the back of the mode switch. This will automatically select the correct 2.5kHz oscillator offsets when USB and LSB are selected. Fortunately, the mode switch connections are easily accessible on the PCB runn- Table 1: Channel Frequencies Channel 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Old Frequency 26 .965 26.975 26.985 27.005 27.015 27 .025 27.035 27.055 27 .065 27.075 27 .085 27.105 27 .115 27 .125 27 .135 27 .155 27 .165 27.175 27.185 27 .205 27 .215 27.225 27 .255 27 .235 27.245 27 .265 27 .275 27 .285 27.295 27.305 27.315 27 .325 27.335 27.345 27.355 27.365 27.375 27 .385 27.395 27.405 ing along the front of the transceiver. They can be identified by checking for the presence of + 8V on each of the terminals, when the appropriate mode is selected. Adjustments A few adjustments are necessary to ensure correct operation. The first step is to adjust the VCO to give PLL lock on the new band of frequencies. This is done by monitoring the DC voltage at test point 2 (see Fig.5) and adjusting L14 for 4.5 volts when channel 40 is selected. New Frequency 28.285 28.295 28.305 28.325 28.335 28.345 28.355 28.37528.385 28.395 28.405 28.425 28.435 28.445 28.455 28 .475 28 .485 28.495 28.505 28.525 28.535 28.545 28.575 28.555 28.565. 28.585 28.595 28.605 28.615 28 .625 28 .635 28 .645 28.655 28 .665 28.675 28.685 28 .695 28 .705 28.715 28.725 Once PLL lock has been achieved, L2 on the oscillator board should be adjusted to set the CHl output frequency to 28.285MHz. To set the USB and LSB offsets, the transmitter should be modulated with a lkHz tone and L3 and Ll on the oscillator board adjusted to give 28.286MHz on USB and 28.284MHz on LSB. These adjustments may interact so it is wise to run through the alignment procedure several times. Finally, the receiver sections prior to the IF stages should be realigned for optimum performance, as should the driver and output stages of the transmitter. This is best done by referring to the service manual for the transmitter. Where to get the parts The 16.6825MHz crystal is available from Hy-Q International, 1 Rosella St, Frankston, 3199. Telephone (03) 783 9611. The cost is $25.23 post paid. Be sure to specify full details as set out in the parts list. BA244 switching diodes or similar can be obtained from Dick Smith Electronics Kit Department (PO Box 321, North Ryde, 2113), or from VSI Electronics, 16 Dickson Avenue, Artarmon 2064; phone (02) 439 8622. F14 ferrite slugs are available from Neosid Pty Ltd, 23 Percival St, Lilyfield, 2040; phone (02) 660 4566. Service manuals for the AX144 CB transceiver can be obtained from the importer, Santronic Corporation Pty Ltd, 27 Garema Crescent, Kingsgrove, 2208; Phone (02) 599 3355. 16 PRODUCT SAFETY RECALL DICK SMITH ELECTRONICS PTY. LTD. NOTICE TO ALL CUSTOMERS TEMPERATURE CONTROLLED SOLDERING STATION Cat T-2000 Sold in all states, may constitute a safety risk as the power switch in some units has been found to be incorrectly wired . The units which may be affected are fitted with a meter having a white, translucent background and bear the legend "AUST DES REG NO 86081" on the front nameplate below the temperature control. Customers are asked to return this product as a matter of urgency to the nearest Dick Sm ith Electronics Store for immediate attention to the problem. We apologise for any inconvenience caused. Further enquiries may be made by contacting Rex Callaghan, Technical Services Division on (02) 888 3200. OCT0BER1988 87