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A synthesised stereo AM tuner, Pt.3 This month, we complete the construction of the AM Stereo Tuner by giving stepby-step alignment details. This job is simplified by taking advantage of the microprocessor control. By JOHN CLARKE The AM Stereo Tuner can produce very good sound quality but to achieve this, the tuner must be aligned correctly. You do not need any special instruments or tools apart from a plastic alignment tool and a multimeter. Alignment involves measuring voltages and adjusting the various coils and trimmers using off air stations as the signal source. The alignment tool can be purchased from your local electronics supplier. Don't use a screwdriver to adjust the coils. Its metal blade will upset the adjustment and you run the risk of cracking the ferrite slugs. Powering up Before switching on, make a final 56 SILICON CHIP check on the wiring and check that the fuse has been installed. This done, set your multimeter to read voltages up to 12VDC, connect its negative lead to the ground pin next to TP2, and switch on the power. The display should light and show 522kHz. If not, switch off and correct the problem before applying power again. The problem could be lack of power to IC1, so use your multimeter to check for continuity between its supply pins and its relevant voltage regulators (ie, between pin 12 and the output of REG2, and between pins 5 & 17 and the cathode of Dl 4). Now re-apply power and measure the voltages at the output of the regulators. Check that REG1 has an out- put of 12V and that REG2 and REG4 are at 5V. REG3 should have about 5.6V at its output. You should also check that the supply rails to the ICs are correct. IC2 should have +5V at pins 6, 12 & 18; IC3 should have +12V on pin 4; IC4, +5V on pin 3; IC5, +5V on pin 14; IC6, +12V on pin 4; IC7, +12V on pin 8; IC8 has no supply rail; IC9, +5V on pin 1. If all the supply voltages are correct, the tuner can be left powered up unless a fault is detected during the following tests or during alignment. The next step is to check the operation of the front panel display and switches. First, press the UP tuning button and check that the frequency readout increases from 522kHz to 1629kHz in 9kHz steps. Each memory LED should light when its respective switch is pressed. Initially, all the memories will be set at 522kHz. To program each memory, select the required frequency using the TUNING buttons, then press the :ME switch and the required memory switch. The :ME LED should extinguish as soon as a memory switch is pressed PHONE LINE AND MAINS FILTER/SURGE PROTECTOR Protection for nswering machines ord/ess phones acsimlles or if the ME switch is pressed again. If neither switch is pressed, the ME LED should automatically extinguish after five seconds. The SEEK control should at this stage send the tuner scanning up the frequency band. However, because the tuner is not yet aligned, do not expect the tuner to lock onto a station. Press any button to stop the tuner from seeking. Check that the programmed memories remain intact when the tuner is switched off at the power switch. If not , check that REG3 is supplying a 5V standby voltage to pins 5 & 17 of IC1 when the tuner is switched off at the power switch but not at the mains supply. (Note: the memories should also remain intact for several hours if the mains supply is interrupted, as indicated in Pt.1). Alignment To align the tuner, we need to make a series of adjustments. These are to ensure that the local oscillator and ferrite rod antenna tuned circuits track each other, and that the range of the varicap tuning voltage is within certain limits. The IF tuned circuits also need to be peaked for best signal output and widest audio bandwidth. Once this is done, the 3.6MHz oscillator is set for stereo reception and the 9kHz twin-T filters adjusted. Some of the alignment process involves switching from a low-frequency station to a high-frequency station and then back again, with the process repeated several times to ensure accurate tracking. This can be done quickly, since the stations can be programmed into memory for quick access. Local oscillator range The first step is to adjust the local oscillator so that it covers the range from 972kHz to 2079kHz. The microprocessor (IC1) does all the frequency measurements for us and controls the local oscillator via a varicap tuning diode. This ensures that the local oscillator operates at 450kHz above the frequency indicated by the display. In practice, all we have to do is ensure that IC1 remains in lock. This lock requirement can be met by ensuring that the varicap tuning voltage at test point TP2 operates between defined limits. The procedure is as follows: • Connect your multimeter between GND and TP2, select 522kHz on the display and adjust 12 for a reading of 0.9V. • Press the Down TUNING button once so that the display jumps to 1629kHz. Adjust trimmer capacitor Cl for a voltage reading of 8V. • Return to 522kHz (press the Up button) and readjust 12 for 0.9V. This done, select 1629kHz and readjust Cl for 8V. Repeat this process until the two voltages are within 0.1 V of the required values. The local oscillator range has now been set. The next step is to adjust the ferrite rod tuned circuit so that the RF stage tracks with the local oscillator. RF tracking The RF section is aligned by measuring the AGC voltage at TP1. During ensures that lightning, power surges and spikes do not reach your ccstly communication equipment as it ccnstantly monitors the PHONE line and the MAINS power line. Simply ccnnect to any existing power point, plug in your fax, phone, video, Hi Fi or any other 240 VAC operated equipment and a series of indicators shows the status of the mains power point while several MOV's and gas arrestors prevents access to damaging energy sources. The CPE P- 1 is fully Telecom and Dept. of Minerals and Energy tested and approved. Imported and distributed by: .ARISTI\ ELECTRONICS Available through the following retailers: Telegrafax PIL. 305-307 Sailors Bay Road. Northbridge. 2063. NSW. (02) 9585137 Geoff Wood Elec. 229Burns Bay Road. Lane Cove. 2066. NSW. (02) 427 1676. ANTRIM TOROIDAL TRANSFORMERS * 15VA t.o 625VA from st.ock with single 240V primary and dual secondaries, 2 x 6V to 2 x 55V. Standard models comply with IEC65(BS415). * 15VA to 2KVA to order with dual 120V primaries and your choice of secondaries. Compliance with AS3108(IEC742). UL506 and other standards available. * High power audio toroids available to order for 100V lines. * Comprehensive data sheet available. * Competitive pricing available for OEM and resellers. HARBUCH ELECTRONICS P/L 90 GEORGE ST. HORNSBY, NSW PHONE (02) 476-5854 FAX (02) 476-3231 APRIL 1991 57 The ferrite rod assembly is adjusted by sliding coil L1 along the rod, as described in the RF tracking adjustment procedure. After the adjustments have been made, the coil can be held in place by melting wax over both ends. the ferrite rod slowly, noting the voltage reading for each movement. Take your hand away before making each reading and have the ferrite rod angled away from the tuner case by at least 15°. Adjust the coil for a minimum voltage reading. • Select memory 2 and adjust trimmer Cl for a minimum reading on your multimeter. • Select memory 1 again and re-adjust Ll for a minimum voltage reading. • Repeat the procedure for the memory 2 station, then check the memory 1 adjustment again. The two settings should now be correct. • Connect your multimeter between TP2 and GND and select the 20V range. Check that the varicap tuning voltage is at least 0.9V when the display is set to read 522kHz. If the voltage is below this, readjust the LZ slug for a reading of 0.9V. • Set the display to 1629kHz and check the voltage reading. If the voltage reading is above 8.5V, the local oscillator range will need to be reset as described previously. This also means that the RF tracking will have to be readjusted as described above. Fortunately, all these adjustments can be done quite quickly. IF alignment Heatshrink tubing should be shrunk over the ferrite rod assembly at the conclusion of the alignment procedure. This will provide a degree of protection against bumps & knocks & will further ensure that coil L1 is locked firmly in position. this process, you need to slide Ll along the ferrite rod to find the point of maximum signal pickup when tuned to a low-frequency off-air station. Cl is then adjusted for maximum signal pickup when tuned to a high-frequency off-air station. Using Cl to peak Ll means that the local oscillator varicap voltage range is altered again, and so this will require further readjustment. But that's 58 SILICON CHIP life. Here's the step-by-step procedure: • Select a local radio station which is near 603kHz and tune to this station. Now program it into memory 1. Select another local radio station around 1305kHz and program this into memory 2. • Connect your multimeter between TPl and GND and set it to the 2V range. • Select memory 1 and slide Ll along The mixer coil (L3) and the IF coil (L4) must now be adjusted so that the tuned circuits peak at 450kHz. When peaked, they will provide the best audio bandwidth for the tuner. • To align the IF stages, connect the multimeter between TPZ and GND and select the ZV range. Now select a radio station (memory 1 will do) and adjust the slugs in L3 and L4 for a minimum voltage reading. • Now go back and realign the RF section (ie, repeat the procedure set out under "RF tracking"). This is done because with the IF coils peaked, you will be able to more accurately peak the RF coil (Ll). After the RF stage has been realigned, the 450kHz IF coils (L3 & L4) should be readjusted. • When these adjustments have been completed, secure the antenna coil (Ll) by melting wax over both ends. This done, fit heatshrink tubing over the entire assembly (including the PC board), as shown in the photograph. Signal level display At this stage, you will be able to Be sure to use a plastic alignment tool when adjusting the various coils on the PC board, otherwise you could damage the ferrite slugs. Note that all the adjustments are repeated several times, as detailed in the text, to obtain the optimum result. check the signal level display for correct operation. Tune to various stations using the Up and Down TUNING switches and check that the signal level display operates correctly. On strong stations, all five LEDs should be lit. You can adjust the angle of the ferrite rod for best signal pickup. 3.6MHz oscillator The 3.6MHz oscillator coil (L5) must be adjusted so that this oscillator locks onto the 450kHz IF. Thi'.s will enable the tuner to provide a stereo signal. The procedure is as follows: • Connect your multimeter across the 2.2kQ resistor on pin 3 of IC2 and select the 5V range. Tune to a local station and adjust L5 until the voltage increases to about 2.5V. If the station is transmitting in stereo, the voltage should then suddenly increase to 4.5V and the stereo LED should light. • Rotate L5's slug slowly clockwise and note the position where the voltage drops to 0V from the locked 2.5V setting. This done, rotate the slug anticlockwise and again note the position where the voltage drops to 0V. Set the slug midway between these two positions. Seek control The seek control adjustment is made with trimmer capacitor C2. This sets the seek sensitivity so that it only locks onto stations with good signal strength. • Press the SEEK button and check whether the tuner stops at a station. If it doesn't, adjust C2 for greater capacitance (ie, plates further in mesh) and try again. Conversely, if the tuner stops at every 9kHz step, adjust C2 for lower capacitance. C2 is correctly set when the tuner only stops at stations with at least one LED lit on the signal strength meter. 9kHz filters The twin-T filters are designed to provide a notch at 9kHz in the audio path, to filter out adjacent channel interference. There are two adjustments to be made for both the left and right hand channels. This can be done using off-air stations at night when 9kHz whistles are prevalent. • Switch the power off and set your multimeter to the kilohm range. Connect the meter probes to the top of the 18kQ resistor (ie, the junction of the two 270pF capacitors) in the left channel and to pin 8 of IC3. Adjust VR3 for a reading of 32. 76kQ. • Similarly connect the meter probes to the top of the right channel 18kQ resistor and pin 14 of IC3 and adjust VRl for a reading of 32. 76kQ. • Rotate VR2 and VR4 fully cl ockwise. This should give a notch which is close to 9kHz. Note: if these trimpots are rotated fully anticlockwise, the twin-T filter will be disabled. • Connect the tuner to an amplifier and tune to a station at night which has a substantial whistle. Use the balance control of your amplifier to select the left channel only and adjust VR4 until the whistle is eliminated. Similarly, select the right channel only and adjust VR2 so that the whistle is minimised. Your new tuner is now ready for stereo hifi listening. In some cases, the ferrite rod may have to be adjusted to give the best reception although this will not be necessary on strong local stations. This simply involves angling the rod away from the rear of the case to increase signal pickup. SC APRIL 1991 59