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Phone patch for radio amateurs, Pt.2 Last month, we described the circuit of our new Phone Patch unit and gave the PCB assembly details. This month, we show you how to check the unit out and give a brief troubleshooting procedure. By JOHN CLARKE & GREG SWAIN When you have completed the PCB assembly, it should be carefully checked for soldering defects and component orientation. The MC34018 should be left out of circuit at this stage. In particular, check that all wire links are in place and that the regulator, transistors, diodes and IC2 are all correctly installed. Now connect a 12V AC plugpack to the board, switch on and check that the voltage on the output of the 7808 3-terminal regulator is close to + BV. The same voltage should appear on the collectors of Q1 and Q4, and on pin 4 of IC2. 62 SILICON CHIP If this checks out, the MC34018 can be carefully inserted and soldered into place. Use a 28-pin socket for this IC if you wish (we did not bother). The PCB can now be installed in the case and the rest of the wiring completed as shown in Fig.3. You will have to mark out and drill holes in the front and rear panels to accept the necessary hardware and the cord grip grommets. If you buy a kit with an adhesive front panel label, then this can be attached to the panel and used as a drilling template. You will also have to drill a number of holes in the front panel to allow sound to escape from the loudspeaker. The output to the transceiver's microphone socket can be run using 4-way flat cable (one lead not used), while the leads to the LIU and to the transceiver's speaker output can be run using figure-8 cable. Use banana plugs to terminate the LIU leads and a 3.5mm mono line plug to terminate the leads to the speaker output. The leads to the trans~ ceiver's microphone socket are terminated in a 4-pin microphone line plug. Note that it will be necessary to instail a loop in the LIU and power leads where they emerge through the rear panel so that the cord grip grommets can get a good grip. The loudspeaker is an 80 miniature type and is affixed to the front panel using polystyrene cement (eg, Airfix). Position it so that it is directly behind the pattern of holes drilled out earlier. When all the wiring is complete, reconnect the AC power supply, switch on and check the voltages again. The voltage on pin 16 of ICl should be close to + 8V while the voltages on pins 20 and 21 should be + 5.4V and + 2.9V respectively. Testing To test the unit, you will need a double adaptor phone socket and the line isolation unit. Plug the leads from the Phone Patch into the LIU, then plug the LIU into the phone socket in parallel with the telephone. Now switch the Phone Patch and the LIU on. You should immediately hear the dial tone via the monitor loudspeaker and the VOX relay contacts should close (if not, try adjusting VR3). Check that the volume can be varied using the Monitor control. After a short period, the dial tone will be replaced by the familiar busy signal. When this happens, the VOX relay should turn on and off with the beeps (ie, on at the start of each beep, off between beeps). Adjust VR3 (VOX sensitivity) and VR4 (VOX delay) as necsssary so that the relay turns on and:off reliably. If all this checks out OK, then the receive mode is functioning properly. The transmit mode can now be tested by feeding an audio signal into the input. You can use a transistor radio or your transceiver for this job. If you now hear the audio via the monitor speaker, it means that the transmit mode is also working. The VOX should be off during this time. The wiring to the front panel switches and microphone socket can be secured inside the case using plastic P-clips. Check that all parts (especially the ICs and transistors) are correctly oriented before applying power. Now plug in your microphone and plug the Phone Patch output into the transceiver microphone socket. Set the Mic/Patch switch to the Mic position and check that you can now operate your transceiver as normal using the microphone and PTT switch. Note that if the Phone Patch is plugged into the loudspeaker socket, any incoming signals will be heard via the monitor loudspeaker. Adjust the level into the Phone The Phone Patch is connected to the telephone lines VIa a line isolation unit which is plugged into a double adaptor in parallel with your existing phone. Patch as necessary to give sufficient volume without audible overload. The final test is to use the unit in a real phone patch circuit. Brief both parties as to correct procedure before switching to Patch and switching on the LIU. Monitor the conversation and adjust VR3 so that the VOX triggers reliably when the party on the telephone speaks. VR4 should be adjusted so that the VOX remains on during the The microphone is now plugged into the Phone Patch circuit but can be switched through to the transceiver using the Mic/Patch switch. FEBRUARY1990 63 Fig.4: here is an actual size reproduction of the PC artwork brief pauses that occur between words in normal speech. Do not make the VOX delay too long though, otherwise there will be a considerable pause when it is the turn of the party at the remote transmitter. Note that when the LIU is switched on, you will be unable to use your telephone. This is quite normal and is due to loading effects. Normal phone operation is restored simply by picking up the handpiece and turning the LIU off. Troubleshooting If it doesn't work, the first job is to determine whether the problem lies in the receive or transmit sections of the circuit. If the problem lies in the receive mode (ie, you cannot hear dial tone on the monitor 64 SILICON CHIP loudspeaker when the LIU is switched on), then the problem could lie in the LIU, around Q4, or in the receive idle circuit (Q6 & Q7). If you can hear receive signals but the relay doesn't operate, suspect a fault in the VOX circuit. If the fault is in the transmit mode, suspect the circuit around Ql, Q2 and Q3. On the prototype, Ql's base voltage measured 5.9V, Q2's 3V, Q3's 2.4V and Q4's 3.9V. Their base-emitter voltages should all measure about 0.6V. The same goes for the baseemitter voltages of Q6 and Q7 when the circuit is in receive idle mode (just disconnect the LIU and the transmitter). Check the voltage on pin 24 of IC1. It should be close to 1.45V when the circuit is in receive idle mode but should be variable up A 4-pin microphone line plug is used to terminate the leads to the transceiver's microphone socket. to about 2.9V by means of VR1 when dial tone is present. 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I- <C a. 0 :::c a. • 0 a: (.) :E :c (.) .... <t When all the adjustments have been completed, a metal shield should be fitted over the VOX circuit to prevent false triggering when the transmitter is activated. The shield is secured by soldering is to 5 PC stakes. w z 0 0. ~ ,_ BEND UP I 50 82 transistor is OK. Note that DB and D9 face in opposite directions on the PCB. If you install them the wrong way around, the VOX won't work. Remember that most faults in projects are due either to faulty soldering, wiring errors, or the use of incorrect parts. Check all these possibilities carefully if problems are encountered. Note that excessive noise from the receiver under no signal conditions can lock the system into the 66 SILICON CHIP <C Cl 2 :E • • • a: !:: 0 z 0 :E ,- ; 16 z 0. :c BEND UP • • • 16 ~ .... M Fig.5: this diagram shows the dimensions of the metal shield. It can be bent up from an 82 x 73mm piece of scrap tinplate. transmit mode if it is louder than the telephone signal. This will not normally be a problem on the VHF/UHF bands, where the transceiver can be effectively squelched with no signal. However, it could present problems on HF circuits where the signal strength can vary widely and make squelching difficult. Finally, a metal shield should be installed over the VOX circuit (IC2), as shown in the photographs. Its job is to prevent the VOX from being falsely triggered by the RF output of • • • • ••• • • • • • • • ••• • • • • Fig.6: this artwork can be used as a drilling template for the front panel. the transmitter. Without the shield, the VOX could have a tendency to lock up each time the transmitter is activated. Fig.5 shows the dimensions of the metal shield. It is bent up from scrap tinplate and secured to the PCB by soldering it to five PC stakes. ~