Location Of Circuit On Board



Fig. 16. Phase comparator. Notes: 1. See Fig. 4 for diagram. 2. Capacitor marked * is a bypass capacitor not shown on diagram.

as shown in Fig. 19. +12V connects near IC13, as shown in Fig. 18, and goes to the VCO through the 68SI resistor (Figs. 17 and 18). A good source of power for the regulator is a 12V car or motorcycle battery.

Before applying power, check the resistance between the +5V and +12V lines to ground. If there is a short circuit, look for a wrong connection or tiny sliver of solder on the board. Don't forget to remove the Molex connecting- strip before applying power, since it shorts adjacent terminals.

From now on, a frequency counter is probably the best test instrument you can have. We used the one described in 73 Magazine in the May-Sept. 197 2 issues. If you can at all borrow one, it is well worth the effort since, even if you don't need it in troubleshooting and everything works well, at least you will be able to check the accuracy of the synthesizer's output at different switch settings.

Connect the counter to the collector of Q6 in the VCO. Disconnect the VCO control line from the phase comparator, and connect it to a source of variable dc voltage, adjustable over the range of about 1.5 to about 5.0V. Check that the output frequency of the VCO at the various input voltages shown in the VCO description last month is in the right range.

If necessary, adjust LI so that the VCO gives the right output. The important thing is to check that the VCO frequency swings over the whole range needed with an input voltage change from about +2.2 to about +4. IV, both on receive and transmit. If you don't get enough swing with this amount of voltage input, try another pair of 1N4001 diodes in the control line. It is possible to get a diode which does not have a large enough capacitance range, although only one diode out of all tke ones we tried gave us trouble in that respect. If this does not help, then you will have to experiment with the values of CI, C2, and LI until you get the right VCO frequency swing. This part of the synthesizer must be aligned first., or you may never get the rest of it to work. Only if the VCO tracks right can the rest of it lock onto frequency.

Now disconnect the variable voltage source from the VCO control line, and measure the output frequency. It should be somewhere near the lower edge of the required frequency swing. Check that the same frequency is also on pin 4 of IC4 (though there may be a slight difference because of loading changes as you move the counter). Then check ICS pin 4, IC6 pin 4, IC7 pin 4, and finally IC12 pin 1. Each of these should have a frequency one-tenth of the frequency on the preceding test point. If you get the wrong reading, try interchanging IC's with another of the same number.

Since at this point you don't have the frequency selection switches connected yet, the programmable divider is dividing by exactly 29,000. Connect your counter to IC14 pin 6 to check the output frequency from the divider — it should be the VCO frequency, divided by 29,000, which makes it probably fairly close to the reference frequency.

Next, check the output of the 10 MHz crystal oscillator and the reference frequency divider — Fig. 3 (last month) tells the frequencies to be expected at various places in the reference divider.

Still without the VCO control line connected, use a scope to look at the inputs to the phase comparator (see Fig. 4). The square wave reference frequency is easily observed, but you will need a very good scope (with triggered sweep and good high frequency response) to see the output from the programmable divider (that's why a frequency counter is so useful — you can tell the signal is there and measure its frequency even though it is too narrow to see). Then check the phase detector output. Since the loop is not closed the phase detector output should be varying al] over the place. Seen on the scope, it should look like a staricase which continuously shifts directions and slopes.

Now comes the b.ig step, tf everything is working so far, connect the phase detector output to the VCO control input with a wire jumper on the underside of the board. Make it as short as possible, and keep it tight against the board. The synthesizer should now lock on to frequency. Since the frequency selection switches are still missing, you are set to 145 MHz transmit. The frequency counter should now measure the correct frequency (145 MHz divided by 24, 18, or 12, depending on whether your output is at 6, 8, or 12 MHz, respectively). The counter reading should be constant, with possibly a flicker in the right-most digit. Listen to the output of the synthesizer on a general coverage receiver to make sure it is reasonably clean.

If the synthesizer locks on to the correct frequency but has a lot of hum or buzzing on the signal, chances are that something is wrong with the phase detector. Put a scope on the output - it should be a pure dc level — a straight line right across the scope screen. If it has a slight sawtooth to it, it's possible that one of the two FETs is shot.

Once you have gotten this far, wire up the frequency switches and package the raj INIOOI


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