Warning: Do not attempt to perform any
of the following procedures if you are not comfortable or competent with
soldering equipment or electronic equipment. I shall not be held
responsible for any mishaps whatsoever, including damage to your receiver
or other equipment.
Fine tuning the FRG-9600
Even though the FRG-9600's frequency range
can be expanded to 950MHz and beyond, the capabilities of the receiver
to achieve this are rather slim. This procedure will assist in tuning
the receiver to increase sensitivity in tuning these frequencies above
The local oscillator will need to be "retouched"
in order to improve the tuning capabilities above 900MHz, for which the
receiver was never factory set. The tuning circuits are located inside
the metal box (manufactured by Sharp) next to the ANT plug. Inside
there are two oscillators, one for VHF and one for UHF. This adjustment
involves a link on the UHF circuit to enable the receiver to operate correctly
on higher frequencies. First, open the top cover of the receiver
and locate the box (hard to miss). Carefully remove the cover.
You should see circuitry similar to the following:
Next, power up the receiver and tune to 460MHz. On the PLL unit, below the IC MC145158, you can view the pin marked TPO2. With a digital multimeter or voltmeter measure the voltage on this test point: It should be 1,1 - 1,5v. The PLL unit is, among the two high printed circuits, the one closer to the metal box. Again, check for the "PLL Unit" etched on the main board. In the above figure, next to the red arrow, there's a horizontal copper strip, with another vertical copper strip soldered at its end. These two strips make an angle of ninety degrees. With the soldering iron, melt the solder which connects these two strips and bend the vertical strip a bit towards the beginning of the horizontal, in order to shorten the loop a bit. A fraction of a millimeter (about 0,5mm) should be enough. Measure again the voltage on TPO2 (with the receiver tuned to 460MHz): It should read around 0,6v. If the voltage is not within the desired range, small adjustments can be made to the copper link. After powering-up the unit, tune the receiver to 950MHz: The voltage output at TPO2 should be 30,5v.
Now's where the fun begins. Connect the digital tested on pin 12 of the IC MC3357, the FM discriminator. The voltage should vary from 0,6v w/no signal received, to about 1,2v w/maximum signal and squelch unblocked. Next, tune the receiver to a frequency around 460MHz, keeping in mind that a signal must be present. A repeater output will suffice. Next to the green arrow in the above figure there are some more pairs of copper strips which are utilized as resonating links. With a non-inductive screwdriver, adjust these strips a little closer or farther apart from each other until you read the maximum value on the tester, while always receiving the same signal. You may repeat these steps for higher frequencies but keep in mind that the corrections on the copper strips is very fine.
Now the reception on 900+ MHz should be vastly improved.
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