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Realistic PRO-37

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Modifying the Realistic PRO-37 with a discriminator output is quite straightforward. It is a pre-SMD scanner; the receiver IC is a DIL type. The scanner contains an MC3361. The unfiltered baseband signal can be found at pin 9 and at TP103 (test point 103) as well. TP103 is connected directly to pin 9. If you use TP103, you don't have to solder to the IC.

To remove the scanner's back, you have to remove the squelch and volume knobs.
A 10 k resistor is connected to TP103. Because the output signal contains a ligh IF component at 455 kHz, as usual with the MC3361, we construct a simple lowpass filter. The resistor's other lead is connected to a 2.2 nF capacitor. The other lead of the capacitor is connected to the scanner's ground. The metal shield of one of the coils can be used as ground. This filter effectively suppresses the 455 kHz signal, but leaves the data signal in which we are interested intact.
Location of pin 10
Connection to the IC
TP103 contains the baseband signal
A simple filter is connected to TP103
A 3.5 mm chassis socket is mounted in the back of the scanner. Because of the short distance between the socket and the discriminator IC, insulated wire can be used.

The joint of the 10 k resistor and the 2.2 nF capacitor is connected to the lug of the chasssis terminal that makes contact to the tip of an inserted 3.5 mm plug (red wire).

The other lug of the terminal is connected to ground, e.g. to the metal shield we used before (brown wire).

Overview of the discriminator output

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