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It's quite straightforward to add a discriminator output to a Uniden Bearcat 855XLT. After releasing the 5 screws in the bottom of the scanner, the two parts can be separated. I also disconnected the loudspeaker connector from the PCB, which makes the two scanner halves easier to handle.

The Uniden Bearcat (U)BC 855 XLT



Pin 11 of the 10421 contains the unfiltered audio

Close-up of the connection to the IC

The discriminator IC, a TK10421, is located on the IF Unit. This PCB is mounted vertically on the main board. TP3 contains a signal that comes from the discriminator. However, this signal is filtered too much to be of use. Probably Uniden mounted an RC-filter with a cut-off frequency of about 2 kHz. This causes the slopes of the digital signals to be rounded.

A limiting resistor of 5.6 k has been soldered to pin 11 of the 10421. The exact value of the resistor is not critical; I use values between 5.6 and 15 k. The resistor has been mounted on the side of the IC, and fixated with a drop of glue from a glue-gun.

The inner conductor of a piece of shielded cable was soldered to the other lead of the resistor. I connected the braid to the metal shield of the coil on the other side of the PCB. The other side of the shielded cable has been connected to a 3.5 mm chassis terminal at the back of the scanner.




The result

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