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Uniden Bearcat (U)BC2500XLT


It requires quite some work to modify a Uniden Bearcat 2500XLT with a discriminator output. First, the discriminator IC, a TK-10930, is a VSOP (Very Small Outline Package) IC. Its pins are spaced 0,5 mm (0.02"). A 'normal' SMD in SOIC (Small Outline IC), like the MC3361, uses a spacing of 1,27 mm (0.05").

Next, there is hardly any space for an additional chassis terminal. The scanner is filled very efficiently with three layers of cicuit boards. The case fits almost seamlessly to the components mounted on the PCBs. However, with some effort the existing earphone/speaker socket can be reused.

Desolder these contacts
The socket has been removed
The external speaker socket's solder contacts
The socket has been removed
First take care of the external speaker socket:
  • Disconnect the cable from the antenna connector. This makes it easier to reach the socket contacts
  • Use a desoldering pump and/or solder wick to desolder the three pads
  • Remove the socket from the circuit board
  • If required, clean the solder pads with a circuit board cleaner
  • Connect two contacts with a wire bridge. If you don't, there will be no sound from the internal loudspeaker
Connect these contacts
Connect two wires to the socket
These contacts are connected
Solder two wires to the chassis socket
  • Solder two pieces of insulated wire to the chassis socket. Cut the third contact
  • Replace the socket upside down in the scanner. You can fix the socket to the metal frame with with instant glue
  • Scratch some lacquer from the circuit board's ground plane and solder the wire coming from the socket's ground contact to it
  • Lead the other wire to the other side of the circuit board
  • Insulate the contacts of the socket with a piece of tape. The inside of the scanner's cover has a metal paint shield, which may cause a short circuit in the socket
Replace the socket
Insulate the contacts with tape
The socket, replaced upside down
Isolate the contacts with a piece of tape

Next, the connection to the receiver IC can be made. For narrow-band FM (NFM), this is a TK10930. The other receiver IC, a TK10498, is for FM broadcast (WFM).

The unfiltered baseband audio can be found on pin 12.

Location of the NFM receiver IC
Close-up of the TK10930
The receiver IC on the middle circuit board
Pin 12 and the PCB via
Discriminator output

Right to pin 12 there is a via. It's connected to pin 12. Solder one leg of a 10 k, 1/8 W resistor to this via. The leg snugly fits in the via. Solder the resistor's other leg to the wire coming from the chassis socket.

Check if the cable from the antenna connector has been reconnected. After that, the circuit boards can be snapped together and the scanner can be closed. Happy monitoring!

The connection to the receiver IC

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