The Johnson 8640(44) is a 10(04) ch. 15w 900mhz radio with full
capabilities. The 864x can a only talk to a repeater , meaning it has no
Talk around (simplex) circuit in the VCO.. The Only difference from the
8640 (10ch) and the 8644 (4Ch) is the firmware.. Since the Ham Only
firmware was based of the 8640, the 8644 will now become a 8640..
The new firmware is only capable with 25mhz split Ham systems.
1 - Replacement Micro Processor CPU w/Ham Only Firmware.
1 - 33pf SMD 1206 cap.
----- Ensure you have a working radio before you start ! ------
_ Loosen the 5 screws on the bottom of the radio and remove the
top black cover.
_ Remove the 6 screws holding the top audio/logic board down and
flip the audio/logic board over and secure the board to the with
2 screws on the left side of the radio. It should look like an open Book.
_ Remove the Micro processor chip of the Audio/Logic board and replace
it with the replacement 'Ham Only' Micro processor. Pin 1 is oriented
towards the front of the radio.
_ Remove 4 screws covering the VCO and replace C720 (9.1pf) with 33pf.
and replace cover when complete. See Figure of VCO.
_ You can now program up the with radio with FCC ch. 1 now =
through FCC ch. 79 now = 927.9875 mhz. See Freq. chart.
_ Check to make sure the VCO voltage is within range at TP 801 (pad
to VCO compartment) Voltages Should be (7-9V) on Xmit and (14-17) On Rec.
_ The Conversion is now complete, you may want to check the radio with
a service monitor to insure the radio is on Freq. and has the proper
deviation for the system you plan on using it with..