This page will describe how to add more bands to the Cobra 148 GTL DX and other radios using the PB010 PCB
This set uses a pair of binary adders to alter the programming on the MC145106 PLL,this programming can be changed to expand it to cover so many channels that the set would need to undergo major modifications to be able go to the new channels !
To add any bands you must disconnect the 8 Volt supply to the band switch and run this through a switch so that when the switch is one way it allows the 8 Volt supply to pass to the switch as normal,when the switch is thrown it will go to a diode bank, an ideal way that this is done is to replace the factory band switch with a new one having more positions as this saves having to use another panel switch.This switch may have as many as 6 complete bands !
The diode bank will control the programming to the binary adders, I shall describe the programming for two bands, the low low and high high bands this will allow the set to cover from 26.065 MHz to 28.305 MHz.
The programming for the low low band uses three diodes, they must go to the pins specified below
On adder IC6 the diodes must go to pins 4 and 2
On adder IC7 the diode must go to pin 6.
Make sure that the factory programming is disconnected as already mentioned otherwise it will go somewhere else !
The programming for the high high band uses four diodes, they must go to the pins specified below
On adder IC6 the diodes must go to pins 4 and 15
On adder IC7 the diodes must go to pins 2 and 15
Make sure that the factory programming is disconnected
as already mentioned otherwise it will go somewhere else !
Now, once all is done check for correct operation, you may get some oscillation on receive, usually at ch 19 or 20 Lo Lo. Cure it by soldering a capacitor of about 180pf across pins 7 and 8 of the PLL.
This shows the diode bank for the high high band.
You see how easy it is !
This exact same method can be used to create two more banks, which will only contain 30 channels between them, it is not really worth the effort of making them ! Plus the set may need further modification by way of broadbanding, this may need to be done after the expansion anyway, but you won't know until you try it and after it has been retuned.
+ 10 kHz shift
This modification will allow the set to use a 10 kHz shift ! In case you don't know what this is it simply goes up 1 channel, it will enable you to get the A channels plus 41 on your highest band (41 on any other band is the same as 1 on the next band up ! view my frequency chart to see for yourself !
All that you need to do is remove JP54 and put a 4.7 K Ohm resistor in its place (located by IC6), then connect a wire onto pin 9 on IC6, run this wire to a switch and connect the switch to a 8 Volt supply, when the switch is turned on the set will go up one channel !
5 kHz Steps
If you ground pin 6 of the PLL through a 10k resistor you will get an alternative range of 25.880 - 27.000 in 5k steps.
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