Service manual

Practical Examples

Cobr

148

So much for the theory. Now suppose you want to carry out an actual conversion of a CB

radio to the American

10 meter Novice band which covers 28.100 to 28.500 MHz. To

make it a little more complicated let's choose the later model of the Export version Cobra

148 GTL DX which uses the PB-010 chassis. A single 15.000 MHz

downmix crystal is

employed and

the three frequency bands are created by using a pair of MC14008 binary

adder ICs between the channel switch and the MC145106P synthesizer.

A full description of the operation can be found in the

Uniden "Export" Service Manual,

which is published by CB City International. Briefly, the channel switch produces codes

from 0F to 3B HEX (that is 15 to 59 decimal) which are fed to the binary adders. For the

Low band, three diodes are switched to add the value 67 to each switch code making a

series which now runs from 82 to 126. For Mid band the value 112 is added to give a

series from 127 to 171 and, similarly, the High band codes run from 172 to 216.

High band covers 23.415 to 27.815 MHz and the obvious question is "why can't we add

still higher values to achieve the codes for the Novice Band?

". The answer is that you can,

but with one drawback, which is that you will still have the "A" channel "hops" and the

odd arrangement of channel 23 which is a damn nuisance when the other party says "QSY

ten

kiloHerz" and you promptly lose him! Notice, by the way, that the codes given above

span 45 channels, not 40, because of the "hops".

It is very easy to work out the code required for 28.105. Since each increment of 1 in the

code will move the rig up another 10 kHz just subtract 2810.5 from 2785.5 = 25 (working

in units of 10kHz). So if we add 25 to the code for 27.855 MHz, which is 216, we get 241

which must produce a frequency of 28.105.

Note that all this was done without considering loop mixing frequencies, VCO frequencies

or anything complicated beyond simple addition! Now we know that CH1 in the Novice

Band requires a PLL code of 241. The switch code is

still 15 for channel 1 so we must add

226 by switching in diodes.

We will still be left with the "hops" but let's look at this in a little more detail before moving

on to the

Eprom method. 226 in Binary is

256

128 64 32 16 8 4 2 1

0 1

1 0

0 1 0