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GM7HUD > SIX 22.12.05 01:46l 72 Lines 4237 Bytes #999 (0) @ WW
BID : 3B3171GM7HUD
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Subj: CB to 6m conversion 7
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<ON0AR<HS1LMV<CX2SA<GB7CIP<GB7COV<
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Sent: 051221/2224z 71796@GB7ESX.#31.GBR.EU $:3B3171GM7HUD [Witham, Esx]NNA V3.1
*CB TO SIX METRES* Part7 - The channel selector logic
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above address.*
In my introduction to this series , in part1 , I mentioned the channel
selector and touched on some of the difficulties encountered when using
switches like these . This rig was designed for the American market and as
a result are originally fitted with a switch that selects frequency
according to the American 11M bandplan . This poses a few problems when any
modifications are attempted to change the operating frequency .
For example , you will notice that there are five spots between channels
where the frequency increments 20kc as the channel is incremented . Also ,
the frequency of channel 23 to 25 are not in sequence . Traditionally ,
mods involving a band change (say , to 10M) would require a piece of paper
with a list of channel numbers with corrosponding frequencies typed on it
attached to the rig , so one would be able to tell what frequency you are
operating on .
In my mods , I inserted an EPROM between the channel selector and the PLL
chip to "correct" all of these anomalies. It makes for a tidy, professional
"feel" to the mod and makes checking your frequency as simple as looking at
the LED readout . As the rig tunes in 10kc increments , one could program
the eprom so that Channel 1 (readout 01) will put you on 50.010 , channel
10 on 50.100 , and channel 40 on 50.400 etc. This is how to go about it :
The output from the channel selector switch will be a 6-bit binary "word"
that will be unique for each of the 40 channels . We will use each of these
unique 6-bit words to adress a specific location in an EPROM where you can
program the 8-bit code of your choice . The EPROM will output this 8-bit
word to the PLL chip and , voila ! you are on your frequency .
First you need to draw up a truth table . Make a list of all the channel
numbers and write down the binary from the channel selector down next to it
(you will find it listed in your service manual , or you can read the value
by measuring each bit directly off the PLL chip . The pins in question are
pin 15 (LSD) down to pin 10 (MSD) , in that sequence) . Next , convert the
binary words to HEX . All EPROM programmers I have seen work with HEX .
Select any channel above 27 (ch26-40 are all in sequence) . Read the full
9-bit binary off pins15-7 and write it down . Write down the frequency the
rig is operating on next to it . Click one channel up and do the same . The
binary should have changed by 1 .
Calculate all the binary codes for the range 50.010 up to 50.400 and when
that is done convert them all to HEX . You now have all the information you
need to program your EPROM . The eprom has an 8-bit output and the
synthesizer chip requires a 9-bit word . In practice you will find that the
state of the MSD pin stays the same throughout the 40 channels , with the
result that pin7 can be wired permanently to either logic1 or 0 .
I use a single 27C64 eprom , as there are a lot of them floating around
surplus . Remember to "pull down" the first six imputs with 1k resistors
and tie all the unused inputs to GND . I used a seperate 7805 3-terminal
voltage regulator IC to power te eprom and all these components can be
mounted on a small pcb or perfboard . I used short sections of ribbon cable
to carry the data from the switch and to the PLL chip . It is of course
possible to extend the basic 400kc tuning by programming another set of
codes for say,50.400 to 50.800 and then use one of the unused inputs to the
EPROM to switch "bands" this way . See you in Part8 where we will start the
mods to the transmitter .
vy 73 de Shawn ZR1EV JF96ic
73 de Andy GM7HUD
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