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GM7HUD > SIX 22.12.05 01:29l 73 Lines 4386 Bytes #999 (0) @ WW
BID : 093172GM7HUD
Read: GUEST
Subj: CB to 6m conversion 8
Path: DB0FHN<DB0FOR<DB0MRW<DB0SON<DB0SIF<DB0EA<DB0RES<ON0AR<GB7YKS<GB7LGS<
GB7ESX
Sent: 051221/2224z 71797@GB7ESX.#31.GBR.EU $:093172GM7HUD [Witham, Esx]NNA V3.1
*CB TO SIX METRES* Part8 - The Transmitter section
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.*
Whenever any modifications or repairs are done to any piece of equipment ,
capable of radiating RF energy , it is of paramount importance that the
regulations concerning spurious and harmonic energy radiation are strictly
adhered to . This necessitates the use of a spectrum analizer to verify
signal purity after these mods are done . I realize that a piece of test
equipment in that class is not standard equipment in most "shacks" , but
one would be able to most probably approach your local PMR company to check
the unit for you to ensure that your output is "clean" before you transmit
into your antenna .
The output from the frequency synthesizer is fed via a capacitive divider
circuit to pin4 of IC3 , where the IF injection frequency (injected into
pin1) is mixed with it to produce the 50Mc wanted output . There are what
appears to be two small chokes feeding pin6 and pin8 . L23 is indeed an RF
choke , but L3 is not .If you study the circuit carefully , it will become
clear that C37 and C38 are in series and connected across L3 to form a
parallel L-C Tuned circuit , which at the moment resonates at round 27Mc .
C37 and C38 also form a capacitive divider to do the impedance
transformation to the imput impedance of the cascode amplifier section of
IC3 (input on pin7) . L3 is wound on a carbon resistor of round 400 ohms to
lower the Q of this tuned circuit and thus improve stability and to
increase the effective bandwidth .
Initially , I used to just take a few turns off the coil , but I have
since decided to make a whole new coil instead and just replace the
existing L3 with the new one . I used a slightly thicker wire , and wound
exactly 15 turns onto a 470 ohm 0.5W carbon resistor . Cover the assembly
with heat-shrink tubing , and apply moderate heat till the shrink-tube
contracts and holds the windings in place . Remove C37 and C38 , and insert
47pF ceramic capacitor in C37 and a 27pF ceramic capacitor in C38 . I have
found that changeing L23 there was no percievable improvement , so , you
can leave L23 as is .
The output of the cascade buffer amplifier is fed into T4 and T5 which
forms a bandpass filter . Remove T4 and T5 and dismantle as described in
Part4 . T4 is a Mutsumi coil, part number 10CA006 and T5 is a Mitsumi ,
part number 10CB003 . Refer to part4 as to the orientation of the coils.
T4: Wind 1 turn onto the secon rib from the top starting at the leftmost
pin . Feed the wire down the gap provided on the former down to the centre
pin , form a pigtail , tin , and solder to the middle pin . Feed the wire
back up through the gap in the former and wind an additional 4 turns on the
topmost rib of the former . Feed the wire back down the gap , tin and
solder onto the righthand pin . Remove the internal capacitor , reassemble
and solder back into the PCB . Solder a 39pF miniature disk ceramic
capacitor across the primary (Ref.Part4)on the foil side of the PCB .
T5:Starting at the leftmost pin , wind 1 turn onto the second rib from the
top , crossover at the gap and continue to wind an additional 4 turns onto
the topmost rib , feed the wire down through the gap and tin , solder onto
the righthand pin . Remove the internal capacitor , replace the black
potcore , reassemble and solder back into the PCB . Add a 27 pF miniature
disk ceramic capacitor to the primary on the foil side of the PCB . All the
secondary windings can stay as is . Remove C42 and replace with a 2.2pF
ceramic capacitor . Remove C43 and replace with a 39pF . Remove C185 and
replace with a 18pf Ceramic disk capacitor .
We will continue with the modifications to the transmitter in part9
vy 73 de Shawn ZR1EV JF96ic
73 de Andy GM7HUD
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