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GM7HUD > SIX 22.12.05 01:27l 71 Lines 4168 Bytes #999 (0) @ WW
BID : 453170GM7HUD
Read: GUEST
Subj: CB to 6m conversion 6
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<ON0AR<GB7ESX
Sent: 051221/2224z 71795@GB7ESX.#31.GBR.EU $:453170GM7HUD [Witham, Esx]NNA V3.1
*CB TO SIX METRES* Part6 - More on mods
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7560
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.*
As probably expected , the story does not end there . If you look at the
circuit diagram , it becomes clear that both the frequency synthesizer and
the carrier oscillator gets pulled LOW on LSB , not the other way round as
implied in Part5 of this series . How could this be ?
The answer lies in the mechanics of mixing . On 27Mc an inversion of the
sidebands take place in the process of mixing the 10.695 DSB signal with
the output from the frequency synthesizer (coming in from the "high side"
of the wanted signal) . The result of the mixing process and the resulting
inversion of the sidebands means that one has to generate a LSB signal at
the IF frequency to realise an USB signal on 27Mc , Hence the "pulling" low
of both signals on both LSB and AM .
When we now convert the set to 50Mc , an additive mixing scheme is
employed , and the sidebands come out the "right way round" , with the
result that the frequency shifting scheme would have to be reversed . In
practice , this is quite a simple matter to accomplish . Some later
articles on conversions like these in the Amateur press , circa 1985/6 ,
would have you believe that it is simply a matter of redoing the labels on
the front panel . This will not work and you will have great difficulty in
getting the rig lined up to be on frequency on all the modes (if at all) .
Do the following and all will be well . You will see a grey wire running
from a point marked (T) next to Q4 in the synthesizer section to another
point also marked (T) on the pcb , next to Q18 . Desolder this wire from
the PCB where it enters the board next to Q18 . Pull it through toward the
mode switch . Next we need to look at the switch itself . There are two
sections to this switch . The one section applies full B+ to the final when
the rig is either in USB or LSB mode and routes the B+ via a hefty 6 ohm
wirewound variable resistor to the final when AM is selected (AM carrier
power adjustment) . Leave this as is , and locate the second section of the
switch which selects USB/LSB/AM . The wiring from this switch goes to
points (30), (19), (21) and (18) on the main PCB respectively .
Point (30) selects AM , point (19) selects USB , point (21) selects LSB
originally and point (18) is common . Swap the wires from points (19) and
(21) on the switch itself . Take two small silicon diodes (1N4148 or eq.)
and tie their cathodes together and solder the free end of the grey wire to
this junction . Shrinkwrap or slide an insulating sleeve over the exposed
sections of this connection . The two "free" ends of the two diodes
(anodes) solder directly onto the tags of the mode switch now connected to
point (19) and (30) respectively . For those who are not sure , the end of
the diode with the thickest colour band will be the cathode (apologies to
those who do know HI).
The modifications to the mode selector is now all done , and you can now
proceed to "net" the transciever . Adjust CT1 to net LSB and AM (one
adjustment for both modes) and CT2 for USB only . To enable the rig to
shift frequency on transmit as well as recieve , using the clarifier as a
RIT/TIT control , remove D5 , and connect the unused end of the clarifier
controll to the emitter terminal of Q44 . Be sure to check the settings of
CT1 and CT2 after this mod as it will affect the 12'o'clock position on the
clarifier . I will discuss mods to the frequency selecting circuitry in
part7 ... see you then .
vy 73 de Shawn ZR1EV JF96ic
73 de Andy GM7HUD
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