With more operators using export or amateur transceivers for 11 meter dxing, the use of frequencies 5 kc below the standard cb channels (i.e. 27.400 instead of 27.405) is becoming popular on single sideband . Typically, operators with regular cb rigs "unlock" the clarifier control and swing the xmit frequency up or down 5kc to get these "zero" channels. However, unlocking the clarifier opens a Pandora’s box of problems because using the clarifier to tune in an off-freq station will then put your transmitter off-frequency too. This paper explains how to modify the popular Uniden 858SSB chassis (Cobra 138/139XLR, Realistic TRC449/457/458, etc.) to select frequencies in 5kc steps rather than the standard 10 kc switching, allowing the transceiver to pick up the "zero" frequencies without unlocking the clarifier.
Many of the older PLL chips, like the PLL02A and UPD858c, were used in applications other than cb radios. Because of this, these chips had an input pin that could be programmed for 5 or10 kc channel steps. On the 858 chip, this is done by manipulating the logic state on pin 7: Logic 0 = 5kc steps, logic 1 = 10kc steps. To perform the modification, first isolate pins 7, 19, 21, and 22,and install 5.1k ohm resistors as shown below.
Then connect the pins to a 4 pole switch or relay so that operating the switch will set the logic on the pins as follows:
Pin 7 - logic 0
pin 19 - logic 1
pin 21 - logic 0
pin 22 - logic 1
The unit will now tune from 27.335 to 27.430 in 5 kc steps on channel selector positions 24 to 40.
******************************************************************
pin 19 - logic 1
pin 21 - logic 0
pin 22 - logic 1
The unit will now tune from 27.335 to 27.430 in 5 kc steps on channel selector positions 24 to 40.
******************************************************************
For additional 5 kc freqs, isolate pin 20 and install a resistor the same way as you did for pin 21. Then set the logic as follows:
pin 7 - 0
pin 19 - 0
pin 20 - 1
pin 22 - 1
pin 19 - 0
pin 20 - 1
pin 22 - 1
The unit will now tune 27.440 to 27.510 in 5kc steps on selector positions 8 - 22, 24 - 25.
******************************************************************
For even more freqs, set the logic at :******************************************************************
pin 7 - 0
pin 19 - 0
pin 22 - 1
and... the unit will tune 27.510 to 27.730 in 5 kc steps on selector positions 1 thru 40.
pin 19 - 0
pin 22 - 1
and... the unit will tune 27.510 to 27.730 in 5 kc steps on selector positions 1 thru 40.
- UNIT 399
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