Service manual

22
T h e M B 8719 us e d in Unid e n Ch a ssis
This PLL is unusual in having only seven programming pins but an internal inverter which
makes it work as if it had eight. The seventh pin controls the MSB which is always "1"
(~64) or "0" (.128). Unfortunately, this system means that the range of codes is limited,
going from 64 minimum to 128+63 = 191 maximum: a range of 127 channels or just three
banks of 40, in practice.
The early export Cobra 148GTL DX uses three crystals with this PLL. The channel switch
is the standard Uniden switch with codes from 15 to 59 but, because the MSB pin on the
MB8719 is wired high, you must add another 64. With HI band crystal selected
(15.93MHz) the channel 40 code is 59+64=123 and produces a frequency of 27.855MHz
(which is not legal, by the way!). The highest "N" code is 191 and you can add 191-
123=68 channels at 10kHz to give a maximum frequency of 28.535MHz with this crystal.
The "Novice" band of 28.10 to 28.50 is, therefore, only just possible. If you need higher
frequencies in the 10 meter band you must change the crystal.
The top channel of 28.495MHz requires an "N" code of 187 at channel 40 and channel 1,
28.105MHz will have a code of 148.
128 64 32 16 8 4 2 1
The codes range from 1 0 1 1 1 0 1 1 = BB HEX (ch40)
to-- 1 0 0 1 0 1 0 0 = 94 HEX (ch 1)
Remember that the eighth bit is generated automatically by the inverter built into the
MB8719 itself. You need, therefore, only seven Eprom outputs connected to the PLL and
the eighth bit can be "0" since it is not connected. In that case, BB HEX becomes 3B HEX
and 94 HEX becomes 14 HEX. Since the MSB is always "0" you can connect it to ground
(cut the copper trace) and use only six Eprom outputs. Getting easier, isn't it!
As in the MC145106P version, the switch codes 15 to 59 (with hops) select the locations
where the new "N" codes are to be found in the Eprom. The channel switch in the Cobra is
always wired to ground and external 47k pullup resistors are used to provide a positive
voltage of 9v. These should be removed from the main Cobra board if Eprom inputs are to
be connected directly and pull-up resistors connected to +5 volts used instead. This is
accomrnodated on the Cobra Eprom board described later.