Service manual

mpl

The TC9109 and MB8733 PLL ICs are both similar in that they use eight of the

connections between the channel switch and the LED channel number display as inputs to

determine the "N" code. In effect, there is a built-in "Prom" which translates the LED code

into the "N" code and the PLL can not, therefore, be fooled. They produce the 40 FCC

approved channels, but no others. Unlike most

PLLs, these produce

the transmit frequency

by doubling the VCO output, instead of by mixing. There is, however, a British PLL

which, although now obsolescent, serves to illustrate another method of using an

Eprom if

the PLL is changed. This British PLL, the LC7137, also doubles the VCO output to

produce the transmit frequency, but its input codes are Binary Coded Decimal (BCD) with

the exception of channel 40 which is 000000 instead of 1000000.

Since these PLLs operate in basically the same manner we can substitute, for one of the

FCC ICs, a board containing an LC7137 plus an Eprom to convert the eight LED display

code inputs to six BCD inputs, instead. The LED codes then become the LOCATION

numbers which tell the Eprom where to find the BCD codes for the LC7137. This example

is of interest, firstly because the LED codes seem to be in no logical order but,

nevertheless, select 40 unique locations for the BCD codes. Secondly, because the British

channels produced are in ascending order, without any "hops", therefore giving a better

basis to convert to the 10 meter band, perhaps by mixing methods or by digital subtraction

(which has nothing to do with getting your finger out!).

Note that the input for pin 17 is high for every channel except channel 40. It seems a

singular waste of a wire to use 8 connections instead of 7, just for the sake of one zero.

Close inspection of the codes reveals that we can eliminate this first connection if we use

the zero on channel 40 to make the pin 15 voltage go high. This is fairly simple to achieve

with a single transistor switch and the result for channel 40 is still a unique code of 1 1 0 0

1 0 0 or 64 instead of 44 HEX. This code does not occur for any other channel, so can be

used to identify the Eprom location of channel 40. The first input can be eliminated and all

the other location codes will have to be modified accordingly. The hardware advantage of

this change is questionable but the example shows, again, that the actual switch codes are

not important, provided that each can be used to generate a unique location code. (When

BCD is translated into HEX, the numbers A to F

do not appear since only the binary codes

for 1 to 9 are used.)