[time-nuts] 3GHz prescaler for Pendulum counters

[Arthur Dent]

 Just a note to say that I bought one of these
prescaler boards for my CNT-81. One problem is
the way the board mounts in the CNT-81 the "in"
connector is toward the back instead of toward
the front like some of the other counters and I
had to make a longer input cable. Pawel does
now know this and maybe a different cable can
be included for these counters in case you can't
make one. Be sure to check on this so there
aren't any surprises.

 When I put the prescaler board in and connected
a signal it didn't divide properly. After getting
the service manual and checking the schematic I
found the problem was the way the motherboard
was set up to detect which prescaler was installed.
There are resistors of 10K, 20K, and 40K going
from the 3 ID pins (16,14,and 12) to a common
100K to ground. On Pawel's board for the PM6681/
CNT-81 pin 16 is jumpered high to +5 to select
the correct divisor ratio with pins 14 and 12 not
connected so they should be low. Unfortunately
the resistors on the motherboard feed the +5VDC
from pin 16 back through the resistors to pins
14 and 12 so all 3 pins are high and this screws
up the ratio. If you have the service manual go
to the schematic "Logic Circuits, unit 1 sheet
2(6)" and look at grid C 11 to see what I mean.
I had to tie pin 14 and pin 12 low to make the
prescaler work as it should & it does work great!
I hate to think what an OEM board would cost so
this is a really good deal and I recommend it.

-Arthur