[time-nuts] 5345A with 10811A or 10544A

[Greg Broburg]

Seeking some idea here as to what happened in the
Voltage trim concept in the 5345A

I have been nursing two 5345As over the last few years.
I kind of took it on faith that HP tied off the varactor
trim Voltage line as there appears that they did not make
a method to tickle this Voltage on either the 5345A with
the 10811A or with the 10544A. One of the 5345As died and
had to be solved. The problem was that its 10544A had died.
The rock is bad on that one. However, since my desire is to
add the Voltage trim adjust circuit that HP appears not to
have installed I made some inspection of pins 5,6, and 7.

It appears that the important item is pin 6 and it is
floating in the 5345A. What gives? The 10544A schematic
that I have shows minus EFC in as pin 6 and option for pin7
or internal 6.4 V for EFC plus. Inside the 10544A that I
have, pin 6 has a wire to the prescribed 20k resistor and
the varactor is installed. pin 7 is not connected.

The second 5345A that I have been running has a 10811A in
it and the 10811A schematic shows EFC to pin 6 (varactor
anode) and varactor cathode to internal reference
at 6.4V same as the 10544A that I have.

Is it possible that HP just let pin 6 float in the 5345A
assuming that the 10544As were to be installed without
the varactor option? The nature of a floating anode is such
that the diode goes into a very undesirable region, low Q
high capacitance. Complete troubleville.

The floating nature of this internal pin means that I
need to restart all of my tuning an observations on these
units. Unless I am missing something here?

My idea is to make a Voltage reference onto one of the
little removable option covers on the back with a pot to
eliminate the cats breath tweaks to the trim cap in the
10811A and to finally nail the zero.

Ha anybody been down this path before? Any suggestions?

Any voices from the distant past of HP engineering?

Regards;

Greg