HP5065A +20V supply and C-field circuit mods

I'm doing some more work on the 5065A and thought I'd share the results
of a 
modification (one of several that will be done on this unit).
I measured the +20V P/S temperature coefficient and got -13.5ppm per
degree C. 
This equates well with the 1N938 10ppm/C and the voltage set resistor TCs

of plus/minus 10ppm/C. However the TC varies if the C-field pot is
changed.
The original circuit has 11.9ma thru the reference diode series resistor
(925 Ohm)for a diode current of 6.8ma and a C-field pot circuit current
of 
5.1ma at midrange on the pot. (zero TC on the diode is specified as
7.5ma)
The diode current ranges from 5.3 to 7.8ma depending on the C-field pots
position. 
Since I am replacing the C-field circuit, the C-field pot circuit has
been 
removed. The 925 Ohm resistor was replaced with a 1ppm/C 1K resistor and
a 
1K precision pot. This allows the Reference diode current to be adjusted 
to compensate for all the elements in the circuit. (The TC of the diode
changes depending on the diode current)
After adjusting the diode current and temperature over many iterations 
and monitoring the +20 Volt output I was able to find a diode current to 
bring the +20 Volt output coefficient to below 1ppm per degree C!
(-.6ppm/C)
So that part of the mod is done.
Next I will be installing a new C-field current source that has a TC of
2ppm 
per degree C.
Then I'll do some long term plots against the Maser to see if the long 
term stability has improved at all.

More to come!
Cheers,

Corby

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