Hi

Sounds like a lot of fun getting everything to zero out :)

One thing to watch for is hysteresis. The tempco may not be quite the same
going hot to cold as going cold to hot.

Bob


> On Aug 9, 2017, at 2:34 PM, cdel...@juno.com wrote:
> 
> 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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