[time-nuts] HP5065A +20V supply and C-field mods
kb8tq at n1k.org
Wed Aug 9 14:48:47 EDT 2017
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.
> On Aug 9, 2017, at 2:34 PM, cdelect at 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
> 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
> 5.1ma at midrange on the pot. (zero TC on the diode is specified as
> The diode current ranges from 5.3 to 7.8ma depending on the C-field pots
> Since I am replacing the C-field circuit, the C-field pot circuit has
> removed. The 925 Ohm resistor was replaced with a 1ppm/C 1K resistor and
> 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!
> So that part of the mod is done.
> Next I will be installing a new C-field current source that has a TC of
> 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!
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