[time-nuts] HP 5065A studies

Bill Hawkins bill at iaxs.net
Sun Nov 14 23:40:26 EST 2004

Have a 5065 with a physical package who's warranty expired
in 88, last cal in 98. 2nd Harmonic was low at 11 on the meter
and Photo current was at the low edge of 25. So I opened the
manual for an earlier machine (thanks, Tom) and began to 
understand how the device works.

The photocell current and the 2nd harmonic meter readings come
from amplifier A7, as does the first harmonic 137 Hz that runs
the feedback control loop. The unit had signs of leakage from a
bad Option 001 battery, so I opened up A7 to look for trouble.

That sinking feeling came over me as I realized that someone
had been there before me, and didn't replace all of the screws.
I went through the calibration procedure to verify that the DC
gain was nominal but the AC gains had been "screwdrivered". I
restored the loop gain to nominal from full up and clipping.

The 2nd harmonic calibration instructions say, "see if the
reading moves when you vary the audio generator output." Or
something like that. So I turned it down to match the first
harmonic gain.

Reassembled it, did it again when the replacement screws were
too long, and finally had normal operation. Well, almost. There
was no 2nd harmonic reading so the logic reset did not produce
a green light. The 2nd harmonic is not used for control, just
as an indication of control null. Turned the 2nd harmonic gain
back up to get 11 on the meter, could have got 15 but decided
not to be greedy. Everything has stabilized on frequency, as
measured by 5 MHz phase angle against a Cs unit that is fairly
phase stable against 10 MHz from GPS.

Observations: RVFR says it should produce 54 microamps photocell
current, actual is 41 plus/minus 1. 2nd harmonic reads 11 with
sharp jumps to the 40s and quick return. Mag field on the physics
package says 270 but 400 is required for the right frequency. The
synthesizer output is correct.

So, what is the lifetime of the RVFR? Does it outlive the 2 year
warranty? Should I squirt contact cleaner into the wire grommets
in order to clean the lenses? :^) Should I take the RVFR apart in
order to clean it?

I don't think the jumps in 2nd harmonic are internal because
they did not show up with an audio generator driving A7. The unit
has been running for more than a day and the green light is still
on, so there have been no drop outs.

Once I get this running, then we can talk about a way to keep its
long term accuracy dead on with respect to GPS (or WWVB).

Bill Hawkins

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