[time-nuts] HP/Agilent 5065A Rubidium Vapour frequency standardproblem

[Bill Hawkins]

Jim,

When a controller drifts out to its limit, the control loop is
not closed. Either or both the measurement and output are not
working.

For the input:

What is the second harmonic reading on the meter? If it's too
low, there's not enough for a measurement.

Does changing the coarse oscillator change the error reading?
Actually, you should be able to see motion if you change the
Fine adjustment by 100.

For the output:

Catch the Control reading before it reaches the end of the scale
after turn-on. The coarse oscillator adjustment should change the
Control reading. Once it reaches the end of the scale it has
saturated, and won't necessarily move when you change the coarse
frequency.

I expect that A8TP3 provides a clue, but I don't have the book
with me.

It's entirely possible that the power cycle on the unit just
discharged the control integrating capacitor so it would start
from zero again. Swapping oscillators may have nothing to do
with it. Of course, a cold oscillator has a different frequency
than a hot one.

Tell us what you read on the meter besides Control.

Bill Hawkins


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Palfreyman, Jim L
Sent: Wednesday, May 07, 2008 11:21 PM
To: time-nuts at febo.com
Subject: [time-nuts] HP/Agilent 5065A Rubidium Vapour frequency
standardproblem

Hi all,

I'm in need of someone with some experience in fault finding a 5065A.
I'm having a go at repairing my university's old unit which up until a
short while ago was working well. I've got the PDF of the manual and
I've been testing and trying various things but I need someone with a
bit of experience with these units to help diagnose the fault.

Symptoms:

A) The unit drifts at about 1 usec/sec.

B)  The CONTROL reading is pegged off the scale.

C) The course oscillator adjustment doesn't affect the CONTROL reading.

D) A8TP3 doesn't seem to produce anything but a straight line, however
A8TP2 is fine.


The fact that the course oscillator didn't seem to do anything made me
suspect the 10811-60109 OCXO. I happen to have a spare 10811-60111 and
so I powered off the unit, substituted the (cold) 10811-60111 and
powered on. The course oscillator control could adjust the CONTROL
reading and within a minute or two the unit was drifting at only 80
picosec/sec.

Voila - problem solved!

However, 8 hours later the CONTROL level was pegging the meter again and
the unit was drifting at 1 usec/sec.

And now the oscillator course control had no effect on the CONTROL
level.

So why is that a luke warm 10811 combined with a hot RVFR allowed the
unit to function, but once the 10811 heated up it drifted again?

The Continuous Operation light operates all the time and the Integrator
Limit light comes on after a while as expected when the CONTROL voltage
is high.

Any hints or tips will be appreciated.

Jim Palfreyman



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