[time-nuts] 5061 Long Time Constant

John Ackermann N8UR jra at febo.com
Fri Dec 14 20:21:54 EST 2007


I have two 5061As that are (for the moment at least) working.  Both have
replacement FTS tubes, and both have the retrofitted 10811A OCXO.  One
is a fairly late chassis, while the other is much older.  One difference
is that the long time constant switch on the older one is a slide switch
in the bottom of the control panel, while on the newer one LTC is an
additional position on the main function switch.

The newer unit runs in LTC mode quite reliably.  Its frequency shift
when switching between short and long time constant is not too great,
and can easily be adjusted with the C field control.

But the older unit is interesting.  When I switch it into LTC mode, it
appears to work just fine, and the continuous operation lamp remains
lit.  However, the frequency shifts upward by about 2x10e-11, and even
pulling the C field control all the way to zer0 doesn't make a lot of
difference.

As I understand it, leakage in the big capacitor that is switched in
line to enable the long (60 second) time constant causes the frequency
shift.

I guess I'm not surprised that perhaps the cap in the older unit is more
leaky, resulting in a greater shift.  But I am surprised that the offset
could be large enough (a) that it couldn't be tuned out via C field, yet
(b) wouldn't trigger an alarm and cause the green light to go out.

Has anyone here experienced anything like this?  Might something else be
going on?

John



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