[time-nuts] OXCO Issues
J. L. Trantham
jltran at att.net
Fri May 10 08:12:34 EDT 2013
Before you completely abandon ship, you might want to open it up.
I have opened two Ovenaire OCXO's (an OSC 85-50 from an HP 5316B and an OSC
49-38C from an EIP 545A) and made repairs. The first was low amplitude and
I just made a change in a resistor value in the output stage of the
oscillator to increase the gain of that particular stage.
The last was off frequency and would not come on frequency with the external
20 turn pot. I opened it up, adjusted the cap and it still would not come
on frequency. I then separated it into its three parts (oscillator, output
board, and oven assembly), cleaned off the dust then 'reverse engineered'
the oscillator and output boards to generate a schematic of each. Once I
thought I understood how the circuit worked, I reassembled the unit, turned
it on to allow it to stabilize, readjusted the cap (with the 20 turn pot set
to mid-range) and it 'magically' worked. It's been stable for a couple of
weeks not. Not sure what I did to fix it but it seems 'fixed'.
They are relatively easy to open by just digging out the epoxy and sliding
out the contents, removing a couple of screws on the sides first.
Might be fun to explore.
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Frederick Bray
Sent: Friday, May 10, 2013 1:47 AM
To: time-nuts at febo.com
Subject: [time-nuts] OXCO Issues
Thanks to all who responded. I have learned a lot.
It looks like the oven is still cycling and changing by about 10 Hz. I
thought I had it licked when it was stable for several hours last night, but
by this morning it was up to its old tricks.
I am going to go ahead and pass along the unit to a friend. Thanks to all
the help I got here, I am going to recommend that he replace the OXCO with
something more modern. It looks like a $25 Morion from ebay will do the
trick and the Cushman will probably do what he needs it for with this
upgrade. Since they are cheap, I might even shotgun most of the remaining
electrolytic capacitors in the main supply before I give it to him, as well
as replace the resistors that are well beyond tolerance.
The one nice thing about the Cushman 5110 is that it uses enough common
discrete components to be quite repairable and most of them seem to still be
Thanks again to all who helped.
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