[time-nuts] Broken Ovenaire OSC 85-50

J. L. Trantham jltran at worldnet.att.net
Wed May 27 02:29:41 UTC 2009


Well, a little work with a scalpel and a small flat bladed screw driver and
I was able to open the package.  Inside is what I call an 'Output Board'
that contains the output circuit and the 'FINE' frequency adjust along with
the 'Oven' which has the 'COARSE' frequency adjust.  Attached to the 'Oven'
is what I call the 'Oven Controller Board'.  With the removal of 3 screws
and the 'nut' on the 'COARSE' frequency adjust, the 'Oscillator Board' can
be removed.

I have 'derived' the schematic from inspections of the 'Output Board' and
the 'Oscillator Board'.  I made no attempt at the 'Oven Controller Board'
since it seems to be operating ok.  More on this below.

I have attached the schematics of the output board and oscillator boards.  I
made some measurements of DC and RF voltages on the output board.  I have
multiple pictures that are too large to attach.  If anyone would like to see
these, I can send them.

At start up, the 85-50 draws about 0.5A then falls to about 0.25A as the
'oven' heats up suggesting the temperature regulator circuit is functional.
The 'COARSE' and 'FINE' frequency adjusts work and the frequency can be
brought spot on as compared to my GPSDO.

I have ordered new transistors in the hope that these are the problem.  I am
a little puzzled by the drop in voltage at the 'Red' wire to the oven.  It
serves both the 'Oscillator Board' and the 'Oven Controller Board'.  2.5 VDC
seems a little low for the oscillator.

Also, the gain of the output stage of the output board seems a little low.
0.1 V P-P goes to 0.36 V P-P.

I have a second, good, 85-50 but would like to avoid opening it if possible.

Any help would be appreciated.  I hope I made no mistakes on the schematics.

Joe




-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Gordon Batey
Sent: Sunday, May 24, 2009 6:50 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50

I had an earlier model 20 mhx ovenaire osc that had very low output.  I
opened it up with a scalpel and found the TTL chip had failed and was only
letting a small amount of the osc signal thru.  I was able to use an
external TTl (74LS20 I think) to restore it.  As I recall I was unable to
readily get to the chip so I replaced it with an external one.  

Good luck.  

Gordon



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