[time-nuts] Broken Ovenaire OSC 85-50

[J. L. Trantham]

An update on the Broken Ovenaire OSC 85-50.

I prepared a 'schematic' of the Output Board and the Oscillator Board
(attached) and I have lots of pictures of the external unit and the insides
if anyone is interested.

I resoldered all connections and replaced all transistors on the Output
Board and the Oscillator Board all to no benefit.  I measured all the
components with an LCR meter and found the 0.01 uF bypass on the 330 ohm
resistor in the emitter circuit of the output transistor of the Output Board
to be low and with a high ESR.  I replaced this with about a 20% increase in
output amplitude but still inadequate.  I replaced the rest of the 0.01 uF
caps on the output board with no additional benefit.  I transiently
disconnected the Red wires from the Oven Controller board and there was no
increase in output or significant increase in voltage to the Oscillator
Board.

Therefore, it appeared that a 'low output crystal' (if such a thing exists)
was the only logical explanation that I could come up with.  That seeming to
be the case, there appeared to be only 4 options.  1.  Toss the OCXO (sorry,
too much effort so far).  2.  Build an external amplifier (seemingly too
much additional effort).  3.  Try to adjust on the bias of the oscillator
transistor to achieve a higher output (seemed too 'iffy').  Or 4.  Lower the
value of the resistor in the emitter circuit of the Oscillator Board to get
more gain out of the last stage in the Oscillator Board.


I replaced the 470 ohm resistor with a 47 ohm resistor and the amplitude
increased to about 0.4 V P-P into a 50 ohm load and was sufficient to make
it a usable OCXO again.

I reassembled, resealed with Epoxy and all seems well so far.

If anyone wants pictures or other info, please let me know.

Thanks for all the suggestions and help.

Joe

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Thursday, May 28, 2009 7:13 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50


Joe

Whilst its easy enough to construct a miniature RF current probe using a
small ferrite core which is slipped over the wire in which the current is to
be measured, constructing one using a split ferrite toroid is more difficult
as such toroids are difficult to come by. In principle one can construct
ones own by cutting and grinding a pair of toroids. Unless one has access to
diamond lapping equipment and diamond saws this is probably impractical.

Thus you will probably have to revert to measuring the RF voltage across
resistors in the circuit to get some idea of the RF current.

Bruce


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