[time-nuts] Broken Ovenaire OSC 85-50

[Mike Monett]

  (This is  a repost to see if I can figure out why  the  original was
  scrubbed. Sorry for the duplicate.)  

  > 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

  Joe,

  Congratulations on getting your system to work!

  A couple  of  things. First, trying to measure the  currents  in the
  circuit with a ferrite toroid won't do you much good. You don't know
  what the  currents  should  be,  and  the  secondary  of  the toroid
  transformer requires a termination resistor. The value  changes with
  the turns ratio.

  Just from looking at the circuit, the RF currents will  be extremely
  low. This  requires a large number of turns on the  secondary, which
  will probably resonate at or below the 10MHz operating frequency due
  to stray  capacitance  from the connection to the  scope.  So  it is
  unlikely you will get any useful progress in this direction.

  However, from the values on your schematic, the output  tank circuit
  resonates at  9.602MHz with a Q of 9.6. So the tank is  already well
  below resonance, which attenuates the output voltage.

  Any stray capacitance you add to the circuit will bring the resonant
  frequency lower, further aggravating the loss in signal.

  The output  tank  is tapped with the 75pF and 91pF  in  series. This
  further attenuates the signal.

  I'd change the circuit to a single capacitor across the tank  with a
  small trim capacitor to tune it to resonance.

  To get  the signal into 50 ohms for distribution, I'd add  a limiter
  if you can tolerate a square wave output, or a good emitter follower
  if you  need a sine wave. Take the output from the collector  of the
  2N2369 to get the maximum signal amplitude.

  Your original  post  mentions an output amplitude  of  20mV.  If the
  normal amplitude is around 2V, this represents a loss of  40dB. This
  is a huge loss in signal. The circuit obviously worked at  one time,
  so there may well be some other hidden problem.

  It is  possible the crystal is damaged, but this  seems  unlikely. A
  crystal oscillator probably won't even start if the signal  level is
  down 40dB.

  You can  check  the oscillator and crystal in  SPICE.  Normally, the
  high Q  of  the crystal will make the analysis very  slow.  It could
  take many  hours  for   the   simulation  to  begin  oscillating and
  stabilize at the final amplitude. The transient analysis  requires a
  very fine  time step for accuracy, and you could run  out  of memory
  before the simulation was complete.

  I have developed a much faster way of analyzing a crystal oscillator
  in SPICE.  Instead  of requiring tens or  hundreds  of  thousands of
  simulated cycles,  this method gives accurate results in only  a few
  dozen cycles.  For more information, please see  "SPICE  Analysis of
  Crystal Oscillators"

  http://pstca.com/spice/xtal/clapp.htm

  You can  estimate the value of the crystal ESR by finding  the  Q of
  your crystal and working backwards.

  I'm attaching a gif of your schematic for reference. This is rotated
  90 degrees and enhanced in LView Pro to improve the contrast.

  Please let me know if you have any questions.

  Thanks,

  Mike