[time-nuts] Broken Ovenaire OSC 85-50

Mike Monett xde-l2g3 at myamail.com
Sat Jul 4 02:19:55 UTC 2009


  > Broken Ovenaire OSC 85-50
  > J. L. Trantham jltran at worldnet.att.net
  > Sat Jul 4 02:07:48 UTC 2009

  > I did  not  realize  that   this  post  would  generate  this much
  > discussion.

  > While I  have  a  BEE   and   MSEE,   I  am  a  practicing Cardiac
  > Electrophysiologist and  it  has been some  time  since  I thought
  > about such  issues.  I   have   learned   a  lot  though  from the
  > discussions and as medicine as we know it winds down, interests in
  > EE will grow.

  You probably  don't realize how ironic it is that you are  a Cardiac
  Electrophysiologist, and you have fixed a broken heart (in the sense
  that a crystal oscillator is the heartbeat of any circuit)?

  > I agree with Warren. It should be fixed before  modified. However,
  > I could  not  find anything that was  'broken'  with  the possible
  > exception of the XTAL.

  You have  done   extremely   well.   No   one   is  criticizing your
  observations or  your  solution. They work, and you can  move  on to
  greater things.

  But we are timenuts. We argue and discuss the most trivial-appearing
  topics, because underneath may lie much greater truths.

  > As I  tried  various things, I did notice from time  to  time some
  > reluctance to  oscillate suggesting that the oscillator  stage was
  > an issue.  With  no  obvious  failure and  no  hope  of  finding a
  > replacement XTAL, I had to choose other options.

  Again, you did good. The subject has left the operating table and is
  on his feet.

  > The real  problem is size. There is no room for  added components,
  > at least on the Oscillator Board which is inside a sealed oven.

  > There is only room for changing components. I would have  liked to
  > try adding a resistor and capacitor to the emitter circuit  of the
  > amplifier transistor on the Oscillator Board but there is no room.

  > I have  pictures if anyone is interested. The Oscillator  Board is
  > inside a  metal  can about 0.5x1.5x2.0 cm that has  a  heater coil
  > wrapped around  the outside and an Oven Controller  Board attached
  > to the outside. This package is surrounded by foam with the Output
  > Board on one face of the foam, mounted to one wall of  the outside
  > metal package. The bottom of the metal package has 2  feed thru's,
  > plus a ground connection. One is 5 VDC and the other is  the 10MHz
  > out.

  > While with  better equipment one might be able to find  a problem,
  > it was certainly not obvious to me.

  > Do XTAL's fail often? If so, by what mechanism? Slowly fade away?

  > Sudden death? I am not experienced enough to know this  answer but
  > in all my years with various HAM gear, I do not recall the failure
  > of a single XTAL.

  Your question is a good one.

  Recall that  many  precision  weigh  scales  deposit  material  on a
  crystal to  measure  the  change  in  frequency.  And  other systems
  immerse the crystal in liquid to look for frequency changes.

  These show  the basic piezo response is intrinsic  to  the material,
  and is quite robust.

  I think  crystals  have to be pretty badly abused  before  they quit
  entirely.

  I remember  grinding  a FT-247 crystal for 40 meters to  move  it up
  slightly and get off the European broadcasts when I was a teenager.

  My transmitter was a single 6AG7 with 600V on the plate, and about 4
  watts into  a  long line antenna. The chirp  from  the  sidetone was
  pretty bad, but the crystal seemed happy no matter what abuse I gave
  it.

  The biggest  problem  with crystal  oscillators  is  overdriving the
  crystal. The  acceleration at the surface is on the order of 4  to 5
  million G, so it is easy to understand how overdriving can result in
  fracture. But  the crystal is so badly damaged it won't work,  or it
  oscillates at a different frequency.

  If you look at the oscillator circuits used during the war, you will
  find circuits using a 300 Watt 813 transmitting tube with  a crystal
  in the grid. This is abuse beyond belief, and a modern crystal would
  simply fracture on the first key-down.

  This has happened many times in the past. Recall the BART failure in
  San Francisco some years ago where a train ran into another  one and
  killed passengers. The complete analysis was never published, but it
  really looks like the crystal fractured from overdriving and  ran at
  a much  higher frequency. This upset the software  timing algorithms
  enough so  the train could not stop as programmed. People  died from
  this mistake.

  I find these problems in my SPICE crystal analysis, at

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

  Most of the circuits I analyze overdrive the crystal by a  factor of
  10 or more. I don't know why they survive.

  I don't  know  what  could   happen   to  a  crystal  to  change the
  piezeoelectric response,  which is an intrinsic part of  the physics
  of the crystal.

  There are  many other flaws that can happen to a crystal,  but these
  usually occur  at  manufacture and probably  would  never  reach the
  field.

  So it is likely there is some simple electronic failure in question.

  > However, none of these were in an oven.

  > Thanks again for all the useful information.

  > Joe

  > None the  less, with a simple change of resistor, the gain  in the
  > amplifier stage increased and, at least so far, problem solved.

  So we don't know what happened in your circuit. But you are still to
  be congratulated  at getting it to work under the most  difficult of
  circumstances.

  Regards,

  Mike




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