[time-nuts] Broken Ovenaire OSC 85-50

WarrenS warrensjmail-one at yahoo.com
Sat Jul 4 06:37:04 UTC 2009


Bruce

4ma? Interesting, Maybe IF the Osc was running, BUT 
My DC calculations of the present values I can read on Schematics come to 1.3ma + 0.26 = 1.58 ma load on the 510 ohm

With it NOT osc, which is almost the present case, 
the Only Dc current  drawn is. 

 1.75 V on 20K  = 0.08ma   (Q1 bias)
 1.04 V on 2.2K    =   0.47ma    (Q1 collector)
 0.365V  on 470 ohm.  = 0.77 ma   (Q2 emitter)
Osc board =1.32 ma

Buffer board
  (2*4.7k) at 2.49 V   = 0.26 ma


Also note that the 4.7 K ohm pot and 4.7K divider voltages show excess current draw somewhere, maybe on the Grn lead bypass cap..

ws

**************

----- Original Message ----- 
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Friday, July 03, 2009 11:08 PM
Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50


> Warren
> 
> My Spice model of the dc levels in the oscillator indicate a current
> more like 4mA in the 510 ohm resistor.
> 
> Bruce
> 
> WarrenS wrote:
>> I just saw your schematic of the rest of the buffer circuit. Sorry I missed it early.
>>
>> The Osc Vcc is shown driven from a 510 Ohm resistor, and with the total current thru the 510 res at under 2 ma, 
>> Looks like Osc Vcc should be at more like 4.0 volts instead of 2.49 volts.
>> If I'm correct about that  (this time) then, It has got to be one of three things.
>> The 510 ohm is wrong, the 5 volts is wrong, or the load on the 510 ohm is wrong.
>> It should be easy enough to measure which one it is.
>> The voltage at the junction of the two 4.7K resistors are WRONG.
>> Looks like it is caused by an excess current draw on the Grn lead. 
>> Check the direction and leakage of the voltage controlled cap diode and bypass.
>> If all else is right, Look for the cause of the extra loading current on the 510 ohm, 
>>  
>> WS
>>
>> ****************
>>  
>> ----- Original Message ----- 
>> From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
>> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
>> Sent: Friday, July 03, 2009 9:55 PM
>> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
>>
>>
>>   
>>> WarrenS wrote:
>>>     
>>>> A couple more thoughts; 
>>>>
>>>> Is the VCC (Red wire) current draw correct?.
>>>> Should be 1.3 ma (per the schematic values) with the 470 Ohm and 
>>>> more like 7 plus ma with the 47 ohm.
>>>>
>>>> AND I don't remember seeing, IS the Freq real close AND can it be tuned by the cap and Green wire voltage?
>>>> If both answers are YES, at least most things are working correctly.
>>>>
>>>> As Bruce said:
>>>>   
>>>>       
>>>>> "if the resistor values are correct this is the voltage(s) that one should get"
>>>>>     
>>>>>         
>>>> With the resistors values shown, I agree that all the other voltages are correct given the 2.49 volts, 
>>>> BUT the 2.49 volt is NOT controlled by any of the values on that schematic, it's just an input.
>>>> AND the output amplitude is not yet being controlled by current cut off.
>>>>
>>>> Something that needs to be asked is are all these the correct values.
>>>> It sounds like you got them by measuring and not from a "should be" schematic.
>>>>
>>>> One last, way out thought.
>>>> It was my belief that very good osc have some sort of AGC to control the amplitude,
>>>> ANY chance that the VCC is used as the input to control the amplitude of the Osc output? 
>>>> If so, could it be that the external AGC that is what broken and supplying too low a VCC voltage to the circuit?
>>>>
>>>> ws
>>>>
>>>>   
>>>>       
>>> The Colpitts crystal oscillator limits the crystal current by being
>>> periodically cutoff for part of the cycle.
>>> There is an optimum duty cycle (usually something like 25%) that
>>> corresponds to minimum phase noise.
>>> The oscillator dc collector current determines the crystal current and
>>> hence the output amplitude.
>>> It is possible to perform AGC by varying the oscillator Vcc but the
>>> output amplifier schematic appears to have no circuitry for this.
>>> Some circuits actually do this, but the oscillator transistor is still
>>> cutoff for part of the oscillator cycle.
>>> The oscillator VCC is fed from a tap in a resistive divider network
>>> (located on the external (to the oven) buffer board) connected between
>>> the 5V supply and ground.
>>> Usually one just varies the emitter (or collector) dc current by
>>> selecting a resistor value.
>>>
>>> Bruce
>>>     
>>>> *****************
>>>> ----- Original Message ----- 
>>>> From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
>>>> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
>>>> Sent: Friday, July 03, 2009 8:21 PM
>>>> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
>>>>
>>>>
>>>>   
>>>>       
>>>>> WarrenS wrote:
>>>>>     
>>>>>         
>>>>>> Here is my  two cents worth
>>>>>> 20 mv output, sure sounds like something is broken.
>>>>>> It should be fixed before it is modified.
>>>>>> The 2.49 volts on the Red input voltage seem LOW, More Vcc maybe.
>>>>>>   
>>>>>>       
>>>>>>           
>>>>> Not so, if the resistor values are correct this is the voltage that one
>>>>> should get.
>>>>> The question that remains: Is the voltage what the designer intended?
>>>>> With the Colpitts oscillator used limiting in the oscillator transistor
>>>>> (usually by current cutoff) is used to limit the amplitude.
>>>>> Just increasing the voltage without checking the resultant crystal
>>>>> current will be within permissable limits can be counter productive.
>>>>>
>>>>>     
>>>>>         
>>>>>> The "Grn" labeled wire, FreqCtrl  input should be about 1/2 VCC  for testing.
>>>>>> If you do 'need' to modify the gain, 
>>>>>> It would seem better to bypass the 470 ohm resistor with a cap in series with the 47 ohms.
>>>>>>
>>>>>> ws
>>>>>>   
>>>>>>       
>>>>>>           
>>>>> Bruce
>>>>>     
>>>>>         
>>>>>> ********************
>>>>>> ----- Original Message ----- 
>>>>>> From: "Mike Monett" <xde-l2g3 at myamail.com>
>>>>>> To: <time-nuts at febo.com>
>>>>>> Sent: Friday, July 03, 2009 11:50 AM
>>>>>> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
>>>>>>
>>>>>>
>>>>>>   
>>>>>>       
>>>>>>           
>>>>>>>  > 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
>>>>>>>
>>>>>>>     
>>>>>>>         
>>>>>>>             
>>>>>> _______________________________________________
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>>>>>> and follow the instructions there.
>>>>>>
>>>>>>   
>>>>>>       
>>>>>>           
>>>>>
>>>>>     
>>>>>         
>>>> _______________________________________________
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>>>>
>>>>   
>>>>       
>>>
>>>
>>>
>>>     
>>
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