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" <[email protected]> To: "Discussion of precise time and frequency measurement" <[email protected]> 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" <[email protected]> >> To: "Discussion of precise time and frequency measurement" >> <[email protected]> >> 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" <[email protected]> >>>> To: "Discussion of precise time and frequency measurement" >>>> <[email protected]> >>>> 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" <[email protected]> >>>>>> To: <[email protected]> >>>>>> 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 >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>> _______________________________________________ >>>>>> time-nuts mailing list -- [email protected] >>>>>> To unsubscribe, go to >>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>>> and follow the instructions there. >>>>>> >>>>>> >>>>>> >>>>>> >>>>> >>>>> >>>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>>> >>>> >>> >>> >>> >>> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > > > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
