Here is a new observation about low Vcc. It is not likely that the Osc Vcc voltage and the Varicap freq control voltage is coming from the same 510 Ohm resistor without a regulator. Something is missing on the schematic, because now any Osc current change would cause the freq to change. And there is the missing 3 plus ma that can not be accounted for in your voltage measurements. The Osc and pot circuits as shown are drawing about 1.5 ma, and there seems to be almost 5ma going thru the 510 ohm. My guess is that this is from a 2.50 volt precision shunt regulator on the Vcc, in the Osc oven, but not shown on the schematic. That should be pretty easy to find. If so, make sure your 47 ohm fix does not cause the 2.490 volts to loose its regulation when the circuit is put back in the oven. The increase temp will cause the osc current to increase.
If the EFC pot is really 10K ohm end to end, that would account for the other strange voltage readings. Also if the Freq as small as it is, is still correct, that would eliminate most of the bad xtral possibilities. ws ***************** ----- Original Message ----- From: "WarrenS" <warrensjmail-...@yahoo.com> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Saturday, July 04, 2009 12:24 AM Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50 I get a 'if working correct' Vcc of about 3.5V If the Vcc is 2.49 volts, I calculate a 3ma unaccounted for current draw from something. So I have same conclusion, one or more of the following is wrong. 510 Ohm too high, 5 volts too low, un-shown current draw thru Leakey caps or extra circuits or bad data. (plus the pot is (and should be) more likely a 10 K ohm) ws ****************** ----- Original Message ----- From: "Bruce Griffiths" <bruce.griffi...@xtra.co.nz> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Friday, July 03, 2009 11:58 PM Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50 > Warren > > The Vcc dc current doesn't change much if the circuit is oscillating (at > least it shouldn't if the circuit operating correctly - the active > devices aren't supposed to saturate as thats adds too much phase noise). > > My simulations were over the 50-100C rang although temperature doesn't > affect the currents and voltages too much. > They also don't include loading of the frequency control pot wiper. > > There is no major inconsistency other than the effect of the > unanticipated loading of the EFC pot wiper. > The simulated oscillator Vcc is actually about 3V when everything is > working properly. > NB its important to use models that accurately reflect the correct > transistor junction area just blindly tossing in 2N3904's increases the > current somewhat as the Vbe is lower for a given current than the actual > transistors. The simulated oscillator Vcc is then closer to 2.5V. > > Earlier circuit schematics didn't include as many actual measured dc > voltages. > > Bruce > > WarrenS wrote: >> 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.griffi...@xtra.co.nz> >> To: "Discussion of precise time and frequency measurement" >> <time-nuts@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.griffi...@xtra.co.nz> >>>> To: "Discussion of precise time and frequency measurement" >>>> <time-nuts@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.griffi...@xtra.co.nz> >>>>>> To: "Discussion of precise time and frequency measurement" >>>>>> <time-nuts@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-l...@myamail.com> >>>>>>>> To: <time-nuts@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 >>>>>>>>> _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
