[time-nuts] Re: James Miller/Brooks Shera 10 MHz GPSDO

[Bob kb8tq]

Hi

The version you have simply has a “box” around a fairly normal 
10811 OCXO. Tearing it apart is possible. It isn’t a whole lot of fun
though. 

I still believe that the GPSDO board should put out +/- voltages
and not just a negative voltage. Positive tune voltages are pretty
common. Bipolar tune is unusual but not unheard of. Negative
only tune is *very* rare.  Since the board is already out in the open,
working on it should be easier than ripping open the OCXO.

Bob

> On Aug 31, 2022, at 11:21 PM, Mervyn Thomas via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi Bob,
> 
> I need to find a schematic to download for my particular version of the HP oscillator
> 
> Merv
> 
> On 31/08/2022 2:59 am, Bob kb8tq via time-nuts wrote:
>> Hi
>> 
>> If you check the 10811 schematic, the varicap is biased at roughly +6.8V internally.
>> The other side of the diode is what the EFC is driving. This gives them the +/- 5V
>> range they specify on the 10811 data sheet.
>> 
>> Bob
>> 
>>> On Aug 30, 2022, at 5:39 AM, paul swed via time-nuts <time-nuts at lists.febo.com> wrote:
>>> 
>>> Pretty clever approach to establishing an offset. But doesn't it matter how
>>> the vari-cap is hooked up in the oscillator circuit? If the veri-cap needs
>>> a negative voltage to work that may indicate the cathode is at ground and
>>> the positive voltage would forward bias the vari-cap. Just thinking about
>>> the possible impact.
>>> Though the cost of experimenting is $0. No harm.
>>> Regards
>>> Paul
>>> WB8TSL
>>> 
>>> On Tue, Aug 30, 2022 at 1:22 AM Mervyn Thomas via time-nuts <
>>> time-nuts at lists.febo.com> wrote:
>>> 
>>>> Stephen,
>>>> 
>>>> An idea I may try -thanks for your response.  The current involved in
>>>> the EFC line to the oscillator is extremely small-button cells would
>>>> last years.
>>>> 
>>>> Merv
>>>> 
>>>> On 29/08/2022 8:42 pm, Stephen C. Menasian via time-nuts wrote:
>>>>> Merv,
>>>>> 
>>>>> If I understand your problem, the 10811's VCO control input voltage has
>>>>> drifted above the output range of the GPSD driver board. If the VCO
>>>>> control terminal current is low enough, you might be able to quickly
>>>> solve
>>>>> this problem by putting 1 or 2 Lithium Ion Coin cells in series with the
>>>>> control lead. If only a few microamperes are involved, this solutin might
>>>>> last several years.
>>>>> 
>>>>> Stephen Menasian
>>>>> 
>>>>> On Mon, 29 Aug 2022 15:56:52 +0800
>>>>> Mervyn Thomas via time-nuts <time-nuts at lists.febo.com> wrote:
>>>>> 
>>>>>> Hi Bill,
>>>>>> 
>>>>>> Unfortunately the 2 HP 10811's I have have no way to poke anything thru
>>>>>> the can to adjust a capacitor.  They are completely enclosed in an
>>>>>> aluminium can with 2 power leads coming out with 6 pin female
>>>>>> connectors on the end which go to the power board as used in the HP
>>>>>> Z3801 GPSDO and 2 thin coax cables with SMB connectors - one marked as
>>>>>> 10 MHz and the other as EFC.  I may have to replace mine with the model
>>>>>> that has the tuning capacitor.
>>>>>> 
>>>>>> Thanks for your response.
>>>>>> 
>>>>>> 73   Merv
>>>>>> 
>>>>>> On 29/08/2022 12:45 pm, Bill Beam via time-nuts wrote:
>>>>>>> I also run a Brooks Shera/HP 10811. Still going after many years.
>>>>>>> You must live with the limited steering voltage.
>>>>>>> The solution is to adjust the HP 10811 tuning capacitor to get it
>>>>>>> back near a frequency that the EFC can handle.
>>>>>>> I have had to do this more than once.  Also if replacing the
>>>>>>> oscillator its tuning capacitor may need adjustment.
>>>>>>> 
>>>>>>> On Mon, 29 Aug 2022 11:25:26 +0800, Mervyn Thomas via time-nuts wrote:
>>>>>>> 
>>>>>>>> Hi All,
>>>>>>>> I'm new to time nuts but here goes.
>>>>>>>> I built a GPS disciplined-  10MHz osc about 15 + years ago.-  It
>>>>>>>> uses an HP 10811 D/Oven Oscillator and the Brooks Shera PCB and
>>>>>>>> software V402N.- The physical construction article was per James
>>>>>>>> Miller G3RUH. This unit has worked flawlessly for all these years
>>>>>>>> but now is unable to hold the freq to precisely 10MHz - it is
>>>>>>>> constantly .29 to.60 Hz high.- The DAC output which controls the EFC
>>>>>>>> into the osc outputs a negative only adjustable voltage between -3v
>>>>>>>> down to 0V and is settable by varying a 10K potentiometer.-  The
>>>>>>>> Osc. now needs abouit +0.46V to bring it down to 10MHz but the DAC
>>>>>>>> does not seem able to output a positive voltage.-  The DAC is an
>>>>>>>> AD1861 (obsolete) and has a +5V applied to pin 16 and a -5V applied
>>>>>>>> to pins 1 and 8. My first reaction was the HP osc. had reached the
>>>>>>>> end of it's life but the datasheet states the EFC control voltage
>>>>>>>> can be between a plus 5V and a minus 5V which varies the freq by 1Hz
>>>>>>>> so it seems it is just over half way through it's life.
>>>>>>>> I feel the Brooks s/ware would have provided some method to ensure
>>>>>>>> the DAC output could be adjusted to swing the full full 5V or at
>>>>>>>> least 3V?- I have the source code but I'm no programmer so would
>>>>>>>> appreciate any help someone can provide please.
>>>>>>>> 73 de Merv-  VK6BMT
>>>>>>>> _______________________________________________
>>>>>>>> time-nuts mailing list -- time-nuts at lists.febo.com
>>>>>>>> To unsubscribe send an email to time-nuts-leave at lists.febo.com
>>>>>>> Bill Beam
>>>>>>> NL7F
>>>>>>> 
>>>>>>> 
>>>>>>> 
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