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

[Mervyn Thomas]

Steve,

It has a glue on label stating it's Serial Number but unfortunately no 
hole under it.  The bench power supply I'm using reads current down to 
1000th of an amp but not down to uA so a good suggestion I put a 
microamp meter in series to see what current is being drawn.

Thanks,  Merv

On 30/08/2022 10:27 pm, Stephen C. Menasian via time-nuts wrote:
> Paul,
>
> That is a valid consideration. Unfortunately, I don't have a schematic of
> the oscillator and can't say, definitely, whether is relevant. In trying
> this out, I would start with an adjustable DC power supply with a
> microammeter in series with its output and monitor the current as the
> voltage is increased from zero. This would be necessary, anyway, to
> estimate the expected cell life. A current rise resembling a a diode
> turn-on knee would serve to establish the limit. I wouldn't ever let it
> rise above 1 mA or so. Some varactors have MAX forward currents in the
> 10s of mA range. If the knee is found at a voltage below the Li ion
> coin cell range, a 1.5 Volt cell might be in order. Also, I would monitor
> the oscillator output frequency as I did this. After all, if one can't
> reach 10 MHz without forward biasing the varactor, the only course of
> action might be to cut the oscillator can open.
>
> Something occurred to me. Does the oscillator have a glued-on label?
> Sometimes these are used to cover access holes to "factory-only"
> adjustments. One can often locate this type of hole with ones finger
> by pressing on the label and feeling for a depression.
>
> Stephen
>
>
> On Tue, 30 Aug 2022 09:39:36 -0400
> 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
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>>>>>> Bill Beam
>>>>>> NL7F
>>>>>>
>>>>>>
>>>>>>
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