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

[Mervyn Thomas]

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.

73Â Â  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-Â  10MHz osc about 15 + years ago.-Â  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.-Â  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.-Â  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-Â  VK6BMT
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>
> Bill Beam
> NL7F
>
>
>
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