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

Mon Aug 29 04:45:20 UTC 2022

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-ï¿½ 10MHz osc about 15 + years ago.-ï¿½ It uses an 
>HP 10811 D/Oven Oscillator and the Brooks Shera PCB and software V402N.-ï¿½ 
>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.-ï¿½ 
>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.-ï¿½ 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.-ï¿½ 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?-ï¿½ 
>I have the source code but I'm no programmer so would appreciate any 
>help someone can provide please.

>73 de Merv-ï¿½ VK6BMT
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Bill Beam
NL7F