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

[Mervyn Thomas]

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