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

[Mervyn Thomas]

Hi All,

Problem with my James Miller/Brooks Shera GPSDO resolved.

The main problem was I had a component (R5 I think) on the board as 4.7K 
instead of 470R.  This was no real problem up until the EFC needed to go 
beyond where it was .  Changed it to a 470 R and put a PCM61P DAC in 
place of the AD1861 and it now holds the 10 MHz  hour after hour with no 
High LED coming on.  There was probably nothing wrong with the AD1861 
DAC but having changed it I'm leaving PCM61P in place.

Thanks to all who responded - much appreciated.

If anybody needs a PCM61P I have quite a few spares and at around US$ 10 
I'll post to anywhere.

Merv

On 2/09/2022 4:45 pm, Mervyn Thomas wrote:
> Bob,
>
> I'm with you on that and although I have 2 10811's I would be loathe 
> to tear one apart - certainly as a last resort only and for 
> information sake rather than being able to fix it.
>
> Merv
>
> On 2/09/2022 4:55 am, Bob kb8tq via time-nuts wrote:
>> 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.
>>>>>>>>
>>>>>>>> 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
>>>>>>>>>> _______________________________________________
>>>>>>>>>> time-nuts mailing list -- time-nuts at lists.febo.com
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>>>>>>>>> Bill Beam
>>>>>>>>> NL7F
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
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