[time-nuts] Z3801A EFC Error

[Dave M]

> Dear John,
>
> On 07/10/2011 04:38 PM, John Ackermann N8UR wrote:
>> I'm getting my two trusty Z3801As up and running after 18 months of
>> downtime and a 500 mile move. Unit #1 came up just fine.
>>
>> Unit #2 has been running for about 18 hours and is working OK except
>> that the health status is showing an EFC error. The EFC value is
>> above 1e6 (currently 1014166) and is trending downward, though the
>> trend has leveled off in the last several hours. I suspect that the
>> health monitor error comes on to indicate that EFC is near the end
>> of its range; if I
>> query for relative EFC, it comes up at about 93% so perhaps the
>> trigger
>> is 90%.
>>
>> Other than the EFC error in the health monitor, the unit seems to be
>> working fine. LEDs show normal activity, the Oven PWR and OCXO health
>> status are both OK, and after initial alignment the PPS seems to be
>> holding within a handful of nanoseconds. (I'm using GPSCON for
>> tracking.)
>>
>> It's possible that the EFC could keep trending downward as the unit
>> stabilizes, and if that continues to be the direction there should
>> be no problem. But if it starts to go up, I'll run out of tuning
>> room soon. Assuming there's no other hardware problem, I'd think the
>> normal way to address this would be to twiddle the oscillator's
>> coarse frequency
>> trimmer to get it centered in the tuning range.
>
> If there is no other systematic source, I would have expected that the
> change of direction in frequency would have already occurred if it is
> there. There is a gentle bump in the 10811s as they heat up, but after
> that there is a stable slope as far as I recall.
>
>> Before I tear into the box, is there a coarse frequency adjustment
>> accessible on the dual-oven 10811? If that's not an option, any other
>> suggestions on what might be going wrong and how to cure it?
>
> I don't recall that the coarse frequency being accessible, but it
> could
> be worth checking. What does rings a bell is the oven settings might
> need some care. Maybe time to do some read-up on the oven assembly
> again, for both of us.
>
> I would let it sit for some longer time before really being worried.
>
> Cheers,
> Magnus

The 10811 oscillator does, indeed, have a coarse frequency adjustment.  It's 
on the top of the oscillator module.  While it's been quite a while since 
I've been inside my Z3801A, the pictures at 
http://www.realhamradio.com/GPS_Frequency_Standard.htm shows that the 
oscillator is mounted on its side, with the top facing towqard the front of 
the Z3801A chassis.  You might have to unmount it to gain access to the 
coarse adjustment.
As others have suggested, I'd let the Z3801A run for a few more days to see 
how it settles down.  If the EFC continues to run to the end, you might 
unhook the EFC to the oscillator and see what its free-running frequency is. 
If it's way off, then tweak the coarse adjustment to bring it back close to 
10 MHz, and then hook the EFC back up and see how it runs.

Cheers,
David
dgminala at mediacombb dot net