[time-nuts] My NTGS50AA failed

[GandalfG8 at aol.com]

Hi Ignacio
 
I have removed a "faulty" oscillator from one of these, not one  of my more 
relaxed moments and quite amazed when the board emerged totally  undamaged, 
but proof at least that it can be done:-)
 
With the oscillator removed there's access to two sets of connector  pads 
that will either accept SMA or SMB connectors and after  conducting tests on 
the original oscillator via flying leads just  soldered to the board I 
decided not to fit a replacement to the board after  all but to fit a couple of 
SMB connectors to make the setup more versatile, and  to avoid the 
possibility of having to remove another oscillator in the  future:-)
Another advantage of these pads is that if the board does get damaged  
removing the oscillator they can still be used as an alternative.
The connectors take the 10MHz from the oscillator into the board and  the 
EFC control voltage out from the board to the oscillator, now there's a  
surprise:-), and oscillator power can be completely external if required.
>From this it would seem that the reference supply from the  oscillator 
itself is not used in practice, certainly not in the "offboard" case  anyway.
 
As others have suggested it seems likely your problem may not be the  
oscillator itself, but it still might be worth removing anyway to make testing  
and fault finding more straightforward.
 
The maximum positive excursion of the NTGS50AA should be 6 volts, not 5 as  
you're seeing, and another indication it might be worth removing the 
oscillator  to see how the board behaves stand alone.
I've not seen what seemed to be the repeated attempts at lock that you  
mentioned previously, but then I wasn't even aware for a long time that the  
control voltage could drive below 3 volts as well as above it:-)
 
This is my note from previous observation of my "faulty"  unit----
 
-------------------------------------------------------------
When first powered it brings up all LEDs and then switches to a green LED  
for a few seconds and then amber. It starts a self survey and acquisition  
process with all appearing ok, and the DAC voltage reported as 3.000002  
volts.
Sometime later, I've seen as short as 6 minutes but as long as 12 to  15 
depending on how long the oscillator has been turned off and allowed to cool,  
once enough satellites are being tracked, the DAC voltage starts to 
increase,  presumably seeking to drive the oscillator frequency to 10MHz, but the 
frequency  doesn't reach 10MHz and the DAC voltage ramps up to 6.000004 volts 
over a period  of approx 30 seconds where it remains.
As the DAC voltage crosses approx 5.6  volts the Red "fault" LED is 
switched on, as opposed to green that would  normally be expected to indicate all 
was well, and Lady Heather's "OSC:" report  switches from Good to Bad and 
highlights red. Similarly "Normal OSC age" changes  to "OSC age alarm" and also 
highlights red.
---------------------------------------------------------------
 
I've attached a Lady H plot that shows this, hopefully it will get through  
OK.
 
The above DAC voltages were as reported by Lady Heather but I've checked  
these and, when the board is working as it should be anyway,  they're very 
close.
 
In my case the problem described above was an oscillator that had  aged 
beyond the upper 6 volt limit, needing approx 6.54 volts to reach  10MHz, and 
once removed from the board I was able to add a simple op  amp level shifter 
to bring it back into range just to prove all else was ok,  which it was, 
but obviously Lady H now indicated the EFC into the level  shifter rather than 
at the oscillator itself.
 
Whilst your problem sounds like it might not be quite such an obvious  fix, 
removing the oscillator would open the loop and make  testing both the 
oscillator and the board much easier, so much as it's  a pain I do feel that's 
probably your best next step.
 
Regards
 
Nigel
GM8PZR
 
 
 
 
 
 
In a message dated 12/11/2014 02:06:54 GMT Standard Time,  
eb4apl at cembreros.jazztel.es writes:

Hi,

Removing the oscillator for testing and replacing it  with other if it 
was the culprit was my first option.  I have a spare  Trimble oscillator 
that probably came from other NTGS50AA since it still  have the foam band 
attached, but this oscillator is really aged, it needs  7.91 V to bring 
it on spot and the maximum control voltage of the NTGS50AA  is 5 V.
I was trying to avoid removing the oscillator but probably it must  be 
done to clarify things.

Thank you,
Ignacio  EB4APL

.
El 12/11/2014 a las 2:40, Mark Sims escribió:
> I  have seen this caused by the oscillator not responding to the EFC  
signal.  Fixed it by swapping in a MV-89 oscillator.
> The  oscillators used in these units don't output an oven temperature 
monitor  signal.                     
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