[time-nuts] My NTGS50AA failed

Thu Nov 13 01:10:51 UTC 2014

Hi Nigel,

Thank you for the suggestions, I was trying to avoid the OCXO removal 
but I think that now it must be done.
I was also playing with the idea of populating the connectors so an OCXO 
exchange could be easily made, this makes possible to try various 
oscillators.
Meanwhile I has been probing and measuring a lot of points and by chance 
I found a very interesting thing:  Probing TP33 (which is close to the 
Trimble chip (U2) and directly connected to pin 76) there is a 1PPS, 10 
us wide signal.  I've checked it and appears to be synchronous with the 
1/2 PPS output so maybe it can be routed to an output, probably I'll 
replace the 1/2 PPS with it, using the existing drive circuit and 
connector since it is very straightforward.
My only concern is if this signal only is there during the anomalous 
condition that I have now, I have to retest it after fixing it.
I had asked several times if anybody had located a 1PPS signal on these 
units, but the responses were negative and I had not probed 
systematically the board before.  Also I'm taking notes of the signals 
found and I'll try to make a partial schematic at least with the EFC 
circuitry.  When I fix it I'll clean the notes and figures and I'll 
upload it to some place.

Best regards,
Ignacio

El 12/11/2014 a las 11:47, GandalfG8--- via time-nuts escribió:
> 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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