[time-nuts] My NTGS50AA failed

Wed Nov 12 20:59:29 EST 2014

Hi

It seems reasonable that there is a 1 pps somewhere on the board. The big question is if it’s the output of the GPS receiver or the output of the OCXO after division. 

Bob

> On Nov 12, 2014, at 8:10 PM, EB4APL <eb4apl at cembreros.jazztel.es> wrote:
> 
> 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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