[time-nuts] My NTGS50AA failed
EB4APL
eb4apl at cembreros.jazztel.es
Wed Nov 12 20:10:51 EST 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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