[time-nuts] My NTGS50AA failed

EB4APL eb4apl at cembreros.jazztel.es
Thu Nov 13 21:06:59 EST 2014


Hello Nigel,

Finally I got enough courage and removed the OCXO, you know what kind of 
task it is..  Fortunately I didn't spoiled the PCB so I'll install 2 smb 
connectors in the provided places, I don't want to return there if I 
need to change the oscillator.
Well, the oscillator is ok, this is in some way bad news because the 
oscillator is easily replaceable, and the EFC voltage remains stuck on 
5.02 V.  Now I'll test the quad op-amp and the related parts.  If they 
are ok the problem is in the Xilinx chip, something that cannot be 
replaced unless I get  parts donor, even that way is very problematic 
since it is a  80 pin chip.  I'll continue posting my findings.

Best regards,
Ignacio


El 13/11/2014 a las 12:03, GandalfG8--- via time-nuts escribió:
> Hi Ignacio
>   
> I'm sure removing the oscillator would make your faultfinding much more
> straightforward.
>   
> When I removed mine I started out quite convinced I wouldn't be able to do
> it, even with the solder mostly removed from around the pins they were
> obviously  quite a tight fit and all I had available at that time was a plunger
> type  solder sucker and desoldering braid, as the bits on my old Pace
> desoldering kit  were well past their best.
> The answer basically was lots of braid, lots of patience, and resisting
> like crazy any temptation to pull against a hole that wasn't fully released,
> but  I certainly wouldn't want to do it that way again in a hurry.
> The desoldering gun I use now would make it easier but the holes are really
>   a bit small for those oscillator pins.
>   
> The good news though is that the connectors are there as a back up and
> fitting them definitely makes it more versatile.
> I had considered repeating the performance on another NTGS50AA  and/or
> NTBW50AA, I really think I should but so far haven't found the  courage:-)
>   
> I think previous checks for 1PPS outputs might have been limited to the
> external connectors but it would make sense if it was available somewhere on
> the  PCB, I'll try to get one powered up later and check your findings, if I
> can find  a bit of space that us amongst the usual chaos!
>   
> Regards
>   
> Nigel
> GM8PZR
>   
>   
>   
>   
> In a message dated 13/11/2014 01:11:30 GMT Standard Time,
> eb4apl at cembreros.jazztel.es writes:
>
> 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_ (mailto: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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