[time-nuts] Newbie with Broken RFTGm-II XO/Rb - Need an Elmer for Basic Advice

[k1jos at att.net]

I posted a specific problem earlier and TVB has been kind to help me off
line but suggested I come back to the time-nuts group for broader advice.
Though I've subscribed to the list for many years, work kept me from diving
in and I am still very much a Newbie, recently retired and finally have the
time to dive deeper into time measurements.   I'm really fortunate to have
acquired from a local EE/Ham estate some nice high-end test gear but  I am
also on the learning curve to fully understand their full capabilities.   
 
A few years back I bought on Ebay a KS-24019 Rb XO pair.  I read all that
was here on the mailing list, made the correct interface cables and the pair
has been working continuously for a few years until a XO Fault.  I have been
monitoring its status with the original RFTGm software running on a XP
laptop.  Here's what I have found out so far:
 
*	The XO red LED Fault appears after warm up, immediately after GPS
Lock it faults with "OSC PLL LOCK FAILURE".  Re-setting a few times same
fault. In Fault mode there is no 15MHz Output.
*	The Rb works well, has a very clean 15Mhz sine 23dBm output  on my
spectrum analyzer without spurs and a good short term Adev on my Fluke
PM6690 counter with a "Very High Stability Oven Timebase; 0.01 ppm/month".
but I have it on EXT REF to my long running TBolt GPSDO. maybe I should be
testing using its better short-term stability internal TCXO?)
*	I checked all the supply voltages in the XO incl the Datum OCXO and
all seem normal per various posting on time-nuts.
*	During warm-up the XO ADC is 4.9v but after it Faults it drops to 0v
*	If I disconnect the interface cable and power cycle, the XO goes
through warm up, locks onto satellites and no longer gives a RED LED Fault
and no more OSC PLL Lock failure.  The XO  has a green LED Online, all the
other LEDS are off, and the 15MHz becomes enabled.  It seems to be in a
stand-alone condition and the software shows a no fail codes only "Locked to
GPS".  
*	There is a coax from the Rb 10MHZ SMA Ref Out to the XO 10MHz Ref
In. but there is no effect on any of these conditions or faults if I remove
it.  So it seems the Osc PLL failure is between the XO & RB 1PPS exchange
(Rb pins 7-->XO pin 8 and XO pin 7 -> Rb pin 8)
*	I checked pin 7 on the XO and Rb interface DB9 connector and both
show a very clean 1pps square wave (400us wide) on my scope.   Is this
correct?  I read that the GPS 1pps was 100msec wide. 
*	If I connect the interface between Rb and XO the XO faults
immediately back with "OSC PLL LOCK FAILURE".  Disconnecting, it goes by to
WarmUp, losses the PLL Fail fault and back to a standalone active state.   
 
My thinking at this point, without any schematic, would be to trace the
signal from the Rb 1PPS into the XO interface input but I'm not sure how to
follow a 400us wide 1pps pulse?    Any advice where to go from this point
would be very much appreciated.
 
Jerry NY2KW


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