[time-nuts] My NTGS50AA failed

EB4APL eb4apl at cembreros.jazztel.es
Tue Nov 11 20:07:30 EST 2014


After 2 yeses of continuous operation my NTGS50AA turned its red led on 
and stopped working. A check with LH shows "OSC: BAD" and "OSC age 
alarm" assuming that the oscillator had aged too much.
I didn't believe this because when working, the DAC control voltage was 
around 2.9 V, in fact near the middle of its range (0-5 V). Measuring 
the 10 MHz output it is high, about 4.46 Hz which agrees with the LH 
I measured the oscillator EFC pin and it is struck at 5.02 V, not 
following the DAC voltages as reported by LH. I think that either the 
DAC is bad or an amplifier after it.  Since I don't have any schematic 
and the oscillator covers the top layer the troubleshooting is difficult.
Has anybody experienced this failure before?.  Does anybody has an 
schematic, even a partial one?

Since the unit now is operating open loop I observed the locking 
strategy of this GPSDO.  First it waits about 12 minutes for warming (I 
don't know if it internally monitors the oven current or uses a fixed 
time).  During this period it sets the DAC output to the initial value 
as stored in the EEPROM (3.0 V).
When it thinks that it is "warmed enough", the DAC is ramped in the 
right direction to intercept exactly 10 MHz (towards 0 in my case). If 
it reaches 0 then declares the alarm and the DAC voltage is set to half 
the initial value (1.5 V). Five minutes later it switch the DAC to 2,25 
V and 17 seconds later it returns to 1.5 V and remains there for 4 min 
and a half.  Then it goes up again to 2.5 V and after 10 seconds it goes 
down to 1.88 V and after some 14 minutes it goes down again to 1.5 V, it 
remains there for about 15 minutes and then it goes down to 1.14 V, 
remains there for 80 seconds and goes back to 2.5 V.
It looks like it checks from time to time if it is able to control the 
oscillator or simply it does weird things once it thinks that the 
oscillator cannot be disciplined.

I will appreciate very much any information.that can help my 
troubleshooting.  These units has more than doubled its price since I 
bought mine and I think that they are vanishing.  The Guatemala's cell 
towers scrap has been exhausted.

Ignacio EB4APL

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