[time-nuts] UPDATE: DATUM 9390-52054 Grief again...

Fri Apr 12 18:48:47 UTC 2013

Sorry for the delay in posting this update.  Things have been hectic, 
and then there was NAB.

Here's what I've discovered:  The receiver started working after 
about 6 hours of just sitting.  However, the 9390's internal Vectron 
oscillator was quite a ways off frequency and did not want to lock 
after trying to stabilize all night.  I had to tweak the adjustment 
screw quite a ways and then, after a while, it locked.  Prior to this 
episode the oscillator had been sitting at only a few E-12.  I 
suspect that this oscillator has had an intermittent problem for a 
long time, and I should not have had to tweak it as far as it wanted 
to go.  The receiver portion has not failed in the few weeks since it 
decided to start seeing satellites again.  Maybe because I had 
tweaked the oscillator?

So, I got brave and changed the oscillator out with a smaller Vectron 
oscillator that I got from my friend Stu, K6YAZ.  This is not a 
commercial, but he sells these oscillator modules on eBay.  See: 
http://www.ebay.com/itm/10-MHz-precision-crystal-oscillator-in-sealed-oven-5-volts-voltage-adjustable-/190820631639?pt=LH_DefaultDomain_0&hash=item2c6dcbc457#ht_848wt_1143

The new oscillator requires 5 Volts rather than 12 Volts, which I 
simply grabbed from the from the power supply's 5 Volt rail.  It was 
not a lot of fun removing the original oscillator from the circuit 
board and the associated fine traces, but once I plucked it from the 
board I put short wire leads in the original circuit board holes for 
the various connections to the new oscillator.  I cut a piece of foam 
about the size of the old Vectron and carved out it's innards to 
accept the new, much smaller oscillator.  This should provide some 
thermal stability and also insulate the new oscillator from the 
circuit board.  I connected everything up, substituting the 5 Volt 
rail lead for the 12 Volt lead.  When I powered the 9390 up 
everything seemed to work and the oscillator was within 1-Hertz, 
except that the output was about 20 dB higher than with the original 
oscillator was when it was working properly.  Once the receiver 
locked up to 4 satellites and started "OSC Stabilizing", the 
oscillator suddenly jumped about 18 Hz low and shortly afterwards the 
DAC went to all zeros.  I let it sit all night figuring that it would 
eventually find itself.  Well, it didn't, it just sat at 18 Hz low 
and the DAC at all zeros, and the EFC (Electronic Frequency Control) 
voltage at 0.19 Volts.  (What a revolting development!)

Very early this AM I got up and put a simple 10:1 voltage divider 
using a series 680 Ohm and 68 Ohm resistor from the oscillator's RF 
out to ground This brought the levels pretty much back to the same as 
my good 9390.  I also substituted a pot for the EFC voltage.  I used 
a 1 K pot in series with a 300 Ohm resistor to the 5 Volt rail.  The 
resistor was on the high side, the bottom end to ground, and the 
wiper to the EFC terminal.  I was able to tune the oscillator to 
precisely 10 MHz long enough to measure the substitute EFC at +2.15 
Volts.  Leaving it hooked up this way I let the receiver do it's 
thing just to see what would happen.  It went through it's normal 
routine and finally the green lock light came on.  Although the 
oscillator wasn't being controlled by the 9390, the 9390 thought it 
was happy and the DAC went to about 28,000 - very close to ideal.  I 
tweaked the pot hither and yon to move the frequency and to see which 
way the EFC voltage went.  It went the right direction to properly 
steer the oscillator and should have worked.

Pondering the situation, I powered the 9390 down and reconnected the 
9390's EFC voltage back up to see what would happen this time.  This 
time it locked up quickly and the oscillator homed in on 10 MHz.  The 
EFC voltage is 2.15 Volts and the DAC settled in nicely at 28302 - 
very close to the recommended DAC numbers.  It's been cooking for 
about an hour now and is presently at 112E-12 and moving closer all 
the time.  When comparing the two GPS receivers I am not seeing the 
random drifts that I had seen in the past.

I suspect that the problem with the frequency jumping to -18 Hz and 
the DAC going to all zeros was the result of the new oscillator 
over-driving the following circuitry.  In hindsight, maybe I should 
have probed a bit with the scope to see if that is a correct 
assumption, but I didn't do that.

By the way, Stu sells the oscillators fully tested and with a note 
giving the EFC voltage for each unit to tune it to 10 MHz.  If this 
turns out to be as good a modification as it now appears, I'm going 
to get more of the oscillators from him.  I may replace the one in my 
other 9390 and for sure will replace internal reference oscillator in 
my IFR-1500 Service Monitor.

Burt, K6OQK

Earlier, I had written:

I notice that when it first turns on there is a message where I 
normally expect to see "System Check ok."  The message 
reads:  N200/S116/DT202KP.  I don't know what this is saying.

Burt

(Note: additional information about the receiver module has been added.)

Burt

Gang,

You will remember several months ago I had some stranges that I 
thought were related to a defective Vectron oscillator in one of my 
DATUM 9390-52054.  That turned out to be a the internal switching 
power supply so I replaced it with an external Cisco unit.  I've done 
this in three units, two are mine and one belongs to Stu, K6YAZ.

Well, I now have a different grief in one of my units.  It had been 
cooking along swell with no problem ever since I replaced the power 
supply.  This morning I notice that one of my units had the lock and 
tracking lights out.  The display said that the signal level was low 
and there were no usable satellites.  The 10 MHz output is also about 
20 or more dB low.  I assumed the power supply is all right because 
the display was working and it said it was 9-E9, not so good, but 
working - sorta.  I swapped antennas and the good Datum was happy so 
I know the antenna is ok.  when I got inside I checked the power 
supply rails and they're within .05 Volts of where they should 
be.  There is 5 Volts on the GPS module and there is 4.96 Volts on 
the antenna Type-N connector (measured with the antenna line 
disconnected.)  Power cycling the DATUM did not resolve the problem.

Does anyone know if the symptoms above will occur if the 10 MHz 
oscillator is defective?  I can understand it not tracking or not 
locked, but could this cause the receiver to not see or indicate any 
satellites?  Since I only have two of these units on line, I'm very 
reluctant to start swapping modules because I would be without any 
working reference, so I need to keep one up and running.  Both of my 
units are on a UPS, and according to the clocks in the kitchen, the 
bedroom, and the old VCR, we've not had a power interruption.

I do not recognize the GPS receiver module, but it has the following 
number on it:  TNL 22880-B.  I have the schematics for the overall 
DATUM 9390-25054, but the GPS module in just a block.  By the way, 
the GPS block on the DATUM overall schematic is marked, "SV6 / 
(TANS)".  I suspect this means something noteworthy.

Any guidance would be appreciated.

Thanks,

Burt

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at att.net
www.biwa.cc
K6OQK  