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

Sat Apr 13 05:42:38 EDT 2013

Burt, what is your DAC setting now it's stabilised? It is normally set so it
is mid-range when setting these units up.
Rob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Burt I. Weiner
Sent: 12 April 2013 19:49
To: time-nuts at febo.com
Subject: [time-nuts] UPDATE: DATUM 9390-52054 Grief again...

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=item2c6dc
bc457#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  

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