[time-nuts] 9390 GPS RX

Rob Kimberley robkimberley at btinternet.com
Wed May 22 05:15:54 EDT 2013


What is the full model number (from label on rear)? It will be 9390-xxxx.  


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Mark C. Stephens
Sent: 22 May 2013 07:56
To: time-nuts at febo.com
Subject: Re: [time-nuts] 9390 GPS RX

Just a Follow up,

I checked the 10 Mhz output of the 9390 and it was... ~12 MHz.
Uhuh, Wiggling some cables I came across one that when pressure was put on
it, the frequency dropped back to 10Mhz.
The was J9 on the main (logic?) board.

Now I need to mention this particular 9390 has a separate board for the
16.368 oscillator.
>From the photo's I posted earlier, it is the 2nd card in from the front on
the right.
>From earlier posts I have gathered this is normally present on the Trimble
GPS monstrosity.
My Point is, this particular 9390 isn't your average model.

With the frequency now at 10 Mhz I tried a number of different antennae /
terminator combinations and eventually I was able to find a satellite.
Just 1? And it dropped in and out all the time.

Watching the 10 Mhz, it was drifting  +/- 500Hz.

That definitely wouldn't do, so I plugged the house standard into the
(external) time base input and the 9390 10 Mhz output became stable.
After a few minutes, I had 2 satellites! Cool.. 5 minutes we had 4 birds and
we are doing "GPS Correction"

Being the inquisitive sort I went in reverse, unplugging the house standard
from the (external) time base input.
Uhuh, we are back to 1 satellite dropping in and out. The GPS receiver
definitely doesn't like the 10MHz reference wobbling about all over the
place like a drunk sailor on shore leave..

Of course I plugged the House standard back in and we are in business again
with 4 Satellites, locked and loading.
No drop outs either. Interesting.

So My question is, (mostly to myself) Is the Efratom FRK Faulty and the root
of the Drift?
And how do I fix it, I ask myself?

I have downloaded the FRK manual for perusal, I believe there is an external
adjustment to bring it into lock area.
It may also be a problem with the PLL loop, fun stuff because I don't have a

You see, the service manual I was supplied with when I purchased the 9390
isn't for this unit.
It's for the much newer varieties.
There is no mention of this particular Trimble GPS receiver with its
multiple sub-boards nor mention of a FRK oscillator in it.

I did a search of earlier time nuts posts and found there are quite a few
time nuts with 9390's, although there appear to be drastic differences
between the different 9390's.
Perhaps I should be going on the postfix, for instance mine is a 9390-55024.

Bah, I just noticed the GPS day is wrong, I think we are ~142 but its saying
278, Is that a GPS receiver bug?

So, Any helpful navigation hints before I embark on this Journey?

Many thanks,

From: Mark C. Stephens
Sent: Tuesday, 21 May 2013 8:42 PM
To: 'time-nuts at febo.com'
Subject: 9390 GPS RX

Hello Fellow time nuts,

I received a Datum 9390 GPS receiver I bought off eBay today. It had a PSU
fault and I am running it off a bench supply until I can get the PSU working
again. All the PSU does is supplies 28V so I can probably adopt a more
recent supply to fit if I can't fix the old one. I pulled the PSU apart and
so far found a blown triac. There is bound to be more 'things' blown but I
want to see if it's worth repairing first. I mean, it's not much use
repairing something that proves to be useless?

Funnily enough in the back of the Manual binder was a second manual. A
service Manual for a Trimble 5000A GPS/TIME frequency monitor. I'll scan it
if anyone needs it?

This particular 9390 is an odd Chap, doesn't look at all like picture in the
supplied manual.

When I opened it up,
http://www.vk2hmc.net/blog/wp-content/uploads/2013/05/9390-insides.jpg  I
saw an Efratom FRK!

I am having a bit of problem identifying the GPS module. It says Trimble
navigation on the board:
I measured the antenna Jack for the expected 15V normally on a Datum 9390,
but found it was 5.1 volts.
I could not get rid of the Antenna feed error until I put a 50 ohm
terminator on it. I tried higher values (75-180R) but the only value that
cleared the feed error was 50 ohms. So I used a BNC 'tee' and terminated the
output of the GPS distribution amp to the 9390. I fear this GPS RX requires
an antenna with a down-convertor -or- a whacking great L1 signal. I always
thought the 9390 had a Trimble svee6 module in them. But this one's
positively ancient.
I have entered my approximate co-ordinates to see if it finds any birds but
so far no luck.

If you know anything about this GPS board, please leave me a message?

The interface convertor from the Trimble to the Datum:
http://www.vk2hmc.net/blog/wp-content/uploads/2013/05/gps-bd-if.jpg is
labelled "Datum TANS interface".


Complete blog post is here: http://www.vk2hmc.net/blog/?p=178

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