[time-nuts] Ball/Efratom MFS-209 Rubidium GPSDO...

[Chuck Harris]

It's Alive!!!

The MGPS module was saying there was an A-to-D fault
and a Feedline fault.  Suspicious of the common element,
I traced the signal path from the GPS antenna connector
to the input of the ADC, and it was a straight DC path.
Sure, there were a few chokes, and capacitors to ground
to filter out any stray 1.5GHz signal, and to keep the DC
circuitry from loading the antenna signal.

I ordered up a new ADC converter, which was an ADC0808 by
National and TI.  A small 28 pin quad J lead surface mount
package.  It finally came, and I swapped it out this
afternoon.

I was rather worried, as the failure mode could have been
simply a faulty converter, or it could be that something
happened on the antenna line that killed the converter.

It was not evident how the GPS module knew there was an
A-to-D fault.  It could have been something like a lack
of an end of conversion flag, or they could have been
using some extras of the 8 analog inputs to measure things
like power, ground, resistor dividers... I really don't
know...

Powering the GPS module on the bench showed an immediate
improvement:  The A-to-D converter fault condition was
no longer set!  It was still showing an antenna line fault,
but since I didn't have an antenna connected, that was to
be expected.

I put the GPS module back into the MGPS chassis, and
installed it in the MFS-209, and as soon as I connected
a good antenna, the antenna line fault indicator went
away.... So far so good.

Some satellites started appearing on the display, and after
waiting for what seemed like forever, and the almanac was
finally updated.

And...

The system status changed to GPS Lock!  And reports normal
operation.

Now to let it cook for a few weeks and see how well this
ancient GPS RbDO performs.

Thanks to all that offered tips and moral support.

-Chuck Harris

Chuck Harris wrote:
> And as I feared, the MGPS display is telling the truth, there
> is something wrong with the GPS module's ability to check the
> feed line for shorts and opens, and something wrong with the
> motherboard's ADC module.
>
> At least the communication between the MGPS and the GPS engine
> is working properly...
>
> -Chuck Harris