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
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