[time-nuts] Re: HP Z3801A project update

[John Ackermann N8UR]

I recently fired up my old Z3801As and ran across a similar problem. 
Art Sepin helpfully reminded me that the old Motorola receivers' 
on-board oscillators drift with time and after a long power off the 
oscillator may drift so far that it can't lock.  The receiver stores 
calibration data in eeprom and after a long power down that data no 
longer works due to crystal aging, etc.

The symptom is that on power up the receiver doesn't seem to get past 
acquiring and periodically losing one or two satellites.  The cure is 
simple -- just leave it running (connected to an antenna) and come back 
a few hours later.  It should be locked, and will have recalibrated and 
stored new constants in eeprom, so you won't see the problem again until 
after another prolonged power-down period.

John
----
On 2/24/22 12:14 AM, ed breya wrote:
> I found a spare Oncore VP GPS RX unit in the parts department last week, 
> and tried it out in the Z3801A. It would not track, even after quite 
> some time in use. The original would pop right up almost immediately, 
> after warm up.This one had some slight differences from the original, 
> around the antenna connector area. It was originally rigged up with the 
> rechargeable Li cell option - I remember removing it years ago. I added 
> a jumper to carry the external battery supply line in, to make it the 
> same as the other. After several swap-outs with the good one, with no 
> improvement, I did a direct comparison, and deleted a couple of parts 
> from the bad one, making it look "exactly" like the good one. I 
> suspected the problem was in the memory chip enable/reset circuit, or 
> the left over Li cell charging part.
> 
> On this last round of checking, it acted just like before, no good. It 
> seemed to operate, and all parts showed OK in SatStat, but it just would 
> not track any satellites, even though it seemed to recognize and list 
> them. I checked the antenna power and such, and all looked OK.
> 
> I was even planning to send a message about this to the group, asking 
> for ideas, but finally, after quite some time (maybe an hour), it seemed 
> to miraculously come alive, and showed satellite tracking. My theory is 
> that either the power up reset wasn't working, or the memory was so 
> corrupted (yet remembered enough) from all the different trials and part 
> changes, that it just took a while to gradually clear enough of it to 
> work. During earlier testing, I found responses to queries like 
> "invalid/data corrupted," to paraphrase.
> 
> So anyway, it seems to be working right now. I'll have to put through 
> some power up and down tests and such, to make sure it really is good to 
> go. It's comforting to have a spare of this part, since it appears 
> replacements are getting hard to find.
> 
> Ed
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