[time-nuts] 9390 GPS RX

Ed Palmer ed_palmer at sasktel.net
Sun May 26 23:32:54 EDT 2013


On 5/26/2013 8:24 PM, Mark C. Stephens wrote:
> Ed, I'd unplugged the Rb from the counter and into the timer for measurement.
>
> During this time it had dropped out of lock again.
> That's why the Allan Deviation is so bad, it was sweeping.
> But I didn't realise as there was no counter attached!
>
> I need to connect something up to the 'lock' pin for a visual indication of lock to avoid further embarrassments..

I've got manuals for both the FRK and the 'FRK (H or L)'.  The FRK says 
that the lock input can handle up to 70V @ 50 ma.  The other one doesn't 
say.  You should be able to run an LED with a dropping resistor from the 
+24 supply.

> I have cracked it open again and revisited the crystal board.
>
> I found that the centre frequency was a tad low so I upped C12 to 27pf (from 22pf)
> With 6v EFC and the trimmer cap in mid position its dead on 10Mhz.

Remember that it doesn't have to be dead center.  After all, that's why 
the thing sweeps in the first place.  It's looking for the signal and it 
will find it if it's anywhere in the sweep range.

> And I slowed down and waited (thanks Ed..) overnight to see if it drifted off anywhere else.
> But at 10Mhz it has stayed.

Very nice! :)  More thought, less action.  Move as though you're 
defusing a bomb.  Ideally, you know what you want to do, why (with at 
least two seperate tests to confirm your reasoning), and what should 
happen when you make the change.  Adjustments are bad enough, be triple 
sure before you touch the soldering iron.  Yes, it's overkill for most 
situations, but if major dollars or an irreplaceable device are at 
stake, it will pay dividends in the long term.

> I will reconnect EFC later on today (being a work day) when I get some time to move.

Work?  What is this Work thing you speak of?  Oh, yes, I think I do have 
some vague recollection from years past.  I think I tried it but didn't 
care for it.  :)

> I also have the sneaking suspicion the external adjustments have been 'got at'
>
> Looking at the trimmer pot through my mag lamp for c-field adjust, I can see a lot of scratches and wear.

I wouldn't be surprised to find that someone played with the external 
adjustment in a misguided attempt to compensate for the crystal drift.  
Totally ineffective.  The C-field is at the wrong end of the system.

> Hopefully the Oscillator current capacitor C11 and the temperature adjustment hasn't been touched.

You mean for the lamp?  They're pretty hard to get at.  I had to adjust 
the lamp temperature on one unit and had a devil of a time finding a 
screwdriver that could reach it.  The reading on the lamp voltage 
monitor will tell you if those adjustments are correct. Remember, move 
slowly and carefully.  :)

> Anyway, Wish me luck ;)

Always :),

Ed





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