[time-nuts] Temex LPFRS-01 / LPRO rubidium reference repair

Ed Palmer ed_palmer at sasktel.net
Sun May 26 16:10:04 EDT 2013

Hi Gerd,

When it locks for a few seconds, make a note of the VCXO control 
voltage.  Watch it as closely as you can during the lock period.  Is it 
very near one end of the sweep range?  Does it then drift off the end of 
the range when lock is lost?  That behaviour would suggest that the VCXO 
has drifted out of range as Bob suggested.  But if the voltage stays 
more towards the middle of the range (like 4D, 4C) that's not going to 
be the problem.  You could also watch the output frequency since it 
tracks the frequency.  Whichever is easier.

If you can run it while it's open, check the temperatures of the lamp 
and the cavity as best you can.  I see that they're specified in the 
manual.  Also, check for obvious things like bad capacitors. Living next 
to that physics package must be a hard life for any capacitor!  The 
negative terminal on the black electrolytic near the flex circuit looks 
a little strange, but that could just be a trick of the light.


On 5/26/2013 1:12 PM, Gerd v. Egidy wrote:
> Hi Ed,
>> I don't have an LPFRS, but if I'm reading the manual correctly, the
>> fourth column of numbers on the 'M' command (4D, 4C on your screen
>> capture) is the VCXO control voltage.  Shouldn't this be continuously
>> sweeping up and down until it finds lock?
> You are right, it sweeps the control voltage up and down and the output
> frequency changes. But that is only happening during warmup. Once the unit had
> this short lock for a few seconds, it stops sweeping and does not start again
> even after losing the lock. Maybe a firmware issue not taking aging into
> account or something.
> The terminal output in my screenshot is from the time after the short lock, so
> you don't see any sweeping in it.
>> Does the output frequency
>> sweep up and down and is 10 MHz somewhere in the sweep?
> It sweeps and is near 10 MHz, but I don't know if 10 MHz is within the
> sweeping range or just near it. Unfortunately I don't have another reference
> reliable enough to tell - that was why I bought this unit in the first place...
> Bobs suggestion in the other posts goes in the same direction.
> There is a trimmer cap and several trimpots on the board. I think I'll take a
> look at the board and try to figure out what exactly they are doing before
> trying to tweak them.
>> The first column (Photocell DC voltage) shows that you've
>> got lots of light from the lamp.
> That is why I have hope to fix this thing, a burnt out lamp would be hard to
> replace.
> Kind regards,
> Gerd

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