[time-nuts] Help with my FTS4050...

[Chuck Harris]

Hi Tom,

At the moment, my time/frequency equiptment is all boxed
up and stored away.  Listening to all this banter about
4060's made me curious, and I dragged my 4050 out just to
see what was left of it.  As it happens, I have a spare
FTS5000 C-Beam module.  I don't know what shape it's OCXO
is in, but I do know that the spare module has a well burned
up powersupply board, so it wouldn't surprise me to find
that the OCXO is in perfect shape.

I would like to fix the off frequency OCXO.  It bugs me
when things can't be used because of trivial things like
this.  The factory was able to set the gross frequency
of this crystal, so there must be a way to do it.  I wish
I could find someone who has done this before... Maybe I
will have to be that someone.

Thanks!

-Chuck Harris

Tom Van Baak wrote:
> Chuck,
> 
> Nice work, especially the Zeeman check. Sounds like
> all the important parts of your 4050 are working fine.
> 
> If you can calculate the dF/dV for the OCXO in the 4050
> you may be able to replace it with a similar OCXO, e.g.,
> a HP 10811. A complication would be the 4050 needs
> 3 or 4 outputs from the OCXO, right?
> 
> Do you have a Rb or GPS-referenced frequency counter
> handy? It would not be hard to measure the two ends of
> the OCXO EFC range. That would not only give you the
> EFC gain but also confirm the OCXO is off-frequency.
> 
> I have some spare 4050 OCXO here and would consider
> swapping with you so you can get your 4050 running.
> Unless the ones I have here are also fatally off-frequency.
> 
> /tvb
> 
> ----- Original Message -----
> From: "Chuck Harris" <cfharris at erols.com>
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Sent: Saturday, January 08, 2005 07:01
> Subject: [time-nuts] Help with my FTS4050...
> 
> 
> 
>>Well, I dragged my FTS 4050 off of the shelf, and back to the bench
>>to see what was left of it.  It hadn't been turned on since it quit
>>in 1995.  It powered up ok, and the ovens stabilized at the proper
>>values, and the beam current returned to where it was when things were
>>normal.  It still won't lock, and the control voltage is stuck at 3.5V
>>which I am pretty sure is its attempt to pull the OCXO up in frequency.
>>The OCXO voltage control pot is fully CCW, and the frequency out is
>>just a tiny bit below 5 MHz... All consistent with an OCXO that has
>>exceeded its adjustment range for aging drift.
>>
>>Ok, so what to do?  The only manual I have is an operation and alignment
>>manual for the FTS5000/100 module that is the guts of the 4050 frame,
>>and it talks about selecting a resistor: A4R64 to bring the DAC back
>>to the center of its range.  It has a table that says the nominal
>>value is 14.7K, and for a unit that has its control voltage at 3.5V,
>>I should replace A4R64 with an 8K resistor.  The only problem is I don't
>>have the schematics/drawings book for the module, so I have no idea
>>which resistor is A4R64.  They say it is mounted on a "bifurcated"
> 
> terminal,
> 
>>but there are no such terminals on the DAC (A4) board.
>>
>>I think the C-Beam tube is working fine, as when I put a
>>Zeeman frequency of 42.820KHz, 1Vrms into the Zeeman port, the beam
>>current goes into a fairly deep dip, and both the amplitude and
>>frequency of the Zeeman signal change the dip's depth, and so does
>>the thumbwheel switches (though very slightly).
>>
>>I suspect that all this source needs to be back up and running is
>>for me to adjust the OCXO to a new center frequency.  I remember
>>trying to do this back 10 years ago when it first quit, and finding
>>that the oven has no external adjustments.  I also recall that the
>>guts of the oven are stuffed into a vacuum bottle, and it looked
>>really scary to try and disassemble the unit.
>>
>>Any ideas??
>>
>>Thanks!
>>
>>-Chuck Harris
>>
>>Chuck Harris wrote:
>>
>>>My 4060 was working like that for a while, but eventially the green
>>>light wouldn't light anymore.  What I found was the OCXO had aged to
>>>a point where its frequency could no longer be dragged back to the
>>>proper frequency with the voltage control.  I know there is an
> 
> adjustment
> 
>>>available inside the oven, but I never got around to finding and fixing
>>>it.  (anyone know how to adjust the crystal osc. without ripping it
>>>completely apart?)
>>>
>>>-Chuck Harris
>>>
>>
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>>
> 
> 
> 
> 
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