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

Sat Jan 8 17:17:09 UTC 2005

I just opened up my spare burned out FTS5000, and removed
its OCXO.  It uses a FTS1200 for that function.  I took a peek
inside, and it is as I remembered it.

The oscillator is housed in a small glass vacuum bottle of
the type used in thermos bottles, or dewars.  The bottle is
foamed into the rectangular metal outer box of the unit.
The top of the oscillator is the oven gear: a power transistor,
and a thermistor setup to cook at a non adjustable temperature.

There are no capacitor adjustments accessable from the open
end of the unit.  I would suspect that you have to remove the
quartz oscillator from the vacuum thermos bottle to get at
the trimmer.  That could, of course, be very simple, or be
very much like opening a can of worms!

It would appear that FTS had no intentions of making the
oscillator mechanically adjustable without complete disassembly.
They probably figured that the internal varactors would
allow enough adjustment to handle any aging.  Maybe they
do?  I have no idea where any of the test points are in
the FTS5000 c-beam unit, because my manual is only words,
with no pictures or schematics.

I am somewhat reluctant to just plug in the "spare" OCXO,
as something burned up the powersupply in this junk '5000
unit.  It could easily have been this OCXO (though it
is clean and sweet smelling inside).

And then, even if I do plug in the spare OCXO, I have no idea
where the various DAC and other trimming adjustments are in
the '5000, as I said, my manual is only words and tables,
no pictures or schematics.

-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
>>>
>>
>>_______________________________________________
>>time-nuts mailing list
>>time-nuts at febo.com
>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>
>>
> 
> 
> 
> 
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> 