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

Brooke Clarke brooke at pacific.net
Sat Jan 8 18:57:05 UTC 2005


Hi Chuck:

Can you make a digital high resolution photo of the A4 PCB?

Brooke

Chuck Harris wrote:

> 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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