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

Chuck Harris cfharris at erols.com
Sat Jan 8 14:22:38 EST 2005


Hi Bill,

The book says that the DAC is capable of a +/- 2.5V swing, then it
says that the control voltage meter reads 1/2 that swing, implying
a +/- 1.25V swing, then it says to compensate for OCXO aging, take
the control voltage meter reading (0 to 5V), and use the graph in
the book to find the DAC resistor that will recenter the DAC.  Well,
my meter shows 3.5V, and the graph shows that for a 3.5V control voltage,
you need a 7.0K resistor for A4R64.  But they don't mark any of the
resistors on the board, and they don't give any parts layout
diagrams... grrr!

Obviously there must be a Volume II for my manual.

I suppose the biggest indication that my oscillator needs some help
is if the control voltage pot, which is there to allow you to run your
standard in a limping mode when the C-Beam quits, is all the way to
one end of its travel when the loop is open, the frequency is just
a tad below 5 MHz.  At the other end of its travel, the frequency is
about 10ppm below 5 MHz.  You can almost, but not quite reach 5 MHz.

OBTW, the DAC has a ovenized voltage reference, LM199, or some such.
I believe the resistor they are talking about skews the DAC range to
fit the proper place in the 10V control range of the OCXO.  But then,
the pot that controls the OCXO in open loop covers the full 10V, so
I guess that still means the OCXO needs to be worked upon.

-Chuck Harris


Bill Hawkins wrote:
> I mentioned the power supplies because everything else may depend
> on those voltages, like a Tektronix scope. OTOH, you would expect
> a separate regulator for the OXCO DAC because of the precision
> required. I didn't say accuracy because the DAC is inside a feedback
> loop, but it does need adequate voltage swing. 3.5 volts seems low
> for the top of the range.
> 
> Of course, the oven has a great deal to do with the center frequency
> of the crystal. If the supply voltage is limited or a heater joint
> has raised in resistance or gone open, then you could not achieve
> normal operation.
> 
> But these are all theories that can be settled by swapping the OXCO
> or the DAC board. You could find the wire that supplies the control
> voltage to the OXCO and try something higher than 3.5 volts, but
> that won't fix anything. It is possible to raise the DAC control
> voltage by one diode drop, using a bias resistor to something higher
> than 5 volts. This must be done carefully without schematics, and
> only if manually raising the OXCO control voltage works.
> 
> Too bad no one knows anything about the 4040.
> 
> Bill Hawkins
> 
> 
> -----Original Message-----
> From: Chuck Harris [mailto:cfharris at erols.com]
> Sent: Saturday, January 08, 2005 10:14 AM
> To: bill at iaxs.net; Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Help with my FTS4050...
> 
> 
> Hi Bill,
> 
> Not yet, but it would be a good idea.  Way back when, when I
> was last working on this unit (10 years ago) they were fine.
> I know that all of the supplies that are external to the FTS5000
> module are ok.  I just opened the case of the '5000 last night
> before bedtime, so I haven't gone too far yet.
> 
> -Chuck Harris
> 
> 
> 
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> 



More information about the time-nuts mailing list