[time-nuts] Tuning a Symmetricom 1111C

Ed Palmer ed_palmer at sasktel.net
Sat Jul 8 14:03:48 EDT 2017

Hi Tom,

I have a Datum 4065A standard with a Datum 1111B oscillator.  I've never 
had to tune it, but it appears to be a clone of the HP 10811 so you 
might get some insights by researching that oscillator. However, if 
you're having issues with the +-15V supplies, I'd STRONGLY suggest that 
you resolve those before attempting to retune the oscillator.

Circuit information for these units isn't available, but reading between 
the lines, I note the following:
- My unit uses an AD9713BAP D/A converter to drive the EFC.  It requires 
+-5V supplies.
- The system monitors +5V, +15V, and -15V, but not -5V so where is -5V 
coming from?  I see an LM320-5V on the mainboard near the D/A converter.

My first guess is that -5V is derived from the -15V supply.  If that's 
true, flaky +-15V supplies will almost guarantee a flaky EFC.  Since 
your unit was able to lock once, it further suggests that retuning isn't 

There's a power supply board bolted to one of the side panels that uses 
a couple of DC-DC bricks.  I found some bad solder joints on that 
board.  I can't remember the details.  You might want to check that out.

Since these units are microprocessor controlled and include multiple 
tests and measurements to ensure that the lock is on the correct signal 
peak I don't think it's going to lock to the wrong peak.

You should also be aware that these units use a STEL-1173 chip to drive 
the D/A converter.  This chip is infamous for failing after a few 
years.  If you search the archives you'll find more info on that topic.  
I found a source for this chip a few years ago, but it looks like that 
source has dried up.


On 2017-07-08 10:00 AM, Tom Knox <actast at hotmail.com> wrote:
> Hi All;
> Does anyone have a data sheet or experience with the Symmetricom 1111C, I have one in a Symmetricom 4065C and the internal diagnostics say it is at the end of it's tuning range. Is there a way to do a coarse manual adjustment, or is it a possibility the Cesium is locking to a side peak? In addition the +- 15 volt supplies have failed to switch on twice during testing, but have switch on after rebooting. On one reboot it locked quickly.
> Thanks;
> Thomas Knox

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