Erik,

For what it may be worth... I have two DATUM 9390-52054 GPS Frequency and Time Standards. A few years ago I had to replace the original Vectron oscillators in one of the units and eventually in the other one. In my case I used a McCoy VCXO that's in an oven. This McCoy oscillator, being about 1/4 the size of the Vectron did not match the original wholes in the circuit board, so like you, I had to extend the wires from the circuit board to the oscillator module, which I mounded upside down in the same location that the Vectron occupied. I also mounted it inside of a foam block that I cut out to enclose the McCoy and trimmed to fit in the same space as the original oscillator and as tall as the vertical real estate would allow. The top cover of my DATUM's push down on the top half of the foam giving it a pretty good seal.


I found that while the EFC control voltage for the McCoy was in the right direction and and the desired tuning voltage center very close to the center that the original Vectron used, the McCoy was much more sensitive to EFC voltage change and what the Vectron seemed to be happy with would drive the McCoy out of range into never-never land. I needed a bigger "Flywheel." In my case I wound up adding a series 18k resistor in series with the EFC source to the EFC terminal on the oscillator and a 47uf capacitor from the EFC terminal to the ground terminal on the McCoy. This made for a much larger "Flywheel" and tamed things down quite nicely. You may have a similar issue. Since doing this on my two units I have had no further problems, even when powering them down and doing a cold reset on them, they come back up nicely and behave well. Normally they are connected to a UPS.

I documented this all in this group. See: https://www.mail-archive.com/time-nuts@febo.com/msg73790.html

I took a bunch of pictures of what I did and I'd be happy to share them. I'll be out of town for about two weeks so if I'm slow in answering, that's why.

I hope this offers some help based on my experience.

Have fun,

Burt, K6OQK


From: Erik Thomassen <rv2p...@yahoo.no>

Subject: [time-nuts] Improved Symmetricom ET-6000
Good evening/morning -depending, group :-)
For the perfectionists of you, pass this post ;-) For the rest, here are my results in improving an Symmetricom Exacttime ET-6000. My problem was that the supposedly stable 10MHz, off the programmable outputs just wandered "all over the place" with the supplied TCXO. Refered to any free running OCXO or rubidium the output was useless even in the locked state.

There was an upgrade-set avaliable that also involved a software change but that was not an option for me.

The original TCXO is relatively easy to remove. The solder holes are left alone, better connection points for experiments exists on the PCB. Check for connectivity the 10MHz output, and Vcorr. The 5V connection i left alone.Before this project arose i had a 5V/10MHz Bliley OCXO unit previously bought off Ebay from China for some USD20.- I did not think about the output being square wave.... The main thing however is that this OCXO matches the Vref range of the TCXO.

Yesterday was the day: Took out the original TCXO. The new Bliley unit, for the sake of the experiment, was set outside the main PCB with long wires soldered between the units. Due to the higher current drain on the OCXO i decided to supply it via an external simple 7805 regulator. This was mounted directly to the bottom of the chassis for good heat dispersal during warmup. 12V was taken from the main PCB.

After power-up things looked usable - but did not work.... No lock, the system searching up to+/- 200Hz trying to find a lock point. Setting the DAC-value to "zero beat" with a similar standard did not help.The combination of quite higher output voltage from the OCXO combined with a very distorted waveform (due to the square-wave output) made the 6000-unit to discard the OCXO.Â

To improve the waveform I used an old trick: A 10MHz crystal in series with the signal. This improved things a bit, I now can see a distorted sinus wave on the output.... A 50k 10-turn potmeter in series with the crystal solved the level problem -.

A new connector mounted on the rear for 24VDC supply and an surplus Rifa 24V to 5/12/-12V converter module from a Instrument Landing System transmitter solved the mains problem.... No more power surges :-)

Inserting a jumper (J27) told at least the display that there is an OCXO in place. After some 6 hours of use now, the internal monitoring system claims that output accuracy for the time beeing is +/- 40 e-12, almost two decades better than the original TCXO, for all what that statement is worth.... The next question coming up: Why not use a simple 12V supply, sinus output OCXO? Yes, why not try that? With the level problems sorted out - a simple potentiometer, the only problem withstanding is the Vcorr/ref voltage. It may have altered with the setting og J27 - I did not measure that, there, the software change probably kics in - or not :-)
Erik/

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California U.S.A.
b...@att.net
K6OQK
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