[time-nuts] Improved Symmetricom ET-6000...

Burt I. Weiner biwa at att.net
Mon Oct 17 13:32:37 EDT 2016


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 <rv2p800 at yahoo.no>

Subject: [time-nuts] Improved Symmetricom ET-6000
Good evening/morning -depending, group :-)
  For the perfectionists of you, pass this post 
;-)Â  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.-Â  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:Â  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 :-)

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California U.S.A.
biwa at att.net

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