[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://firstname.lastname@example.org/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.
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.Â 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.Â 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.Â The new Bliley unit, for the sake of the
experiment, was set outside the main PCB with
long wires soldered between the units.Â 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....Â 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.Â This
improved things a bit, I now can see a distorted
sinus wave on the output....Â 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?Â 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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