On 2/19/2013 12:54 PM, Magnus Danielson wrote:
On 19/02/13 19:36, Ed Palmer wrote:
Hi Stu,
Did you find the long message threads from April & May of last year
where I was trying to fix my 304-B? Look for "Antique Rubidium
Standard", "General Technology Corp model 304b" and "Antique Rb
Standard". Lots of good info from guys like Magnus Danielson, Paul Swed,
Ed Breya and others.
To cut to the chase, no I haven't got it working. The second harmonic is
still missing. I haven't given up. I still have a few puzzles to work
out. I work on it for awhile, decide I need more equipment, wait for it
to show up on the auction site, wait for it to be delivered, fix it
and/or learn about it, make some measurements, get frustrated and walk
away for awhile, repeat until wallet is empty and workshop is full. :-)
Answers to your current questions are interspersed below. We'll talk
more, but it's probably best if we take it offlist. I'll contact you
directly.
On 2/19/2013 12:36 AM, Stewart Cobb wrote:
Guys,
I'm repairing a 1960's vintage lab-grade rubidium standard, General
Technology Corporation model 304-B. Apparently Tracor bought GTC soon
after this unit was made, because references to this as a "Tracor
304-B"
seem to be more common. I've made some progress, but now it seems like
time to consult the hive mind.
The unit appears clean, but it doesn't lock. I've read through old
comments on the list regarding this unit, and I've downloaded a copy
of the
manual and schematics available at
<*http://sundry.i2phd.com/ServiceManual_304b.pdf>*
That file seems to contain a complete copy of the manual text, but some
schematics are missing. In particular, the schematics for the
sweep/acquisition board (A8) and the three boards inside the physics
package (the lamp oscillator (A13), the SRD driver (A12), and the
photocell
preamp (A11)) are not shown. Does anyone know where to find copies of
those schematics?
I have a hard copy of the manual. I'll scan those schematics and send
them to you.
Could you put them on a suitable server?
I don't know what server would be suitable. It's not like there are
hordes of people who are desperately waiting to get their hands on this
manual. Any suggestions?
A frequency counter (GPSDO reference) shows that the crystal oven
warms up
as expected. The output can be centered on 5 MHz and the sweep circuit
covers a symmetrical range around 5 MHz as expected.
Really? That surprises me. Drift in the OCXO should have caused an issue
over this much time. On my unit, some kind soul adjusted the oscillator
via the adjustment on the oscillator itself. Unfortunately, that changes
the oven temperature which does change the frequency, but it also moves
the oscillator away from the crystal's turnover point. It still works,
but for proper operation, the oscillator has to be opened and components
changed to bring the oscillator back to it's proper frequency at the
proper temperature.
The ovens for the
lamp and filter cell appear to warm up properly as well, judging from
test
points available on the A1 oven controller board. The test point
voltages
don't quite match the ones in the PDF manual, but it looks like those
readings were typed into each individual manual after being read off
the
particular unit that came with that manual.
That's one of the remaining puzzles on my unit. It appears that one of
the thermistors on my unit has drifted badly and the RF cavity (and
therefore the resonance cell) are running about 20C colder than they
should. My next round of testing will investigate that.
That would indeed help to keep the Rb-85 and Rb-87 peaks misaligned,
with weak return signal as result.
I tested the temperature early in my investigations. Raising or
lowering the temperature reduced the maximum amplitude of the error
signal so I left it alone. But the current temperature of 56C seemed
suspiciously low. The correct temperature isn't stated in the manual.
I was finally able to find the data for the thermistor and the current
hot resistance should correlate to a temperature of ~72C which seems
much more reasonable. So I'm going to boost the temperature to that
level and then go hunting for a signal.
The test point on the A5 board shows that 155 Hz resonance detector
modulation is within spec. The A6 filter-amplifier board test points
show
the system attempting (and failing) to detect 155 Hz and 310 Hz
resonance
signals coming back from the photocell.
The manual says that the A7 RF pre-driver board (the x14 multiplier)
should
be supplying 70 MHz at +13 dBm to the SRD driver inside the physics
package. That would be about 2.8Vpp, assuming a 50-ohm system. Instead,
it's supplying a clean 70 MHz at about 100mV into a 50-ohm load. My
best
guess is that the final amplifier transistor on that board is blown,
possibly from being operated with only a scope probe as a load
(infinite
VSWR). Replacement transistors are on order. Any other thoughts?
I've measured my A7 output at +12 dBm. I've also found that the level
isn't critical. Levels as low as +5 dBm didn't reduce the amplitude of
the error signal.
Once sufficient, levels should be set somewhat below peaking, and
preferably for best stability. Signal level shift causes frequency shift.
Is that drive level to the SRD or signal level in the cavity at 6.834 GHz?
It's a bit obscure actually. Anyway, if you can have a servo-loop to
maintain a stable level over time, it will reduce the drift.
I don't think that this unit has that kind of servo-loop. It's from
1969 so the technology might not have reached that level yet.
Any other comments on how to get this box working again?
Invest copious amounts of time, money, and frustration. Odds of success
are unknown, but probably not great. The effort will either keep you out
of the bar or drive you to it. But you will learn lots about how a Rb
standard works! ;-)
Indeed. You guys are lucky, on one of my rubidiums I have a bit of a
challenge, as the schematics and much of the documentation is in russian.
When I'm reading documentation or, in particular, the research papers on
this stuff, I often start to wonder what language these people are
speaking. It makes my head hurt! :-)
Ed
Essentially the same concepts thought, so I just need to concentrate
some more.
Cheers,
Magnus
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