[time-nuts] repairing General Technology (Tracor) 304-B rubidium standard

Ed Palmer ed_palmer at sasktel.net
Tue Feb 19 13:36:29 EST 2013


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.

> The main power supply voltage on my unit seems to have been deliberately
> adjusted lower than spec (18.54 V actual, versus 20 +/- 0.1V specified in
> the manual).  Replacing a resistor on the regulator board (that had smoked
> from overload due to the low voltage) didn't change the voltage much.  I
> had to crank the trimmer across half of its range to get the voltage back
> within spec.  Nothing in the regulator circuitry seemed to have drifted
> enough to change the setpoint that much.  Is there a reason why a tech
> would have deliberately set this voltage lower than spec, or did it just
> drift down over the years?

I can't think why the voltage would be intentionally adjusted down. 
Probably just drift.

> 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.

> 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.

> Obviously, the box won't lock until the RF input is the right level.  But
> it also requires the Rb lamp to light.  Corby Dawson posted to the list
> back on 12 November 2009:
>
> "Tracor bulbs fail with a different mechanism and last maybe 10 years."
>
> Anyone know what that "different" failure mechanism is?  Is it repairable
> in an ordinary lab, like the heat-gun trick for LPRO bulbs?  If not, is it
> feasible to build a "Frankenstein" replacement using something like an LPRO
> or FEI bulb?
>
> Is it possible to tell whether the lamp is lit without opening the physics
> package?  If not, are there any tricks to opening the physics package?  Any
> precautions to take before doing so?

No, you have to open the physics package to see the lamp. Disassembly is 
fairly straightforward.  The photocell, preamp, and 70 MHz amp are at 
the end where the cable exits the unit.  The lamp is at the other end.  
There's enough slack in the cables that you can remove everything except 
the preamp without unsoldering cables. The leads from the photocell to 
the preamp are the exception.

As Corby mentioned, the lamp uses a high voltage pulse to ignite the 
lamp.  There's a hair-thin wire to carry that voltage to the lamp. If 
you touch the wire, it will probably fall off - unless it's already done 
so.  I was able to replace it so don't panic.

Corby also mentioned the relay click.  That will only occur during the 
automatic sweep.  So the lamp will only light after it's warmed up and 
even then only when the sweep circuit starts it's sweep at the lowest 
frequency.  That's when the relay clicks.  Once the lamp test point has 
stabilized and the sweep has cycled once or twice, you can disable the 
sweep if you want to do other tests.

> 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!  ;-)

Ed

> Cheers!
> --Stu
>
> Side note:  This unit was built during the era of "elastic seconds"
> (roughly, the 1960's).  It contains a board (A9) which digitally offsets
> the output frequency in increments of roughly 7E-10, without changing the
> rubidium resonance frequency or the C-field.  There's also a note in the
> manual saying that annual changes to the definition of the second may
> require replacing the rubidium resonance cell in the physics package with a
> new cell calibrated for the new second in the new year.  Leap seconds bring
> their own problems, but compared to dismantling your lab instruments every
> year, they're a breeze.




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