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

[Bob Camp]

Hi

Keep in mind that the Tracor's came out before the Teflon coated bulbs. Without the Teflon, the Rb will combine with the glass of the bulb. The ones I have seen slowly turn a black. As they darken, the light transmission drops enough to stop the standard. Since it's a chemical reaction, there is no simple way to reverse it.

Bob
   
On Feb 19, 2013, at 1:36 AM, Stewart Cobb <stewart.cobb at gmail.com> 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?
> 
> 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?
> 
> 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.  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.
> 
> 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?
> 
> 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?
> 
> Any other comments on how to get this box working again?
> 
> 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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