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

Magnus Danielson magnus at rubidium.dyndns.org
Tue Feb 19 06:25:59 EST 2013

On 19/02/13 07:36, 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?
> 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?

Trimming might have drifted. OK signal in? On a HP5060 (similar vintage) 
I had to start by tuning the crystal filter of the OCXO to get a strong 
5 MHz. I ended up having trouble with the multiplier board.

> 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?

The photodetector should detect the lamp intensity as a DC component. 
The modulation doesn't cut out much of the light, so the 155 Hz and 310 
Hz is there only when things are properly aligned.

The tricky part is that not only the frequency needs to be aligned, but 
also the temperature of the lamp with the temperature of the filter and 
resonance cell. Check the oven settings on it. The temperature alignment 
is needed to make the optical pumping align up and work well, if you 
don't too little pumping is achieved, and too few atomis is in the right 
state to later alter state with your applied signal and hence gives a 
much weaker signal.

> Any other comments on how to get this box working again?

Check the return signal yourself. It could be that you have the 155 Hz 
and 310 Hz, but you need to re-align the phase of the synchronous 
detector, but this only comes after getting the 70 MHz operating 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.

The R&S XSRM is built such that you can take out the lamp without even 
powering anything down. There is a tool for it with comes with the 
module. That's how they should be built... unless they are built like my 
KVARZ rubidium, in which they have built a rejuvination mechanism into 
the lamp to start with, after analysing the issue.


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