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

[Stewart Cobb]

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.