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 <[email protected]> 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. > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
