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

[Magnus Danielson]

On 19/02/13 19:36, Ed Palmer wrote:
> 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.

Could you put them on a suitable server?

>> 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.

That would indeed help to keep the Rb-85 and Rb-87 peaks misaligned, 
with weak return signal as result.

>> 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.

Once sufficient, levels should be set somewhat below peaking, and 
preferably for best stability. Signal level shift causes frequency 
shift. It's a bit obscure actually. Anyway, if you can have a servo-loop 
to maintain a stable level over time, it will reduce the drift.

>> 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! ;-)

Indeed. You guys are lucky, on one of my rubidiums I have a bit of a 
challenge, as the schematics and much of the documentation is in 
russian. Essentially the same concepts thought, so I just need to 
concentrate some more.

Cheers,
Magnus