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

Ed Palmer ed_palmer at sasktel.net
Wed Feb 20 10:46:39 EST 2013


On 2/19/2013 12:54 PM, Magnus Danielson wrote:
> 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?

I don't know what server would be suitable.  It's not like there are 
hordes of people who are desperately waiting to get their hands on this 
manual.  Any suggestions?

>>> 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.

I tested the temperature early in my investigations.  Raising or 
lowering the temperature reduced the maximum amplitude of the error 
signal so I left it alone.  But the current temperature of 56C seemed 
suspiciously low.  The correct temperature isn't stated in the manual.  
I was finally able to find the data for the thermistor and the current 
hot resistance should correlate to a temperature of ~72C which seems 
much more reasonable.  So I'm going to boost the temperature to that 
level and then go hunting for a signal.

>>> 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.

Is that drive level to the SRD or signal level in the cavity at 6.834 GHz?

> 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.

I don't think that this unit has that kind of servo-loop.  It's from 
1969 so the technology might not have reached that level yet.

>>> 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. 

When I'm reading documentation or, in particular, the research papers on 
this stuff, I often start to wonder what language these people are 
speaking.  It makes my head hurt!  :-)

Ed

> Essentially the same concepts thought, so I just need to concentrate 
> some more.
>
> Cheers,
> Magnus
>



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