[time-nuts] 5061A Insufficient Cs Beam Current Modulation (long)

[Michael Ulbrich]

Hi Paul,

thanks a lot for your advice!

I hadn't looked into the c-field adjustment yet, since I considered this
to be the final touch on an otherwise correctly functioning and well
aligned instrument. Question is whether this 5061A can ever be brought
back near that target. If so, there's still some way to go, I guess ... ;-)

Tried the c-field adjustment nonetheless and it worked out as expected.
Found three peaks over the tuning range of the c-field dial and set to
highest middle peak. At 6.1 the dial reads a bit outside the expected
range of 4.0 to 6.0, but that shouldn't be something to worry about.

Basic situation didn't change any: No 274 Hz signal and tiny 137 Hz
signal mostly buried in noise from the AC amplifier with gain HI and
loop gain nearly at full cw. Instrument somehow tracks phase but control
reading tends to wander, no lock, no stable operation.

Will post my findings if I make any progress.

Best regards ... Michael U.

On 07/03/20 22:56, paul swed wrote:
> Michael bringing old Cesiums back to life is interesting business.
> I have one thing to check. The c field settings. If the tubes been changed
> that absolutely matters. Usually a resistor on the regulator baord of the
> oven controller. I had a different tube in a unit and the straps were not
> changed to match. It would lock and such. But was always slightly off
> frequency.
> Regards
> Paul
>
> On Sat, Mar 7, 2020 at 3:00 PM Michael Ulbrich <mul at rentapacs.de> wrote:
>
>> Dear time-nuts,
>>
>> I've had the chance to work on a 5061A over the last couple of days. I
>> guess it's a fairly old unit with "singing" oven controller and 105 5
>> MHz crystal oven assembly. Last digits from serial number are "0964" -
>> not sure if that indicates the date of mfg. It definitely has been
>> tampered with before as evident from some missing screws, a module
>> cover  and pulled coaxial interconnects. All considered it's in a quite
>> miserable state, but anyway - I couldn't resist taking the opportunity
>> and started fiddling with the unit.
>>
>> Basic checks revealed a standard unit (no options) with a standard beam
>> tube 05061-6077 and a complete set of modules. Probability that it's not
>> the original CBT is quite high since the screws on the hold-down straps
>> were loose and the oven controller taps for transformer T4 were not
>> wired according to the tube label (that I actually found out much
>> later). Uhm, and - most obvious - the -2500V connection did not fit
>> since both plug and chassis socket are of the same - female - type ...
>> but a carefully placed clip lead actually worked ok here.
>>
>> The owner had told me that the unit must have been stored for years,
>> maybe even more than a decade ...? So I was positively surprised when I
>> found out that it pumped down the cesium tube within a day and the unit
>> stays on since with the Cs oven enabled and an ion current about 2 ticks
>> from zero. OSC oven and Cs oven meter readings come down within 30
>> minutes from powering up and stay within reasonable limits. 5 MHz
>> reading is present and rises until crystal oven has reached its
>> operating temperature.
>>
>> Initially I could get only a faint reading in the MULT position of the
>> circuit check switch. Frequencies and levels from the Multiplier A3 and
>> Synthesizer A1 are ok although levels seem a tad on the low side. 137 Hz
>> side bands of 90 MHz are visible on an 89441 vector analyzer if MOD is
>> switched on. As a final resort I started to tweak the pots in the A4
>> harmonic generator and got the MULT reading up to about 20 on the meter
>> scale. That seemed sufficient for the moment.
>>
>> Next step "BEAM I" - again no deflection on the meter scale. Using an
>> external known standard I set the internal crystal OSC close to 5 MHz
>> and (literally) grabbed the magnifying glass to watch the meter needle
>> while tuning the coarse frequency adjust - and there it finally was: a
>> tiny movement showing a minuscule peak of about one tick deflection with
>> barely noticeable adjacent peaks of half that size! Hahaa ... here we
>> go! It took quite some time (and manual reading) to find the "Beam I
>> Meter" adjust accessible from the front panel. With that I was able to
>> bring up the resonant peak meter reading to 35 or so, with clearly
>> visible valleys and lower peaks on both sides of the main resonance.
>>
>> Now on to "OPER" and closing the loop with 137 Hz modulation on. No
>> deflection in position 2nd harmonic. The manual led me to AC amplifier
>> A7 which was thoroughly checked and found without fault. No 2nd harmonic
>> at test output A7J2 and no fundamental at A7J6. Switching A7
>> amplification to HI and cranking up loop gain does help in a certain but
>> not satisfactory way. Now I get a 2nd harmonic reading in the
>> 20-somethings but this obviously is just noise since the meter reading
>> isn't steady and it does not depend on modulation being on or off. A7J6
>> shows a narrow-band filtered signal with strong components around 137 Hz
>> but these also are mostly of random nature as excited by the amplified
>> CBT and amplifier noise. Moving off the main resonant peak (MOD off,
>> Loop open) at 250 by +/-50 to either 200 or 300 on the 10e-10 frequency
>> dial does not change a lot. Only if the 10e-10 dial is fully turned CW
>> or CCW using the full range of +/-250 the 137 Hz component at A7J6
>> becomes stronger but still on a very noisy background.
>>
>> Funny thing is that the unit actually "tracks" (I wouldn't call it
>> "locked", though ...) the changes on the 10e-10 dial and develops a
>> control voltage for steering the main osc. Even Alarm goes off and the
>> Continuous Operation light comes on and (mostly) stays on, but because
>> of the cranked up loop gain it e.g. doesn't detect if modulation is
>> switched off, since the noisy narrow band signals (137 + 274 Hz)
>> obviously are somehow able to trick the Logic Assembly A14 to see a
>> "locked" operating condition.
>>
>> Now (finally, thanks a lot for your patience ...) onto my question:
>> Isn't it so that a CBT with clearly discernible resonant peaks and
>> valleys should mandatorily deliver a sufficiently modulated beam
>> current? - provided input signals to and harmonic generator A4 itself
>> are within specs. Or might there be a condition internal to the tube
>> which will show "normal" resonant behavior (not strong but sufficient
>> IMHO) and at the same time prevent the beam current from being
>> modulated? My impression is, that the tube is not overly noisy, since
>> the outputs A7J2 and A7J6 do not show excessive noise from the tube but
>> only from gain fully cranked up. But maybe I'm wrong here?!
>>
>> I did check the beam current "peak to valley" ratio according to the
>> manual and found with a 10 megohm DC voltmeter from A7J1 to ground: 66
>> mV peak and 22 mV nearest valley. The peak value should correspond to a
>> peak current of .66e-8 A which is bit lower than the .8e-8 A given as an
>> end of life indication, but OTOH the manual says the tube may well work
>> beyond these EOL specs albeit with a somewhat degraded performance.
>>
>> I also did a complete realignment of the A4 harmonic generator according
>> to the manual instructions. All alignment steps could be carried out
>> with the expected results and finally a beam current near saturation
>> (plateau with dip) could be established.
>>
>> My apologies for bothering the list with a such a long post, but since
>> being a mostly lurking member for some years now I learned that nearly
>> nothing could be more on-topic than bringing an old Cs beam frequency
>> standard back to life ... ;-)
>>
>> Thanks in advance ... Michael U.
>>
>>
>>
>>
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