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

Mon Mar 9 01:16:54 UTC 2020

Michael as long as it aligned roughly in the middle thats good.
Different tubes have very different C field currents and that was my issue.
I did not push enough current for the tube in the unit. Changing to the
correct resistor took care of it.
That was also the final issue with Frankenstein some 5 years ago. Still
working.
Regards
Paul

On Sun, Mar 8, 2020 at 7:44 PM Michael Ulbrich <mul at rentapacs.de> wrote:

> 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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