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

[paul swed]

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.
>
>
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
>