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