[time-nuts] HP5061B Peak to Valley Ratios

[John Miles]

> We are having trouble getting beam current on our #2 HP5061B.  Ion
> current has gone down in a week from 10 to zero. It looks like the rf
> chain is working normally.  Both HV power supplies had to repaired.
> Beam current is all the way up and which gives 2 on the meter. We are
> preparing to do the Low Frequency Coil test mentioned on page 5-22
> section 5-175.
> 
> I got this idea from that procedure.  On our good #1 instrument we set
> the beam current at 20 with normal lock.  The middle coax J2 which
> carries 12.631 mc to the harmonic generator was removed next.  The
> beam current instantly dropped to 2.  We set the beam current with the
> front panel control at 10 to 30 and all three settings dropped about
> 10 to one when the cable was pulled.  This ratio seems to be a good
> indicator of beam tube quality.  

Other useful cues are the effect on the beam current when you turn the modulation off -- it should rise a bit -- and the stability of the meter needle in the beam-current position as an indicator of tube noise.  In that regard, excess noise from A13CR1 could potentially be mistaken for noise in the tube itself.  It could stand a bit more bypassing IMHO.

> I don't see it in the manual, but the
> fine five turn pot oscillator control works much better than the
> course adjustment for setting peak beam current.  It also works better
> for setting the control voltage to zero.  The push to turn course
> frequency control slot has been chewed up by the previous owner.

It's a good idea to remove that contraption altogether.  You don't want anything poking into the 10811's trimmer access hole or otherwise touching it.  Drill a hole in the panel that will let you reach the 10811's trimmer directly with an alignment tool.   It will then be easy to follow the guidelines in the manual -- i.e., set the OCXO trimmer to the central peak with the pot at 250.  

The intent was probably to achieve a consistent operating point at a linear spot in the EFC curve, while discouraging technicians from setting the 5-turn pot near either extreme.  By specifying a midscale setting for the pot, they were able to maintain a consistent damping factor between units while incidentally making it easier to return to the central peak after any temporary adjustments.   The 10811-60109s are all labeled with a factory-selected resistor value for the same reason.  Overkill, but that's how HP rolled.

-- john, KE5FX
Miles Design LLC