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. 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. We are planning to get an oscilloscope display of the main and secondary lobes similar to page 4-31 Figure 4-43. If it works we will post it. It should show both the dc and ac parts of the beam current as the 137 cps sweep goes from one extreme to the other. Will anyone having any experience with the Low Frequency Coil test kindly post on the subject? This is NOT the Zeeman frequency test.

> 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