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    User name jroche/jianglai

    Log entry time 02:12:38 on December 6,2002

    Entry number 53147

    This entry is a followup to: 6689

    Followups:

    keyword=HV troubles, flaky CFD connectors,etc

    We tried to take 1 hr fastbus run with 20uA, HV fullgain setting. Could not turn
    on the HVs. Called Kaz and reset the magnet power supply current alarm. Still
    didn't work. I (jianglai) and Louis went upstairs and discovered both "snipers"
    had red light. Looked like the left sniper wasn't connected to anything. The red
    LED on the right "sniper" indicated detector 6-16 had gone bad. Reset the
    "snipers".

    Reduced the current to 5uA. Full gain. HV tripped again. Same problem. Reset
    Herbert's sniper again. Switched to reduced gain. HV tripped again. Asked for beam
    off. Was able to turn HV back on @ 2uA.

    We kept running at 2uA with reduced gain, fastbus-30Hz interleaved running mode.
    Seemed like detector 3-02, 3-10, 4-16, 8-16 didn't show up any MT TDCs hits and
    some of the CFD TDC hits are missing.

    Julie came in and we went upstairs to check detector channels 3-02 and 3-10. PMT
    signals looked good -- 400 mV signal size. Checked ADC spectra of these
    PMTs, looked OK. So Julie suspected something was wrong with the connector
    --- from the splitter to CFD. Replugged it and everything was back on track!

    For the missing hits in 4-16 and 8-16, we recycled the corresponding HV crates.
    Somehow HV tripped again during the recycling. The error light on the sniper
    seemed indicating 6-16 went wrong again.

    Louis and Julie discovered that the "6-16" channels on the left "sniper" was
    actually connected to 4-16. Maybe that caused trouble? Anyway, reset the
    sniper and restarted the HVs.

    With reduced gain, we raised the beam current to 5uA, then 10uA. At 10 uA
    beam, we measured the French base anode current.
    2-04: 5uA
    4-04: 5uA
    4-16: 16uA
    8-04: 5.5uA
    8-16: 18uA
    Comparable with previous measurement.

    Raised HV to 15uA, then 20uA. HVs looked happy. I (jianglai) am really confused
    why it kept tripping when I tried to run at 2 and 5 uA. But there was a peroid that
    the beam position was very unstable (Herbert's looked OK though) and maybe I
    just got bad luck.

    Then we went to reduced gain. 2uA then 5uA beam. Everything was stable.

    Before Julie went home, she left this note. Klaus and her measured I_ano with
    reduced gain (elog 6689). 7-16 had 40uA @ 16uA beam.
    So with full gain at 20 uA, I_ano of detector 7-16 would be:
    I_ano = 40uA*4*(20uA/16uA) ~ 200 uA! So we need to reset the sniper threshold
    before we go to higher beam current with full gain.