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    User name stevensm

    Log entry time 16:13:12 on January24,2002

    Entry number 80570

    keyword=collimator readback

    At 0730 I responded to the faulty collimator readback on the left spectrometer. Initially the readback was "inf"; apparently the adc was seeing a higher voltage than it was designed to take on channels 60 and 63. The collimator reads back on channel 63. There were no wires connected to channel 60. The voltage reading back from the collimator position transducer was 8.461VDC, across the low to high sense input of the adc. However, there was 24VDC between sense high and earth ground. The source of the 24VDC seems to be the collimator box wiring, probably on the board edge connector for the now defunct positioning system (the originally installed motion control system which was too easily killed by the radiation in the area of the collimator position encoder). In addition to the erroneous readback, the standing 24VDC on the adc channel 63 caused the input preamplifier board to overheat with the VME crate power off. We decided that it was preferrable to disconnect the problematic cable from the adc, and provide a "work around" solution to the problem of position readback. We set up a benchtop dvm and read it with a tv camera. The collimator responded normally to the controls, but there was a question about the accuracy of the collimator "slit" and "sieve-slit" positions. For that reason we manually positioned the collimator in the "open" position, verified that position visually, and disconnected the collimator drive system. During the next available maintenance period, we will investigate the source of the problematic 24VDC on the collimator position readback.