RobertS & HughR

Earlier in the maintenance period while Jim was centering the BRS, I exercised the BNCs and connectors attempting to correct the glitching reported in previous logs: 33648, 33767, 33976, & 33999.  I thought this had corrected the problem but my spectra had failed to resolve the problem.

The first attachment trends the problem data channels, ADC_0_9 & ADC_0_10, and others nearby in the the ADC rack--they happen to be magnetometers, for twenty days.  On 31 Jan I made the first attempt at centering the masses.  At this time starts the positive going glitches on the PEM STS channels.  Also the unused ADC_0_11 changes DC level and gets much noisier.  Notice too on 2 Feb when the STS masses were successfully centered, the ADC_0_11 changes DC level again.

After the BRS was centered, Robert and I went back to the VEA and tested a couple things.  See the second attachment, one hour of second trends:  The seismometer was unplugged from the host box and the Y channel was disconnected from the ADC.  After that Robert exposed the back of the host box and looked for issues--he thought he smelled some insulation.  During this period, the glitching disappeared from the nearby ADC channel.  When returned to service, the glitches returned as well.  The spectra looks as it did before: with enough resolution, these 46 second period glitches show a comb at lower frequencies and there is a 1/f shelf beginning at 50 or 60Hz.

Our suspicion is this host box is troubled and we should swap it out to confirm.  We'd like to exchange the Host Box from the HAM2 STS.