i500 problem: low sensitivity of C13

i500 currenly has a hardware problem (likely a damaged broadband preamplifier) which significantly reduces C13 sensitivity. Troubleshooting and repair will be taken at early next week. At this time, users are recommended to use i400 or vxr400 for C13 spectrum measurements. The i500 can still be used for H1 detected experiments (routine 1D H1 spectra, 2D COSY, NOESY, HMQC, HSQC, HMBC, etc.) with no performance loss.