If you suspect a corruption in your MSD tune file, the current process is to delete Inst 1/Add Inst 1. There is a nifty command that may solve this without the hassle. Try the command [freshtune]. This should reset the currently loaded tune file to default.
If this does not work then open explore c:\msdchem\X\59XX and rename the tune to .old <atune.u to atune.old>, them mschem\mssetup\59xx and copy the correct tune (dc pol pos or neg) to the orginal directory you were at then rename the file to .u removing the .pu or .nu from the file name.
Applies to : 5973, 5975