76 second periodic error is a very common problem on these mounts.? It's due to the RA worm bearings being mis-aligned.? Michael Herman has a fix for this, which is to install belleville washers and sand down the outer race of the bearings until they are a slip fit.? Your only hope, before resorting to this, is to loosen and reset the bearing blocks.? PEC will not help you because 76 seconds is not some interval of the 240s worm period.
I had this problem and the belleville washer was the fix.
