Check the pins in the recess that the batteries tabs connect on to when you insert it, if they are definitely OK then it could indicate that the changeover connection contacts located on the input socket are faulty, (not closing when power plug withdrawn) so try "rapidly" pushing the circular power plug in and out a number of times, as that can sometimes cause the "internal" contacts to close properly. If you find this gives results, then give a quick spray of electrical cleaner (i.e: WD40) into (just!!) the power socket as this will clean the contacts. (rotate the camcorder whilst doing this)