Charging problem

[knew1]

I have a problem with my bateries taking a charge. I believe the problem is in the connection. When I plug the charger in, a short hum then stops. I push the plug all the way in......nothing. At times, I can lift the seat or wiggle the cord and the charger starts. Lately, it just won't do anything. Once, I left it all plugged in, went in for supper,came out and the charger was working. My batteries are fine I think. When charged, they last days at 15 to 30 minute usage intevals. I think I have a marathon. The charger is model #26984 Total Charge 3. Mfg date1989. After examining the contacts, they appear fine, but one is fixed and has no give like the other. The receptacle is an Anderson, but it is not in a housing in the frame. It is affixed to a piece of steel. The DC cord is old and appears to have been replaced at some point before I got it. 3 questions........does this sound like a fix that new plugs and receptacle might take care of? Does it matter what size I use.....SB50, SB175, or SB350? Could the problem be something different? Help is greatly appreciated.