I work at a cable TV station, where I primarily service Sony U-Matic decks, so if you're still having problems I might be able to help.
The most common mechanical problems with Sony U-Matic decks are old, hardened pinch rollers (creases the tape badly), and worn or sticky gears driving the loading ring.
Inspect the pinch roller, and if it's hard, shiny, or dented looking, it must be replaced. A good roller looks dull, with no cracks, and the rubber should be fairly soft.
Next check the white plastic gears to the left of the loading ring. The worm gear in this assembly wears down with use, and can cause the loading system to jam. I recommend cleaning off all the old grease with alcohol, then replace it with new silicone based grease. If they are badly worn, they too should be replaced.
And lastly, if you feel the system control board my be at fault, inspect the component side of that board (it's the large board through the bottom cover), and look for any brown glue used to hold the components in place. Over time, this glue actually becomes conductive, and it can cause system control malfunctions, and the glue should be removed. This has been the issue with five or six of our U-Matic decks.
Hope that helps.
