Unusual semi-modern DEC terminal - "MT 505"?

Most of the time it's just a bad contact at the wiper of the horizontal hold preset inside. Take off the terminal cover (extend the 'leg' all the way down, put the terminal on it's screen, take off the 3 (?) plastic caps and undo the screws under them). The horizontal hold preset is marked on the paper cover on the video PCB. Note its position, then turn it several times from one end to the other. Put it back to the original position. Plug the terminal into the mains and power up (you don't need a keyboard). If the picture locks, power down amd put the cover on. If not, turn the horizotnal hold preset until it does, and set it to the middle of the range where the picture is locked. Power down, wait a bit, power up nd check it locks. Then power down again and put the cover on. If you can't get the picture to lock by adjusting the preset, then you have a more tricky fault to find :-) -tony