Having recently moved the roof rack and therefore the antenna mounting and also re-termination of the coaxial cable I wasn't too concerned to find the Yaesu ATAS120 antenna has stopped tuning when I fitted it for the weekend camp out. I didn't have time to fault find so had a radio free weekend.
Previously when the antenna has failed to move it has been simply down to a bad DC connection at the mount connectors. Fault finding showed supply voltages on the feedline where adequate and the RF termination likewise. Next I fed the two levels of test voltages directly to the antenna connector (9 to 15volt should make antenna increase in length, 6 to 8 volt makes it retract, centre pin is positive!) but it proved to be dead.
Next step is to ease up the antenna bottom rubber boot to expose the 3 screws holding the base to the shell. With screws removed and a careful wiggle the base of antenna pulls out revealing the control board and two supply wire still attached up into the shell for the motor drive.
The whole area including the control PCB shows sign of being in contact with water at some point (common problem with the atas120). Slightly spot of corrosion at the motor wiring termination on the board but a clean with a fibre brush sorted that to reveal nothing of concern. Sign of a dry joint where the central pin of connector meets the circuit board, quick supply of test voltage to this point showed caused the antenna to function correctly. So simple rework of the joint with the soldering iron and some fresh solder and the antenna was back to its normal self. Touch of vaseline on the shell joints and a over wrap of self-amalgamating tape to the other joints on the antenna shell where water may ingress. 6 to 40 mtr tunes perfectly well with the normal marginal tune on 2m and awful on 70cm - good all working as per normal.
Another write up of an ATAS120 repair: http://radcomms.net/ATAS120.html
and here: http://kb5wia.blogspot.com/2009/12/atas-120a-disassembly.html