I got the front panel off the mount and had a good look around. All seemed good including the board capacitors. I then tried Richard's advice and connected a PC type power supply directly to the mount. The mount appeared to work normally after a lot of tests. The problem appears to be a connection hidden under my control trolley. Years ago I was given a good long dc lead which was intended for plugging into a car cigarette holder. I have never used it this way but used a floating cigarette type socket to connect it. As you know these plugs have a center contact tube which is spring loaded, making virtually a point contact and just asking for corrosion in the long term. I have got rid of this arrangement and hardwired the lead to the 12.5volt 5 amp power supply. I have also replaced the DC plug at the mount end with a new one. Testing shows that all is now ok, so thanks for your help.
Peter
