Technical

Hello all,

I have a problem with two of my Hornby controllers - both behave exactly the same:

When flicking the regulator lever to the left, the controller switches to 0v output and stays there. It does not then jump up to the fixed 8V output for moving the regulator. The LH green bulb on the controller is illuminated and the voltmeter shows 0V output. Unsurprisingly, nothing happens at the loco. When I release the lever after a second or so, the voltage jumps back to the normal heating voltage (depends on I, II or III heat setting).

If I hold the regulator lever to the left for a longer time (4 or 5 secs or more), then the controller will give up trying and flashes the left hand bulb, but still no 8V output.

If I flick or hold the regulator lever to the right then everything works as normal.

I opened up the controller, suspecting one of the 4 main power transistors would have died. They are nice and easy to get at, wired in a H-bridge arrangement in two pairs fixed to two heat sinks. I unsoldered them, took them all out and tested them using the diode setting on a multi-meter. Apparently MOSFETs can be checked this way, but it's not 100% reliable. All 4 appeared to test out OK, so I re-soldered them back into the controller. Still the same - no change.

I have ordered some brand new MOSFETs just in case and will fit them when they arrive. In the meantime, does anybody have an idea what else it might be ? Alternatively, does anybody have a circuit diagram for the controller. I'm confused because everything works fine in one direction, but not the other.

Fortunately, all the heating / current control settings on the controller work fine, so I can use them with a Live Drive unit.....phew.

Many thanks,

David