In my controlling interest #1 post I mentioned the change of turntable control system to the Model Railway Electronic Group (MERG) turntable control unit.Since I first built Fisherton Sarum the turntable was powered via a Frixinghall motor and gearbox that did not have any form of automatic indexing. This meant that the rotational speed of the table was not easy to control and track alignment had to be done by eye (sometimes hitting the mark easier than at othertimes) via a toggle switch.

Friend and regular operator of Fisherton Sarum and electronics wizard GWMark of this parish, has very kindly built for me a MERG turntable control kit which drives a stepper motor and gearbox.

This clever bit of electronics can give up to 63 different positions (I only need 8 i.e. each end of 4 roads, on Fisherton Sarum) to an amazing precision of 0.03 degrees.

The turntable is simply operated from the panel via a rotary switch (a binary hex coded switch, for those that understand such things) to select the desired road and then pressing the start button. The controller then automatically works out the shortest distance to the desired alignment. When turning clockwise the deck is stopped automatically at the correct place or if turning anti-clockwise it over runs slightly before stepping back clockwise to the correct position ensuring any slack in the gearbox is taken up. The controller has a learn mode to allow the desired positions to be accurately set up which it then remembers.

The typical bank holiday weekend weather, just passed, encouraged working inside rather than other pursuits so the installation was completed. Installation has required a few slight changes to the control panel: firstly to incorporate the rotary switch, mounting inside the control panel the controller PCB card and changes to the wiring harness to get the required five wires to the stepper motor and gearbox that is being mounted directly under the turntable itself, replacing the previously installed traditional motor, gearbox and worm and wheel drive.

It is imperative that the link between the gearbox outlet and the table deck is positive and has no possibility of slip. The Peco deck is designed to be an interference fit on to a Meccano shaft so to ensure no unwanted movement is possible I have drilled through the boss and the shaft at 90 degrees and pinned it with a short section of Brass wire. This shaft matching the gearbox output shaft has had a flat filed onto it and a brass collar joins to the two with grub screws ensuring a positive connection.

Programming the turntable in learn mode has not gone 100% to plan and I did not yesterday manage to get it working correctly, so need to some additional advice. Once I have mastered the programming I will upload a video of the turntable under its new control in action, so watch this space.

This is an extract from my blog at www.grahammuz.com and further information and additional images can be read here.