Thanks very much - I've acquired a smashed up HSI and will be dismantling it when it arrives to find out how this works.
If there are 3 drums then the LHS drum needs to display 0-3, the remaining two need to display 0-9. The middle one actually displays 0-9 for the first 300 degrees being displayed, then 0-5 for the next 60, then goes back to 0-9. Hence there are 36 separate states to display and hence the left-hand 2 dials are linked together as it makes no sense to have a separate dial for the hundreds unit to revolve at 1/36 of the speed of the units dial when the tens dial is also having to rotate at this speed too.
Last edited by sarboy w****r; 21st October 2012 at 11:15.