Hi Derek,
The fault will be more than likely be down to the stop plate assembly (this is behind the search unit, that you will need to remove)
Turn the Mechanism to the A1 position by using the scan switch.
Leave the service switch in the off position.
Remove the search unit.
Look at the stop plate assembly you will see the 200 mark on the plate is opposite a notch in the frame holding the search unit.
Make a mental note of this position, it will need to be in the same position when replaced to get the timing of the mechanism correct.
Remove the central circlip that holds the stop plate assembly in place.
Check out the microswitches are working correctly
Check out that all the soldered joints on the back of the plate are good (no dry joints USA call them cold joints)
If all is ok then re-assemble and test
If you have manual now check that the adjustments for the switches on the stop plate assembly.
Best of luck.
Regards
Alan Hood
ami-man
UK
alan-hood@datex.co.uk