On 09/02/12 14:18, Andrew May wrote:
On 09/02/2012 13:45, Mike Tomlinson wrote:
In , Andrew May
writes

As far as I can see the initial reference position is found by moving
the carriage to the back where there is possibly some sensor, optical or
otherwise.

This is known as 'homing'.

Then step back a known number of steps to achieve the
reference position. This all works fine. It is when it tries to move
back to this position after focussing that it fails to get there and the
problems ensue.

Would you say it can drive in one direction OK but not the other? If
so, the drive transistors to the stepper may be failing.


It will drive quite happily in both directions when turned on and
finding the home position. It is only when doing the scan that is seems
to stop in the backwards direction.

Other obeservation. The sound appears to come from the stepper motor but
I cannot be absolutely certain bit is seems to indicate that it is being
prevented from moving.

There is also a long spring attached to the rear end of the carriage
that goes around a wheel at the front and back to the rear of the
chassis. It is not clear what this does. It looks as if is should be for
returning the carriage but the carriage is driven in both directions by
the screw.

I have tried removing this spring and everything seems to work with it
removed. Replacing it triggers the failure again. There is no sign of
any interference between the spring and the carriage but the spring does
not move smoothly around the wheel so it may be that it is applying too
much pull on the carriage and causing it to stop.

Would seem to indicate nylon wheel or it's axle binding somewhere.

Then when it runs
forward again it lets a bit more spring out and stops it earlier the
next time.

When I get a chance tonight I will try a bit of lubrication on that
wheel to see if that helps.

In the meantime dos anyone have any ideas about the purpose of the
spring. It can be seen at the bottom of the photograph on page six of
the document here if anyone is interested.

http://www.vad1.com/photo/dirty-scan...2000-cleaning/

Note: that this is the LS-2000 but the mechanism looks almost identical.

Andrew

Looks as if it is intended to take up any slack that develops in the
lead screw/nut.

HTH.

DerekG