Nikon Coolscan III problems

[Andrew May]

I have a Nikon Coolscan III (LS-30) film scanner that has worked flawlessly since I purchased it new about twelve years ago. Now, however it is giving a few problems in that the carriage appears to stick.

It operates by moving the scanning carriage forward and backwards over a transparency or strip of film using a stepper motor. A second stopper motor moves it in the vertical direction for focussing.

I have stripped it down and cleaned and re-lubricated the rails which seems to be the standard maintenance procedure but to no avail.

What should happen (I think) is this. When the scanner is first turned on it moves the carriage to the far rear and then steps it forward to some sort of reference position located so that the sensitive parts are out of reach when the film carrier is removed. Here it remains. There is an autofocus function in the software that can be initiated at any time and moves the carriage forward until it is somewhere within the frame and performs a focus adjustment. It then returns the carriage to the reference position. All well and good and this action can be performed repeatedly.

What is actually happening is that in moving the carriage back to the reference position after performing the focus it stops short by about

5mm with a short but loud screeching noise. At this point the reference position is out by 5mm so repeating the action moves it 5mm further forward. What is interesting is that this time it still gets pulled up short with the same sound so now is 10mm short. I conclude it is not a physical obstruction otherwise it would return to the same place and I would expect it to happen on start-up as well when it is moved all the way to the rear. Eventually after several of these focusing actions it is so far forward that when moving forward it rams into the start of the screw and cannot be dislodged by the stepper motor. It needs manual intervention to turn the stepper to move the carriage back a little. Turn off. Turn on and all moves back to the original reference position.

Before I decide whether to replace this or send it off for repair I should like to have a go at fixing it but having tried the obvious would appreciate some pointer towards what the problem might be.

Andrew

[N_Cook]

What sets the reference position ? a slotted opto device?

[Brian Gaff]

How does this carriage know its relative position, and I suspect if the drive is slipping its somewhere in that drive and thus the slippage is misaligning the sensor.

Brian

[Andrew May]

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. 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.

Andrew

[Andrew May]

Perhaps I should add, if I haven't mentioned it before that this is a screw drive not a belt drive. So no belt to slip.

[The Natural Philosopher]

is it a twin screw?

I've seen nasty problems with a laser cutter when one belt on one side slipped a cog and the whole thing went out of square - putting a BIG strain on the whole mechanism.

[Paul D Smith]

...snip...

Check if there is a tiny microswitch somewhere. I've recently seen a bluray drive which did something similar because a microswitch, which was activated by the movement of the carriage, got stuck. The mechanism used this to set its initial alignment and so the carriage thought it was "fully forward so need to move back" when it was actually fully back with the motor squealing because it could not move the carriage any further.

As others have hinted, dust in/on an optical sensor or other sensor-knacking could be causing the same effect.

Paul DS.

[Mike Tomlinson]

In article , Andrew May writes

This is known as 'homing'.

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.

[Andrew May]

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. 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.

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

Andrew

[Andrew May]

Sorry, wrong link. It is in the second photo on that page or on page six of this document which does shoe the LS-30:

[Tim Downie]

I would guess it's there to remove any free play in the drive system. That is, make sure that the there's no microscopic movement caused by there being no loading on the gears at any point. (There's a proper technical term for this but I'm darned if I can remember what it is at the moment. Whiplash maybe)

Tim

[Andrew May]

That would make sense and would explain why it is not particularly strong - just strong enough to keep the carriage on one edge of the screw. I can't imagine it being strong enough to impede the carriage if it is tensioned too much so I will look at something interfering with its movement.

Andrew

[Mike Tomlinson]

In article , Tim Downie writes

Backlash.

[Tim Downie]

Ta.

Tim

[N_Cook]

I'm just wondering if something has gone wrong with microstepping. Course back and forth movement and then at near focus dropping into microstepping mode and not enough drive/wrong phasing / interaction with that spring

[The Natural Philosopher]

backlash, or simply lash.

[Andrew May]

It cold well be. I had a closer look last night and found the following with the spring removed:

When starting up and finding the home position it will move the carriage backwards and forwards with sufficient force that it is not possible to easily stop it.

While scanning it moves the carriage from the rear towards the front, basically from the home position over the slide and does this with sufficient force that it is not easy to stop.

Having finished the scan it returns to the home position but this time just putting a finger in the way will stop the movement and results in a squealing sound.

What I can not figure out is, if this is a micro-stepping problem, why it is using micro-stepping to return the carriage to the home position. I can understand it when scanning but not when it has finished. But then I have no experience of stepper motors.

I assume the motor is OK. Is this indicative of a problem with the drive circuitry? I don't have a service manual or circuit diagram but the PCB which I assume does the driving is not very large and could probably be traced out.

Andrew

[N_Cook]

microstepping

If this was mine , I'd connect a small spring or cord with that spring to reduce its effect, but allow continued use while you obtain info & stepper/driver replacement

[Andrew May]

That is pretty much what I was going to do although since I can't see any particular reason why the anti-backlash is in one direction and not the other I was going to re-route the spring to pull the carriage the other way. That way it is assisting in the weak direction but still doing its job.

In the longer term I do need to work out what is wrong or at least see if I can get a circuit diagram of the driver board. However it is a SCSI device so there may be something to be said for cutting my losses and getting something USB based that I can use with the laptop. For electronic devices they do seem to hold their value well on the second-hand market.

Andrew

[John Williamson]

Could it be wear or dirt on the carriage or tracks, with the off centre force from the spring making it wedge in position? As the problem is in one direction only, I'd be suspecting a mechanical cause rather than electonics failure, bearing in mind the tolerances involved.