[LargeFormat] Durst 138S focusing problem

[Clive Warren]

At 11:12 13/06/01 -0700, Don Wilkes wrote:
>Dear Clive, and anyone else with a 138S...
>
>I'm having a problem with focus 'backlash', and can't seem to find a way to
>adjust the beast.  If I overshoot very slightly while focusing (through a
>grain scope), and then try to go back just a teensy bit, it overshoots the
>mark terribly.  This has become pretty frustrating, as you may well imagine.
>
>The manual is pretty mute on the subject, and I don't see anything in the
>way of a tension adjustment (apart from the lock-down), so I'm a bit stuck.
>  Any suggestions?  Lubrication, maybe? If so, with what -- silicone, 
> lithium??
>
>At first, I thought it was just because I was using the Beast to blow up
>some 35mm work, and had the bellows pretty well jammed to its limits, but I
>have the same problem when projecting 4x5.  Rats.

Hello Don,

Good to hear from you - shame about the 138S.  This is not a problem I have 
had on my machines - now have another bench model which is a bit of a 
mixture BTW.

 From memory (the machines are not at this location) the focusing knob 
moves small rollers against the centre column. Pulling the knob as a lever 
allows gross movements up and down the column.  Could it be the case that 
the spring which holds the rollers against the column is misplaced or weakened?

Lubrication is almost definitely not the solution - you may even have 
lubricant on the centre column or rollers which could  well cause the 
symptoms you describe.  Try cleaning the column and rollers.

Just to be technical for a moment - backlash means that the focusing knob 
turns without any corresponding movement of the head up or down the column 
- is this also happening?

Cheers,
             Clive