Hey Jose,
The fine focusing gears on the Saunders tend to wear out over time. In my case it was exacerbated by the use of the focusing extension wand that puts additional strain on the gears. Fortunately you can replace the whole setup at a fairly reasonable price by ordering the fine focusing attachment for the 6700 series enlarger: http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=45757&is=REG&addedTroughType=search
Note that it works on the 4500 series too. Also, make sure the chassis friction screws are not set too tight as this tends to strain the gears as well.
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