I recently aquired an old 504 and had a chance to use it for the first tijme today. I discovered while attempting to get a final focus using my grain focuser that the focus control (the big knob on the left) is very "notchy". It would sit "here" or "here", but not in between where the best focus was. A bit distressing - I've been trying not to make blurry images. Before I start pulling this thing apart I thought I'd ask if anybody out there has a simple solution or barring that, some first-hand knowledge of what I should watch out for as I start pulling things apart to cleaning and lubrication. So, you legion of loyal DeVere users... what should be my next approach in solving the notchy focusing? Thanks in advance!