I think my old 730 has much the same hook and race as your 850 (I think the 840 is the rotary hook high speed version) and the same top tensioner as on your model. I would take out the bobbin case, hook and needle plate to see if there might be something that might need to be cleaned out. When you pull the bobbin thread (after it's been take up through the needle hole) there might be slighly more tension, but could it be you need to turn the hand wheel a bit to get the needle bar in top position? Tension tend to be very easy and forgiving on these models. The first drop-in bobbin case came in year 1900, when Singer model 66 was introduced, it has been improved a bit over the decades, with or with out fancy features, but basically much the same.