Auto Paper Size Sensing

The cassette trays, both integral and add-on, all have auto size sensing. It's based on sensing the position of the tray's backstop. The backstop is attached to a sliding key inside the left wall of the tray. That key's position determines the combination of three plastic 'fingers' that protrude from the left side of the tray. Here's a view of them.^[1]



Pictured is the letter size setting -- the two lower fingers protrude, the upper finger is retracted. Those fingers act on leaf-spring actuators in the tray's cavity, pictured in the following photograph.



The actuators, in turn, act on three small snap-action switches on a circuit board (printed circuit assembly -- PCA) that's in behind the System PCA. Here's a view of the size sensing PCA in a T640 that's had its system PCA removed. (All the earlier Ts and the old Optra S are much the same.)



And here's a component-side view of the PCA itself.



At the right side are the three little pushbutton switches that inform the system PCA of the tray's size setting.

The switches tend to get flaky after long service, and size sensing becomes erratic; e.g. if the second switch up doesn't make when actuated, the machine will sense A5 size instead of letter size. It's easy to check the switches thanks to the paper size reporting feature in the Paper Menu. When you're viewing a tray's size on the display, you're seeing the actual size being sensed. If you remove a tray that's set for letter size and push on only the bottom actuator, the display's size reading should change to A5. To check for flaky size sensing, simply pull the tray out a bit and shove it back in several times slowly. If you even once see the size reading change from what it should be, you know you have sensing switch trouble. I've been able to get switches working by taking out the size sensing PCA and soaking the switches with WD-40. Then I've actuated them many times, dried off the PCA and reinstalled it. I don't know how long the effect of that treatment lasts.

There is a work-around for a machine with flaky size sensing, and that is to disable the size sensing feature. Invoke the CONFIG MENU. Scroll down to the item 'SIZE SENSING' and select it. There, you'll be able to change the size sensing from 'Auto' to 'Off'. With that done, when you go to the user's 'Paper Menu', you'll be able to specify the size for the tray and it won't matter that the size sensing is faulty.

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[1] From the Optra S through the T640, the fingers are the same. So, any scrapped machine of those models is a source of replacements for damaged ones.

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