Cause: Worn/damaged gear teeth on the change gear.
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Here's a view of the gear-train.
The rearmost gear (right side of the photo) is the tractor drive gear. Those usually last the life of the printer.
Below and in front of the tractor drive gear is the tandem change gear that provides friction/tractor-feed selection. In front of that gear is the idler gear.
The change gear leads a hard life, and is prone to getting chipped teeth -- random line feed losses ensue. Here's a close-up of a change gear with a missing tooth.
The damage to a tooth needn't be that severe for line feed losses to occur. Even slight tooth damage that can barely be seen will cause trouble so, when the symptom shows up, replace the change gear and the idler gear just in front of it.Change Gear P/N 51228001.
Idler Gear P/N 51238501.
Remove the reset leaf spring and the tractor drive gear. (One side of that gear's square hub is a claw that holds the gear on the shaft.)
Remove the idler gear and the change gear. (NOTE that the idler's extended hub goes inboard; the change gear's small end goes outboard.) Clean their spindles.
Grease the spindles and the gears' bores liberally. Do not grease the gears' teeth; grease on the teeth will just attract and retain paper dust. Grease the reset leaf spring's working surfaces.
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Wear in the gears' bores will cause the same symptom as damaged gear teeth do. Gear mesh has to be full and flawless for this gear train to work reliably
On printers that have seen long, hard service, the change gear's spindle may be worn, and even new gears won't provide reliable line feeding -- the worn spindle results in marginal gear mesh. The only solution is to replace the chassis. That's actually quite doable, but it does take a complete teardown and transfer of components.
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