True Fitness Excel Series Owner's Manual page 54

Treadbelt Tension
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Turn both rear roller adjustment bolts counter-clockwise until
treadbelt just begins slipping when walking on it, then turn both
rear roller adjustment bolts clockwise in equal quarter turn
increments until treadbelt stops slipping.
Note: Be sure to run on treadbelt to ensure that the treadbelt does
not slip while under load.
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ARE AND AINTENANCE
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X C E L E R I E S
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W N E R S U I D E
Treadbelt
Tension