i have 2 servos 7.2 volts,the rated current is 0.35 ampere and 3.5 ampere stall.i want to convert it to be continuous,so i will remove the potentiometer and put two resistors equal in value instead of it, the question is ,what is the value of the suitable resistors to put
The total resistance of the two resistors should be (roughly) equal to the pot element resistance. There is insufficient information from the servo specs to know what is used internally, but they tend to use a common design with monolithic drivers that are compatible.
Usually the pot is 5K, but check it (may be marked). If it's 5K (nominal resistance, pots tend to have a large tolerance so it might measure something like 5.5K or 4.5K), just use a couple 2.49K 1% resistors.
That way the feedback controller in the servo thinks the servo is at mid-position and a command of less than mid-position will cause the servo to rotate continuously in one direction and a command of greater than mid-position will cause the servo to rotate continuously in the other direction.