K8048, PIC 16F84, Timer0

I did purchase the kit for our apprentice to collect experience with PIC 16F84. It does work but we have problems to configure Timer 0.
We did use a sample for Timer 0 of sprut.de

If we change the Prescaler (Option Reg) it does not have any effect.
We do count up a variable, dependent on Timer0 interrupt, and then we write the value to Port B. The frequency does not change.

Does somebody have a nice example for PIC 16F84 using the Timer 0 ?

Stell XKOM