if you're operating the IC in astable mode, attach a diode in parallel across the resistor b/w pin 4 & pin 7. The purpose of the diode is to bypass this resistor during charging so that charging occurs only through R76 instead of R47 + R76. Acc to the pricnciple of operation of IC555 as soon as control voltage is reached the capacitor will discharge through R76 only till threshold level. Now if charging & discharging is ocuring through the same effective resistance, Duty Cycle will be 50%
Ton = 0.693(R47+R76)C but R47 is bypassed for any positive going signals and hence effectively, Ton = 0.693(R76)C = Toff.
There's your 50% duty cycle