The combination of a resistor and a capacitor determines the frequency of many simple circuits. Resistance (R) and capacitance (C) determine the duration of each wave cycle, or period. Since the period is inversely proportional to frequency, then larger R and C will give you a lower frequency. So if a circuit's frequency is too high for your needs, the fix is usually a matter of replacing the resistor or capacitor with larger-value components. Since changing resistance sometimes has unwanted side effects, it's safest to simply change capacitors until you reach the frequency you want.
Examine the circuit schematic carefully and look for a capacitor that serves the timing function. The schematic may give you a formula for calculating frequency based on the values of the capacitor and its associated resistor(s). If the schematic has no formula, then simply assume that the circuit's frequency is proportional to the capacitor's value: doubling its capacitance reduces frequency by half.
Calculate the value of a substitute capacitor. Use the following formula: Cn = Co x ((100+P)/100), where Cn is the new capacitor's value, Co is the value of the original capacitor and P is the percentage by which you want to lower the circuit's frequency.
For example, if you want to lower the frequency by 25 percent, and the original capacitor has a value of .1 microfarads, then .1 x ((100+25) / 100) = .125. The new capacitor's value will be close to .125 microfarads.
Select a suitable capacitor from your assortment.
Turn off all power to your circuit. Locate and remove the original capacitor. Heat up the solder joint until the solder melts, then draw it off with the desoldering pump. Desolder both of the capacitor's leads and remove the capacitor from the circuit. If the capacitor has polarity marks (+ or -), make sure you put the new capacitor in with the same polarity. If the old capacitor did not have polarity marks and the new one does, refer to the schematic for polarity. Orient the capacitor's negative lead toward ground or the negative power supply line.
Solder in the new capacitor. When the solder cools, turn the circuit on and test the new frequency. You may need to repeat steps 2 through 5 to find an optimal frequency.