Pitch control is one of the most common and useful features that can be added to a majority of electronic, sound making device. It’s also one of the simplest to find. MOST simple audio circuits use a resistor mounted on the circuit board to determine the clock speed. Clock speed is the rate at which the circuit processes information. Basically the clock speed is the pitch.
Some circuits use other means of setting the pitch (clock speed) such as crystal or inductors. These are harder to modify. The following examples only apply to circuits which use a resistor to set pitch.
The first step in installing a pitch adjustment is to find the pitch resistor.
Below are a few rules and simple steps that will help you find the pitch resistor.
THESE ARE JUST GENERAL SUGGESTIONS. THERE ARE ALWAYS EXCEPTIONS AND ANOMALIES.
LOOK AT THE BOARD:
The following technique most often works with very simple circuits like the example circuit to the right.
-Find the “BRAIN” of your circuit. This is usually a black blob. This blob is a cheap form of integrated circuit (or IC). These are also called “gum drop” ICs. An IC is a collection of microscopic electrical components working together to perform complex functions…like making sound. Some circuits use different styles of IC which look like flat black boxes with lots of metal legs. If this is the case, look for the box with the MOST legs. This will probably be the audio processing IC.
-Once you’ve found the brain, look for the resistor placed closest to the brain. 9 out of 10 times this will be your pitch resistor.
-Test your theory by touching the leads of the pitch resistor while the circuit is making sound. If you’re right, the pitch should jump up or down.
LICK THE BOARD:
If you have a more complex circuit with lots of components you can try the “lick and poke” method.
-Lick the tip of your pointer finger.Turn on your circuit and activate a sound.
-While it is making sound, start touching the board with your pointer finger. Eventually you should hear a jump in the pitch when you touch a certain part of the board. Narrow down your search until you can identify a single point or two that effect the pitch. Maybe use your pinky instead.
IDENTIFY THE PITCH BASE:
In most cases there is one lead coming off of the brain that is the pitch base point. By connecting this point to another point on the board (usually the positive or negative power supply) through a resistor you can set the pitch.
-Once you have found your pitch resistor you will need to determine which end of the resistor is connected to the pitch base and which is connected to the power supply.
You can usually do this by looking at the board to see if one of the leads goes to the power supply.
OR you can use a continuity tester to test both ends of the resistor to see if one is connected to the power supply. Or you can link your fingers and hold a metal pointer, like a jewelry screwdriver” and touch both leads of the resistor. Touching the end connected to the pitch base should result in a noticeable jump in pitch. Once you have found the pitch base, follow the diagram to the right labeled “voltage divider method”.
-It is less common, but sometimes the case that the circuit has two pitch base points. In this case both points will be sensitive and effect the pitch when touched. If that is the case, follow the diagram labeled “attenuator method”
INSTALLING PEAK BUFFERS:
It is of utmost importance when installing a pitch adjustment that you install peak buffering trim pots or resistors. A buffer will limit how high and low the pitch can go.
I usually use potentiometers to find the correct buffering value, then replace the pots with resistors.
A buffer on the high end is most important. Turning the pitch too high for too long can burn out the circuit. Sometimes turning the pitch too high for just a second will fry it. In some cases no buffer is needed at all, but this is very uncommon.
NON RESISTOR BASED PITCH ADJUSTMENT:
Some circuits don’t use a resistor to set the pitch. in these cases they often use a crystal which has a set (non adjustable) frequency. You can replace this crystal with a variable high frequency oscillator circuit like the one sold HERE by GetLoFi