B
DE
C
Figs. B and C: Search the circuit board for the audio
generation IC, which is often a black blob “gumdrop.”
Fig. D: The pitch resistor is usually the resistor closest
to the audio generation IC, often on the back.
Fig. E: Replace the pitch resistor with a potentiometer
of a greater value. A resistor substitution box makes
the process a lot easier.
1. Find the pitch resistor.
First, open the device you intend to hack. Make sure
you put all the screws in a safe place! Examine the
circuit board and try to find the audio generator or
“brain” of the circuit. This is usually a black blob
covering a proprietary chip; these are also called
“gumdrop” ICs. With my Voice Changer toy, this was
on the green, solder-pad side of the board rather
than the yellow, component side (Figures B and C).
If your circuit has no gumdrop, the biggest chip
is likely to be where the audio signal is generated.
(If your circuit has no chips at all, you’re probably
dealing with an analog circuit. This is a different
beast altogether, and it requires different steps to
modify than the ones outlined here.)
Once you’ve found the brain, look for the closest
resistor, which is usually on the reverse side of the
board (Figure D), because 9 times out of 10 this will
be your pitch resistor. Test your theory by touching
the resistor’s leads while the circuit is making sound.
If you’re right, the pitch should jump up or down.
If you hear no change, lick the tip of your index
finger and touch other parts of the board to see if
they affect the pitch. When you find a spot, narrow
down your search (start using your pinky) until
you’ve found a single point or two at a resistor that
alter the pitch. If you still can’t find this, your circuit
may not be using a resistor to set the clock speed.
2. Swap in a potentiometer.
Once you’ve found the clock speed resistor, make
a note of its value by reading the stripes. Turn the
circuit off or remove its batteries. Unsolder the
resistor and remove it from the board, then solder
2 wires about 8" long to the same contacts. These
wires will attach to a potentiometer.
To find the best value of potentiometer to install,
I use one of 2 different methods. The quick and dirty
method is to just multiply the value of the original
pitch resistor by 10 and try a pot that’s roughly
the same value. If the original value is 15kΩ, for
example, replace it with a 100kΩ potentiometer.
Clip the pot to the leads with alligator clips, one to
the middle wiper leg and the other to either of the
fixed contacts. Then power up the circuit and see
how the pot performs. You may need to try a few
values before you find the right one.
WARNING: Don’t turn the potentiometer
all the way up at any point during your testing.
Turning the pot to zero resistance can fry your
circuit.
129 Make:
