Position the board where ■ FIGURE 4
it will be mounted and —
while holding it firmly in place — mark
the hole locations by inserting the tip
of a pencil into each hole and tracing
its circumference. Lift off the board and
press a sharp metal point into the
center of each of the four circles you
just drew. Then, drill the holes with an
1/8” drill bit. You’ll also need to drill
holes for the jacks, the volume control,
the on/off switch, and the battery
holder. For the jacks, find a drill bit that
is slightly larger than the threaded part
that goes through the sheet metal. If you use a pot with
any anti-rotation tab, first drill the main hole for the pot.
Then, insert the pot and rotate it so that the tab scribes a
circle around the mounting hole. Drill the tab hole at
some point on that scribed circle.
The metal battery holder is mounted to the box using
four 2-56 pan head screws. I used small screws to avoid
having the screw heads cut into the battery. Before
inserting a battery, double-check the wiring from the
battery holder to the PCB. You really don’t want to apply
reverse polarity to those chips.
Testing the Amplifier
■ FIGURE 5
Before applying power, clean the board with rubbing
alcohol and an old toothbrush, and give it a careful visual
inspection. Look for the usual suspects:
of BLK/WHT 24/2
wire as their part
number 173164. You can also take two lengths of wire
and twist them yourself. (I’ve seen people use a low speed
hand-drill to do the twisting.) Attach the 2.5 mm plug to
one end of the cable, and remember which color is signal
(the tip) and which color is ground. Then, solder the
electret mic to the other end. (Remember, electret mics
are polarized; solder the ground wire to the pin that
connects to the metal case of the mic.)
Plug in the headphones, plug in the mic, turn the
amplifier on, and listen. Verify that the volume control
works. If it all works, turn it off and unplug the mic and
headphones. Unsolder the mic and proceed to mount the
amplifier box to the wood base.
Mounting the Enclosure to the
Wood Base
• Bad solder joints
• Broken copper traces
• Shorted copper traces (especially IC pads)
• Reversed polarity on capacitors
• Wrong value capacitor
• Diodes in backwards
• ICs in backwards
• Wrong IC in a socket (if you used sockets)
• IC (or socket) lead bent under and not in hole
• Missing components (especially monolithic caps)
Once you’re sure there are no obvious problems,
measure the resistance from +V to ground. If it’s low or
zero, there’s a problem. If you used sockets, pull the chips
out one at a time. If the short goes away, you’ve located
the problem area. Otherwise, look for shorted traces.
Test the amplifier before
mounting the box to the
wood base. Insert a 9V
battery and button up the
enclosure. You’ll need a 20
inch length of twisted-pair
stranded wire. Each wire
should have a different color. ■ FIGURE 6
Jameco sells a 100 foot roll
Figure 8 is a photo of the wood base. It’s a tool called
a “float” and is used to smooth out freshly poured concrete.
It’s 15 inches long by 3. 5 inches wide by 0.4 inches thick.
It’s model 8029 sold by Lowes under the TASK FORCE
brand as number 024530 (for under $5). The handle is
offset, so the aluminum enclosure fits nicely on the long
end while the reflector mounts to the short end. The
■ FIGURE 7
December 2008 37
