rest of the values are not critical.
Be sure to use metal film 1% resistors where
specified. Carbon 5% resistors can be used for the
rest of the circuit. Be sure to check all your wiring
and definitely use the 14-pin socket for the
MCP6024 so you can replace it if needed. Check all
the ohms between pins against the schematic before
applying the battery, and then check the plus input
of U1C (pin 10) compared with ground. It should
read about 2. 25 volts. The voltage across C1 should
read about 4. 5 volts; higher for new batteries, lower
for used batteries. If that all checks out, turn the
power off and install the MCP6024, being careful to
observe the pin 1 placement. If you turn it around
180 degrees, you will fry it upon applying power.
I printed multiple copies of the label file
available at the article link and used one for the drill and
filing guide on the enclosure (Figure 5).
Align a label equidistant from the sides and ends of
the aluminum “U” channel (top). Tape it in place on four
sides. Use a 9/64” drill bit for all the holes. Drill two
holes in the slide switch rectangle and file out to the
lines. Next, two holes need to be drilled on each side of
the aluminum U channel. Each hole needs to be 1/2”
above the bottom of the side flange.
Drill the electrode input jack and the output (Scope
Out) jack hole using a 15/64” drill bit (for a tight fit) or
1/4”drill bit. The output jack should be below the Scope
Out on the label. The electrode input jack should be
about 1.75” from the front of the unit.
Building the electrode cable assembly is next. The
goal was to make it simple to connect the cable to the
unit using low cost parts. That is why the standard
miniature stereo phone plug and jack were selected.
Cut a piece of two-conductor 28 gauge C1228 Carol
coaxial cable about five to six feet long. Slide the plug
cover over the wire before stripping and soldering the
wire to the 1/8” phone plug. Solder the red wire to the
phone plug tip lug; solder the black to the ring lug and the
ground wire to the sleeve crimp lug. Crimp the ground
wire around the cable jacket, then slide the plug cover
over the plug and screw it tight.
Strip the other end of the cable jacket about .75” from
the end. Strip 1/4” of insulation from the red and black
wires. Leave the ground wire, but remove the aluminum
foil wrap. Cut three 18” lengths of Carol C1156 RG174/U
50 ohm coaxial cable. Remove 1” of the jacket on one
end of each cable. Push the braided sheath toward the
jacket. Using a right angle dental probe, carefully separate
the braided wires and pull the center conductor through
Stretch the braided sheath out. Strip 1/4” insulation off
of the center conductor. Slide a 1/2” length of 1/8”
Last, solder together the ground shields from the three
coaxial cables and the center conductor of the Smith
electrode cable to the ground wire of the two-conductor
coax cable (see Figure 6). Next, place a 4” length of 1/4”
diameter heat shrink tubing over the three coaxial wires
and slide it over the soldered joints and their heat shrink
tubing. About 1.25” should cover the two-conductor coax
wire. Apply heat to shrink the tubing tightly into place,
securing the three coaxial leads to the two-conductor
Finally, strip about 3/8” from the center conductor of
each cable and tin the wire with solder. Trim the braided
ground shield off. Slide the outer insulation sheath so that it
covers all of the ground wire’s braided shield. Insert the
center wires of the coaxial cables into the electrode clamps
that attach to the adhesive electrodes.
To do this, disassemble the electrode clamps by
snapping off the housing, then remove the actuator lever.
This releases the upper spring clamp. Remove the bottom
metal strip from the clamp. Thread the center conductor
into the slot in front of the spring clamp and replace the
metal strip into the housing; then snap it in place. Place
the upper spring clamp and replace the actuator lever.
Last, snap the top housing cover over the lever on to
the clamp housing. Use a small jeweler’s screw driver to
press the coaxial cable to the side of the clamp housing,
wedging it under the rib. Fill the end of the clamp housing
with hot melt glue to strain-relieve the coaxial cable and hold
February 2017 27
■ FIGURE 5. Label used as drill guide for machining
■ FIGURE 6. Cable assembly connection.