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10. Install C19, .01uF disc capacitor (marked .01 or 103 or 10nF). This
capacitor prevents DC voltage on the microphone from entering the
ISD1020 IC chip. A capacitor allows AC signals such as voice to pass
through, but blocks any DC voltage.
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11. Install jumper JMP7. Use a scrap piece of clipped off component
lead wire bent into a wire staple shape. Jumpers act as electronic
"bridges" carrying signals over other circuit traces on the circuit board.
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12. Install R16, 470K ohm resistor (yellow-violet-yellow). This part
combined with C14 provides for proper timing in the automatic gain
control circuitry internal to the ISD chip.
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13. Install C14, 1uF electrolytic capacitor. Observe correct polarity.
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14. Install C9, another 1uF electrolytic capacitor, with correct polarity.
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15. Now that your soldering skills are good and warmed up, it's time to
install the 28 pin socket that U4 (ISD1020A) is inserted into. Sockets
aren't absolutely necessary here, but they are a good idea. However,
they are difficult to install and care must be taken to do this task
correctly! Carefully insert it into the PC board and look at the back side
of the board to verify that all 28 pins are sticking through the board
before soldering. After you check that all 28 pins are properly in place,
solder all pins. Be very careful in soldering since some of the pins are
close to traces and a little too much solder can lead to a solder short.
When you are done, double check all of your solder joints for bridges,
especially around pins where there are traces running nearby. When
everything checks out OK, insert U4 into the socket, checking the
orientation, and making sure that none of the pins are bent under.
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16. Install R21, 10K ohm resistor (brown-black-orange). This part also
helps shape reset pulses.
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17. Install D2, 1N4148 type small signal diode (glass body with black
band). Install it with the cathode band as shown. Diodes are polarity
sensitive and their cathode bands must be installed in the correct
direction. This diode insures that only positive-going reset pulses will
appear on the ISD chip.
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18. Install jumpers JMP6 and JMP9 using scrap component lead wire.
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19. Install R14, 10K ohm resistor (brown-black-orange). R14 and C8
provide a reset signal to microprocessor U3 upon power up.
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20. Install jumpers JMP2 and JMP4.
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21. Install R15, a 10K ohm resistor (brown-black-orange). This resistor,
combined with C1,5 and 17, filters some of the "bounce" from the control
CB-1 • 9
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