R70 limits the current drawn by D17 to a safe level. This LED usefully assures
that the otherwise-silent PTT circuitry is indeed functioning and obviously has
nothing to do with the RF output of the transmitter.
The PTT circuit may also be activated at pin 3 of the Packet I/O jack. A direct
short to ground is not necessary. The author noted very positive PTT action
with resistance as high as 100K from pin 3 to ground. This is a highly reliable
and efficient PTT system provided that careful attention is given to correct
selection and installation of all parts: resistor values, PNP transistors, zener
diode orientation, U4 installation, correct wiring of microphone jack.
Stage M: ASSEMBLY PROCEDURE:
Install the following parts:
M1: C86, .001 uf.
M2: C83, .001 uf.
M3: C88: .001 uf.
M4: C77, .001 uf.
M5: C23, .001 uf.
M6: R57, 2.2K (red-red-red).
M7: R59, 100K (brown-black-yellow).
M8: R58, 270 ohms (red-violet-brown).
M9a: R50, 10K (brown-black-orange).
M9b: R49, 10K (brown-black-orange).
M10: R51, 47K (yellow-violet-orange).
M11: R56: 47K (yellow-violet-orange).
M12: R61: 2.2K (red-red-red).
M13: R37: 100K (brown-black-yellow).
M14: C93: 4.7 or 10 uf. (Watch polarity!)
M15: C89: .001 uf.
M16: C24: .001 uf.
M17. Install U4, a 14 pin DIP IC, type LM324, which contains all 4 op amp
sections of this circuit. Orient the notched end as shown on the board. If
you elect to provide a DIP socket, use the same care as in soldering the
IC itself.
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