Service manual

Page 7
DTMF BURST TONE MODE
Model 805 microphones are shipped with the "Continuous Tone" mode enabled. To
change to a "Burst Tone" mode, remove jumper JPR4 on the 805 printed circuit board.
Refer to the Component Layout illustration for location of this jumper. When the 805 is
in the Burst Tone mode, each time a key is depressed a burst of the applicable DTMF
tone is generated for a period of 150 milliseconds, independent of how long the user
holds the key depressed. This feature is provided, as some systems require this type
of DTMF signalling.
CIRCUIT DESCRIPTION
Each time a key is depressed on the keypad the normally low output of U3 goes high,
forward biasing CR2 and causing C19 to charge. The charge on C19 causes both
inputs of U1C pins 8 and 9 to go high resulting in the output on pin 10 to go low. A low
on either input of U1A results in a high output. With input pin 1 low, the output pin 3
will be high. This high is routed to pin 5 of U1B. When pins 5 and 6 of U1B are high,
the output pin 4 of U1B will be low. This is tied to pin 2 of U1A. With a low on both
outputs, pin 3 of U1A will stay high and the circuit is now latched, causing Q1 to
conduct and turning on the sidetone audio amplifier. Pin 3 of U1A also causes CR5 to
be forward biased which in turn causes Q2 and Q3 to conduct and automatically key
the transmitter and illuminating the PTT LED. C19 will keep the circuit latched for
approximately 1.5 seconds after each key has been pressed. This time delay allows
the next key to be pressed before the transmitter is dropped.
With jumper JPR4 in place, the tone generated will be a tone burst of approximately
150 ms per digit, regardless of how long the key is depressed. With this jumper
removed, the tone will be sent for the duration of time that the key is depressed.
To adjust the length of the tone burst, change the value of R16 (270K). A higher
resistance will result in a longer tone burst length.
The tone output goes from pin 16 of U3 through RV1 (the tone level adjustment), R1
and C5 to the level matching point at the junction of R8 and R24. The audio is then
brought to the microphone output amplifier U4B pin 6. When a high level of audio
output is required, jumper JPR1 is removed. This allows U3 to operate without
negative feedback and raises the available audio level. RV1 adjusts the tone deviation
level. C4 can be increased or decreased in value to change the column/row tone radio.
When the PTT switch is pressed, manual keying with voice audio is activated. The
PTT switch S1 connects R2 to +5VDC which causes Q4 to turn on, activating the
microphone audio path through U4A. Pressing the PTT switch also causes CR6 to be
forward biased that in turn causes Q2 and Q3 to conduct, automatically keying the
transmitter and illuminating the PTT LED.