TINY-2/MICRO-2 Technical Ref., 2. Ed 1989
- 18 -
Modem Receive Bias Adjustment
Trimpot (R9 100k) is the modem bias adjustment. The factory procedure below is used to adjust
the modem bias. It normally results in a value of 2.65 to 2.70 VDC on pin 7 of the TCM3105 mo-
dem chip (U16). The setting does not seem to drift over time but may need to be changed it a dif-
ferent modem chip is substituted. If it is necessary to adjust the modem bias, the following proce-
dure should be used.
1. Prepare to perform an audio loopback by jumpering pins 1 and 4 of J2, the radio connector.
2. Give the commands:
CAL
<CR>,
KD
<CR> to provide a 'dithered' transmit signal for loopback
demodulation. When the adjustment is completed, type '
Q
' to discontinue the tone. NOTE: The
watchdog timer will cause the PTT circuit to reset approximately one minute after the 'KD' Keys
are depressed. If you need a longer adjustment time, type '
Q
'<CR> to return to the
cmd:
prompt and restart the procedure.
3. Observe the signal output on pin 8 of the TCM3105 (U16) with an oscilloscope. It may be nec-
essary to increase the modem output level by adjusting trimpot R12 to get a strong enough
signal for decoding. The signal should be a square wave. Adjust R26 to cause the positive and
negative portions of the square wave to be of equal duration.
4. Enter
Q
, remove the jumper from J2 and readjust R12 if needed.
Carrier Detect Level
Trimpot R26 should be adjusted to cause the DCD LED to illuminate when a weak input signal to
the modem is present and to go out when no signal is present. See the section on audio input lev-
els.
Crystal Oscillator Adjustment
You may adjust the oscillator with the variable capacitor C17, to within a 5 kHz range (typical) from
its 4.9152 MHz nominal frequency to move a harmonic a few kilohertz off of a favorite operating
frequency. The setting of this frequency is not critical. A 30 kHz error at the oscillator frequency
will result in only a .6 % error at 1200 baud.
