10
Pin 5
During transmission of CWID, the level becomes H in synchronism with CW code. When the
level is H, the SW OUT FSK is turned on, and the FSK TTL OUT is set to L level. In CW mo-
de, this pin operates in synchronism with pin 2.
Pin 2
When sending CW, the level becomes H in synchronism with CW code. When the level be-
comes H, the SW OUT CW is turned on, and the CW LED is lit.
Pin 9
When the level becomes H, the gate of CW monitor OSC is opened, and an 800 Hz monitor
sound is delivered to the AFSK OUT and internal monitor Speaker.
Pin 19
When the level is H, the Output of SW OUT FSK is maintained in the mark. However, since
this Output port is not used in the present Software, the level always remains at L.
Pin 16
This is designed as an output for changing over the rate of frequency division of IC4. This
function, however, is not used in the present Software. Internally, this pin is used as key
operation mode memory of telegraph key of CW, the level becomes H when the telegraph
key is pushed down.
Pin 12
Though it is designed as a means of control of display screen changeover by Software, this
function is not used in the present Software. Therefore, the level is always L.
3.3 Actions of CW audio demodulator
The IC42 is a Norton amplifier having four logic circuits of identical function contained in one pack-
age. In this system, two of the four logic circuits are used to compose an AGC amplifier for CW
and band pass filter. The other two logic circuits are not used.
The circuit of the AGC amplifier block is identical with that of AGC amplifier for RTTY on the MO-
DEM PCB, whereas the circuit for CW uses 10 µF C14 in order to increase the time constant.
This AGC amplifier is actually responsible for both AGC action and limiter action.
The output of the AGC amplifier is fed to the band pass filter on the next stage by way of coupling
capacitor C37.
The resistor R28 is for adjusting the input level to the band pass filter. The waveform at test point
TP-5 of the band pass filter output attains the maximum amplitude when the output amplitude
from the AGC amplifier becomes 90 % of the maximum value as an 800 Hz signal is fed from the
AF IN, and the peak of the waveform is slightly deformed at 100 % input, in normal state. This re-
sistor is varied in the range of 220 to 330 kohms in each manufacturing lot.
If the resistance is too small, a misdecoding may be caused by noise signal where there is no sig-
nal.
The variable resistor VR5 is for setting the center frequency of band pass filter. It is adjusted to
800 Hz at the time of manufacture.
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