Audio Response: GE Century II

This is the audio response as seen on pin 4 of J910 ("Filtered VOL/SQ HI") Although this should be unfiltered audio, there is still a definite deemphasis present, although far less than fully deemphasized audio. The frequency response has a bump at about 3kHz; this was present even after realigning the IF and detector per the service manual.

The output level is about 280mV peak-to-peak for a 2200Hz tone deviated 3kHz.

This is the audio response at the speaker output. Here, the 3kHz bump is gone, probably masked by the full deemphasis circuit. The deemphasis in the Century II is very aggressive, at 6.5dB (instead of 5dB) between 1200 and 2200Hz.

This is the transmitter audio response. It is also more aggressive than expected, with 6.43dB preemphasis between 1200 and 2200Hz -- quite a close match to the receiver performance.

One thing to note with this radio: although the audio input is quite sensitive, it was designed to work with a pre-amplified microphone and there's a 560 ohm resistor (R901) that provides voltage for the pre-amp in the mic case. This results in the audio input impedance being quite low. Although most TNCs can drive this radio, some -- notably the PacComm PicoPacket -- can't. The cure is to clip R901, which is located on circuit board on the top side of the radio. With the cover off the radio, and the back of the radio facing you, R901 is mounted parallel to the side of the case, just to the right of a cluster of caps and a diode. All you need to do is clip it out. Although removing R901 only has a modest effect on the input sensitivity, it greatly increases the input impedance and will allow any TNC to properly drive this radio.

With the deviation control on the channel element set per the manual (with maximum, clipped, deviation set to 4.5kHz), it takes 47mV RMS of audio to generate 3kHz deviation on a 2200Hz before removing R901, and 28mV after clipping it.