Service manual

Page 23
This is one thing that enables high speed QSK (full break-in) operation.
There’s one other thing that must be done during non-full-duplex transmit. The receiver cannot
help but pick up a little of the transmitted signal, and we do not usually want to hear that in the
speakers. Therefore the audio output must be muted during transmit. This is done by Q34
(Sheet 8). The various audio outputs (from the AM detector, SSB/CW detector and FM detec-
tor) are amplified by U4 (pins 1,2,3) and fed into P-channel JFET Q34. R100 provides a load,
while C127 makes the AC voltage on the gate follow the source, providing a 0V gate-source
voltage that keeps the transistor turned on. As long as the HRcvEn line is high (meaning the
transmitter is off), the drain of Q31 will be low, allowing the AC waveform to keep Q34 on. As
soon as HRcvEn goes low (and we’re not in full duplex), Q31 goes high, forcing Q34 to pinch
off. The time constant formed by C127 and R102 assures that the drain-source junction pinches
off slowly, which eliminates an audible click that occurs when an audio signal is switched off
abruptly.
Once the audio signal makes it through Q34, it is fed to the back panel and to the internal PC
after being attenuated. The signal is quite strong at this point and must be attenuated so as not to
overdrive a PC sound card. In addition, an output impedance of about 600 ohms is desirable.
This is done via R44 and R136. The signal is also passed into two Digitally Controlled Pots
(DCPs) (U15), which are programmed via the serial bus discussed earlier. These particular pots
are used either to feed the monaural receiver signal to both speakers (normal mode) or to re-
move it from the left speaker and drive only the right speaker (dual receive mode). In addition,
pressing the Mute button sets both pots to 0 to remove the receiver signal from the audio path to
the volume controls.
Audio mixer
Sheet 9 on the Receiver board takes audio signals from the Receiver, Sidetone from the Con-
troller, PC Audio from the internal PC or Line-In audio from an external source, and mixes
them in audio mixer U11 (pins 1,2,3, 5,6,7). Capacitors C95 and C96 provide one pole of fre-
quency rolloff at about 4KHz. The right channel drives Line-Out to the transmitter (and note
that since this can include the Sidetone as well as the receiver, it is possible to transmit code via
SSB or AM, and to re-transmit receive audio if full-duplex is enabled).
Volume controls
The outputs of the audio mixer are fed into two more sets of DCPs, one for the speaker volume
controls and one for the headphone volume controls. The output of one speaker channel is buff-
ered and level shifted by Q16 to provide an AntiVOX output that is then fed into an A/D con-
verter in the Controller’s Keyer microprocessor.
The other half of U11 is used to drive the headphones, and one additional pole of frequency rol-
loff is provided. The speaker audio is amplified by U18 and U19 to provide 1.5W of stereo with
high drive capability. Since the speakers have good audio frequency response up to 20KHz and
there’s only one pole on the amplifiers, the audio response has a fair amount of treble. This
helps maintain good frequency response when the PC sound card is played through the speak-