Service manual

Page 24
ers. However it can cause the receiver to have higher pitch than may be comfortable. The solu-
tion is to mute the receiver and use the line out to the internal or external PC sound card, and to
run the audio through a DSP such as SiliconPixels’ ChromaSound software, which is included
when you buy the PC option. Another alternative is to use the graphic equalizer on the sound
card to reduce high frequencies. One extra pole of frequency rolloff on the speaker outputs is
provided by R162/C153 (left channel) and R147/C6 (right channel).
FM
FM receive is accomplished via an FM receiver chip, the NXP SA615 (Sheet 3). This chip re-
quires a local oscillator and a wideband input signal. To accomplish this, LO3, at 9.455MHz, is
split into two paths (Sheet 5). One path drives the mixer that it used for AM/CW/SSB (U5) and
it is also fed into pin 4 (FMOSC) of the SA615. The input signal is fed in at 9MHz, which pro-
vides a 455KHz difference frequency on the output. This 9MHz signal is derived from the
70.455MHz first IF by routing it through a 70.7MHz low pass filter (to remove sum products
from the U6 mixer), and another Gilbert cell mixer, U22, which provides 10dB of gain too.
LO3, at 61.455MHz, combines with the 70.455MHz IF to provide a final difference frequency
of 9MHz. The output is bandpass filtered by a 500KHz bandwidth filter and fed into the SA615.
The mixer output of the SA615 is a wideband signal at 455KHz. That signal is split into two
paths by transformer T2. One side of the transformer buffers this output and transforms the im-
pedance from 1800 to 50 ohms via Q16 and then feeds it to the IF Output connector on the back
panel. The other side of the transformer passes though a 20KHz ceramic filter and back into the
SA615 for detection. A squelch input is provided by output B of DAC U23 (Sheet 8), whose
level is set by the FM squelch control on the front panel. Comparator U29 compares this volt-
age with that of the RSSI (relative signal strength indicator) output of the SA615 to provide the
squelch signal. The detected audio output is fed to the audio amplifier stages by R30 (Sheet 8).
Detectors
Sheet 7 shows the AM, CW and SSB detectors. AM, CW and SSB are present at the XFO1 sig-
nal. AM is routed through diode D16 and D19 when control signal ModeAM is high. The AM
signal is then given one last boost by U26 before being detected by the half-wave rectifier
formed by D6. R22 and C15 filter the high mixing products out, leaving only audio. R196,
R197 and D15 form a biasing circuit to bias D6 exactly at its diode voltage (about 0.6V). This
allows small signals to be detected, since they do not need to overcome the diode drop.
CW and SSB signals are routed to the product detector/BFO (U7) after passing one final 20KHz
ceramic bandpass filter to clean up any remaining mixing products. U7 is another Gilbert cell
mixer with >10dB of gain. LO4 is set to 455KHz (+/-, depending on the pitch and/or sideband
that’s desired).
Metering
CMOS switch U21 (Sheet 7) selects either the S-meter or the FM relative signal strength indica-
tor (RSSI) or the output of DAC U23 (TxMeter) to drive one or both analog meters. TxMeter is