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World of Ham Radio 1997
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1997-02-01
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From: ylo@osiris.kbfi.ee (Ylo Mets)
Newsgroups: rec.radio.shortwave,rec.radio.info
Subject: Satellit 500 sync. detector fix
The information used is Satellit 500 service manual
obtained from Grundig in Finland, and Sony data sheet on
CX857 AM stereo chip that Mr. Gruber at ARRL kindly sent me.
Grundig Satellit 500 is known for its defective synchronous
detector. The reason for this is too high DC voltage at the
output of the detector chip CX857. The situation can be
considerably improved by connecting a 24-33 kohm resistor from
each output (pins 7 and 8) to ground. If you don't have the
service manual, the correct points are positive pins of
electrolytic capacitors C825 and C826. These capacitors can be
found next to the larger shielded box on the RF board, on the
side close to the loudspeaker. After installing these resistors
suppression of the unwanted sideband improved in my receiver
from 14 dB to 20-26 dB, and I think the distortion is reduced too.
Some hints for opening: the best screwdriver is Pozidrive #1.
Ordinary Phillips does not fit well, and the screws are quite
tight for the first few times to unscrew. The back cover should be
lifted at the bottom side, there are plastic hooks at the top
side. The RF board can be removed completely after unscrewing
the 5 screws and unplugging all connectors. There MAY be an
extra capacitor soldered between the RF board and the shield of
the processor unit, in the vicinity of the antenna socket. It
must be then unsoldered too.
The board is manufactured using surface mount components, so
certain caution and fine tipped soldering iron is necessary.
The procedure should be undertaken only if you are sure that
your detector is defective too. This can be determined by
viewing the suppressed sideband signal of about 500 Hz with an
oscilloscope connected to line output, it must be severely
distorted. Another symptom is bad suppression of the unwanted
sideband. At frequencies between 50 and 200 Hz there is another
reason for the distortion of the beat signal: feedback from
audio stages to the frequency of the first mixer oscillator.
This can be improved by connecting a 2000 microfarad or larger
capacitor from the AM +3.5 V to ground (pin 11 in connector A).
The detector can be switched to selectable sideband synchronous
mode also, but here rather strong phase noise of local
oscillators creates clearly audible background (about 20 dB
below audio). Also, the shielded oscillators exhibit strong
microphone effect, resulting in audio feedback at higher volume.
To achieve this mode, pin 3 of connector C on the
RF board can be connected to ground via about 1 kohm resistor
and a switch. This resistor is useful only for avoiding extra
interference from processor, also to avoid shorts in case of
errors. For receiving normal SSB transmissions this connection
should be broken.
The unwanted sideband rejection can be further improved, but
for this the service manual, a signal source (signal generator
or a transmitter with clean carrier) and an AC voltage meter or
oscilloscope is needed. The procedure is balancing the summing
resistors after the audio phase shift circuits. Resistors in
question are CR834/CR832 for LSB and CR833/CR831 for USB.
The results on my receiver are following (the frequency of the
best suppression depends on actual component values in the phase
shifters and is probably different for other receivers):
Freq. [kHz] USB [dB] LSB [dB]
0.2 15 14
0.5 16 15
1.0 26 24
1.6 48 42
2.0 36 36
2.5 29 28
3.0 25 24
3.6 22 20
For proper balance CR834 was reduced by 10% and CR833 by 20%,
but I am sure this is different on each sample. The best
suppression value says also something about the distortion.
WARNING: you repeat these procedures at your own risk. Any
warranty will be void after these modifications.
Ylo Mets,
Estonia.