grundig: 500; Satellit
Giovanni Bruzzi, Italy brought this article. But he used copy & paste to bring it in from an other editor (like word) which did only show his first sentence (for certain browsers - like IE 8):
"This article was written by Ylo Mets, I'm only reporting it because I've found interesting."
The correct way is to copy (yes) and to place the curser into the field (yes) but then to click the pictogram above with the "T" instead of using "paste" (for instance by control/v). The "Text button" is the 3rd after the scissors.
The original post from Giovanni did even not allow a second post ... Well, after writing that: The whole can be seen at least with Mozilla Firefox - but I let it be to be able to analyze the text codes one day.
Institute of Chemical Physics & Biophysics,
Grundig Satellit 500 is known for its inferior synchronous detector. There are two problems with it:
1) excessive distortion, especially in the SSB mode, and
2) its synchronous SSB mode cannot be activated via the keyboard and processor.
For the excessive distortion there are two reasons:
1) too high DC voltage at the output of the detector chip CX857 (this may have slight effect on normal AM also), and
2) low frequency (50-200 Hz) feedback to the VCO of the first mixer (this affects only SSB).
The voltage at the outputs can be corrected in two ways. The easier way is to connect a 24-33 kohm resistor from each output (pins 7and 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 the suppression of the unwanted sideband improved in my receiver from 14 dB to 20-26 dB, and I think the distortion is reduced too.
The correct way to do this would be to install two omitted components: one 1N4148 diode in place of a jumper under the metal shield of the detector, and one 33 kohm chip resistor from pin 7 of the chip to ground (there is a place for it on the board). These will have similar effect, perhaps result in slightly better unwanted sideband suppression in SSB mode without additional balancing (as described later). These components are shown on the factory test circuits of the chip and also in the article on synchronous detection by Mike Gruber in QEX, Sept. '92, pp. 9-16.
The feedback from the audio stages to the frequency of the first mixer oscillator can be reduced by connecting a 2000 microfarad or larger capacitor from the AM +3.5 V to ground (pin 11 in connector A). This reduces also the hum in synch mode, when operating off the mains. The better way could be building a separate better 3.5 V regulator and switch it with AM +5V.
This could improve also the operation of the synch detector with NiCads, which seem to have a bit too low voltage now.
For switching the detector to selectable sideband synchronous mode the pin 3 of connector C on the RF board must be grounded and the wire from the pin 4 of connector B must be disconnected.
This can be done with a switch with two groups of contacts. This switch will then affect only the USB and LSB mode. With the switch activated the SSB mode will mean synchronous SSB. The ground connection can be done via 1 kohm resistor, this avoids problems in case of false connection, and also reduces extra interference from the processor. The detector circuit uses the phasing method to select one sideband, and therefore the 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.
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.
Care must be taken also when soldering the connector wires.
These wires go directly to the microprocrssor pins, so static and other voltage differences between the tools and the radio should be avoided. The simplest way is to disconnect the radio from everything during soldering.
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
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 units):
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.