- Satellit 500 Synchronous Detector fix
Subject: Satellit 500 sync. detector fix
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.
Info gathered on usenet (rec.radio.shortwave ,etc.) :
Formerly Grundig's flagship portable SW receiver, this is still an excellent
piece of equipment. Excellent sensitivity and selectivity. Digital display with
two separate clocks (only one displayed at a time), signal strength meter,
frequency readout, and four-character user-definable "name" for each
memorized station. 42 memory positions. Tunes to .1 kHz increments. Selectable
wide/narrow bandwidth for AM mode reception. Tunes MW in 9 or 10 kHz steps.
Keypad frequency entry. In addition to AM, has three receiving "modes"
for LW, MW, and SW: USB, LSB, and "Sync," which should more accurately
be described as "fine tuning," as this implementation of synchronous
detection operates differently from the Satellit 700 or Sony 2010. Selectable
automatic/manual gain control. Covers LW (150-353 kHz), MW (528-1611 kHz), SW
(1612-30000 kHz), and FM (87.5-108 MHz).
Excellent audio output, with separate treble and bass controls. Stereo through
headphones. Line-level out (mono), external antenna, and 12 VDC-in jacks.
Local/DX switch. Built extremely rugged. Power supply included.
An all-around great performer. Now that it's discontinued, many outlets are selling this unit at close-out prices, making this radio an excellent value for the money. Purchased from Universal Radio for $379 in December, 1992.
I have a Grundig Satellit 500 and am basically satisfied with it. I miss the 512 memories of 700 and 'SSB clarify' control sometimes.
Yes, its 'sync' is exactly fine tuning in 100 Hz steps. However, it is rather simple to make it work similarly to '700 and '2010, because the detector chip is the same in all three. Only a switch with two pairs of contacts is needed. Another design fault, the excessive distortion in SSB mode, can be partly cured by adding two resistors. For serious DX-ers it may be important that the internal ferrite rod cannot be disabled on MW and LW in both 500 and 700, so the in-house noise may become a problem on these bands.