GRUNDIG Satellit 700 modification
Mikael Nordman
I purchased the Grundig Satellit 700 in 1993
and found it has some annoying 'features'.
During the years I have tried to get rid of them by some minor
modifications.
Here is the result.
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WARNING !
These instructions are for people who are experienced with radio
electronics.
Do not attempt to modify your radio if you are not familiar with radio
electronics,
(and with tiny surface mounted components).
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1.Sensitivity
There are two problems in the sensitivity area. Bad sensitivity with
ferrite-rod
aerial, and low sensitivity through the external antenna connector.
1.1 Low sensitivity in the 0-5 MHz range
Immediately before the first mixer there is a capacitor. It has been
selected
to be so small that low frequencies are attenuated. By just shorting out
that
capacitor, the sensitivity in the 0-5 MHz range is improved remarkably.
I have not noticed any problems after this modification. With the
ferrite-rod, reception has much better sensitivity and signal/noise
ratio.
With an external antenna, reception in the 1.6-5 MHz range is also
greatly
improved.
I have not noticed any dynamic range or intermodulation problems due to
this
modification.
This is propably because the preselector reduces the signal levels
reaching the first mixer.
1.2 Low sensitivity in 10-30 MHz range using the external antenna
connector
The solution is the reduce the attenuation in the preselector.
The preselector has four frequency ranges which are switched in and out
with PIN diodes. Each range consists of a double tuned circuit. To
reduce
the attenuation, the two top ranges are converted to single tuned
circuits.
This improves the sensitivity in two ways. First, the actual attenuation
is reduced. Second, the selectivity is reduced, and as a
consequence attenuation from inaccurate preselector tracking is removed.
As a result signals and atmosperic noise, that could not be heard before
on 14 or 21 Mhz amateur bands, are now clearly audible.
I have not noticed any dynamic range or intermodulation problems due to
this
modification. A single tuned stage preselector is enough in the 10-30
MHz
range since there are not so many strong stations there to cause
intermodulation problems.
2.AGC on USB and LSB
The AGC is not suitable for receiving SSB signals. The attack time is to
long
and the decay time is too short.
Another problem is that the AGC requires quite a lot of signal input
before
it starts to reduce the gain of the receiver.
Both of these problems lead to detector overdrive and distorsion in the
USB/LSB modes.
2.1 AGC threshold.
The AGC threshold can be lowered by using germanium or schottky diodes
as AGC voltage rectifiers instead of silicon diodes.
2.2 Attack and decay times
These can be adjusted to suit both SSB and AM. The attack time is made
very short and the
decay time is made longer.
To do this change the Manual Gain Control has to be disconnected.
Actually the MGC is not needed any more, since the AGC works well enough
anyway.
3.Intermodulation when using the whip antenna.
The whip antenna amplifier is dual gate mosfet. The drain voltage of
that
mosfet is only about 2.5 volts. By lifting the drain voltage to the
available 4.5 volts, the dynamic range of the amplifier is increased.
That is done by adding a toroidal RFC in paralell with the drain
resistor (820 ohm). The RFC should be about 100 microhenry to be
efficient
down to 1.6 MHZ.
4.Widened BFO frequency adjustment range.
By adding a resistor and recalibrating the BFO centerfrequency,
the BFO can be
made to tune +/- 700 Hz, instead of the usual +/- 150 Hz.
This allows variable SSB bandwidth, which can be useful.
5.Unsolved problems.
These modifications greatly improve the Grundig Satellit 700.
There are still some problems left which, if solved by Grundig,
would make the Satellit 700 a very good portable receiver.
5.1.Synchronous detector low-frequence noise.
5.2.Improving the opposite sideband rejection
5.3.Allowing a real SSB filter to be used.
5.4.Widening of the AM audio response
5.5.Reducing the synthesizer noise
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The above views are purely my personal views and do not in any way
reflect the views of the company I work in.
No responsibility is taken for destroyed Radios, if You follow the
above instructions.
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Good luck with the modification work
Mikael Nordman
mikael....@era-lvk.ericsson.se