Amstrad SRD400 Channel Expander Control
The Amstrad SRD400 satellite receiver has proved to be a reliable workhorse since its
introduction over six years ago. Despite its technological age and 48 channel limitation, it
can still give excellent sound and picture quality. In fact the only real limitation is its tun-
ing range.
With the introduction of the Global Communications “ADX-plus” Channel Expander,
even this limitation has been removed. You can now watch programmes from Astra’s
fourth satellite, “1D”, along with the best. The ADX can shift the entire band of frequen-
cies 500MHz higher. However, the ADX has to be selected manually when you want it to
do this — a drawback for “couch potatoes” like I am. (like me?)
The only switching function which can readily be used on the SRD400 is the
TV/SAT button. However, this selection can not be stored on a per-channel basis. The
ideal solution would be to allocate a specific number of channels for Astra 1D pro-
grammes and to have the ADX switch on automatically when each of these channels is
selected.
Help is at hand! This article will explain in detail a simple, low cost solution.
My initial thought was that it would be
a simple matter to select a number-display
segment which lit up only for a specific
range of channels. The top left segment of
display three was chosen as being ideal in
that it lights only for channels 40 through
48 (see Fig.1)
The initial enthusiasm was dampened when a glance at the circuit showed that the
LED segments are multiplexed. They are actually pulsed by the display driver IC which,
in turn, receives serial data, from the microcontroller, that would be difficult to interpret.
In addition, the common cathode of each seven-segment display unit is strobed. There is
no steady DC voltage or even usable pulses with respect to the zero volt reference.
Luckily the old bonce, being entirely uncumbered by folicular growths, remains fair-
ly cool and immediately (well, seven months after the original idea to be truthful) came
up with a solution. An optocoupler hooked across the relevant LED segment would give
a pulse output which could be “smoothed” and used to control the ADX. Thus was the
present design conceived.
Since every SRD510 power supply uses a CNY17 optocoupler (or equivalent) I was-
n’t short of these. The optocoupler LED was connected between pin 16 of the LED dri-
ver, IC151 on the front panel, and the centre leg (collector) of the PNP strobe transistor,
Q152. With the optocoupler transistor emitter grounded and a 1k
Ω
resistor between col-
lector and 12 volt supply, the goal was achieved. The pulses from the optocoupler are fed
via a 10k
Ω
resistor to a PNP BC557B (anything will do) and smoothed by a 100µF elec-
trolytic. The BC557B is connected via a 270
Ω
to the ADX
The circuit is shown in Fig.2
Page 9
Fig.1