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AMSTRAD SRX 100/200 CONVERSION TO 23 CMS ATV

The SRX 100/200 satellite receivers are easy to modify for ATV use.  They will work (poorly) unmodified, but the following mods improve the video output and allow audio reception.  Remember that the unit feeds around 14-17 volts up the aerial feeder originally to power the satellite dish LNB; this can be used to power a pre-amplifier.   If no pre-amp is used and an aerial with a DC short across the feeder is employed then this voltage must be disabled.  To do this, break the link shown in the diagram below (this could be made switchable).  Access to the internal circuitry is simple.   Remove the three philips screws inset in the base of the set.  Some models have a fourth screw at the rear of the base.  Slide the top cover towards the rear a couple of inches and lift and remove. The front panel is held in place by three plastic clips on top and two below.  Ease these up and remove the panel; the circuit board behind the front panel can now be hinged forward.  Disconnect the multiway connector at the left front of the board followed by the black earth lead immediately behind.   The earth lead is held in place by a plastic block which will separate when pulled upwards with pliers; discard this and, when refitting the earth lead later, solder in place.  Remove the single philips screw from the centre of the board.  The board may now be removed by easing back the two plastic retaining clips at the left of the board.  Component numbers are silk screen printed on both sides of the board.

You may wish to carry out just the video mods initially and replace the XTAL at a later date. Once the video mods are complete, some improvement to the overall sensitivity is possible by very careful adjustment of C70 in the RF front end. It is easier to adjust if the screened lid is removed first; you will require a trimming tool, a small screwdriver will not suffice. If you lose the picture entirely don't panic; small adjustments will bring it back. You may also need to adjust the manual tune position to re-gain the picture. Ensure that you have an off air signal available before making this adjustment. Remember that on its own this unit is a little deaf; it will work fine if you have a good (line of sight) path to the transmitter, but otherwise may need a pre-amplifier.

Video Modifications:
Replace R702 with a 10K resistor
Replace R704 with a 1K2 resistor
Replace VR701 with a 1K preset

Audio Modifications:
Replace X302 with a 16.7 MHz HC18 crystal
Change C301 and C304 to 22pF
Change C303 to 390pF

Manual Tuning:

Manual Tuning is recommended and can be achieved as follows: Referring to the diagram below, locate the link below IC104 and directly above R122. Break the link and connect the left side to the wiper of a 10 K pot. Each end of the pot is connected via a 36K resistor to the can of the modulator at one end and pin 8 of IC105 at the other. The resistors should be soldered directly to the pot. This will allow tuning between approximately 1200 - 1350 MHz.  (Note that this prevents the use of the digital frequency display.)

Board Layout - Click to enlarge

Author: Trevor Day G3ZYY

 



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