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Article #9: Useful Amstrad SRX 100/200 Modification

copyright ©2000 Paul Blitz


(an old article that may be useful to someone: whilst originally for the SRX200, the idea may well work on newer boxes too. Apologies for the "naffness" of the presentation, but it's a quick way to get it online!!)

I suspect that many Hospital Radios who have a satellite receiver (we use it
to get IRN from Sunrise Radio) use an Amstrad SRX 100 or SRX 200... after
all they are cheap, and easy to get hold of.

However, they DO have one minor irritation: if the power has been off, then
they revert to "standby" mode, needing someone to then hit the correct
channel button to get Sunrise Radio again (and pressing buttons means
"special training for presenters" ).

This little modification will make the receiver power up, tuned into a
(selected) channel. We selected it to power up to channel 4, as that was the
easiest position to fit the mod on the board, avoiding adjacent connectors

Not too surprisingly, this modification is supplied on an "as-is" basis: it
is up to you to be happy that you can do the mod, taking the relevant safety
precautions (you will be potentially working on live mains-powered
equipment) - we can not be held responsible if you blow up yourself or your

I do NOT reccommend this mod for any electonics "beginners", or anyone who
feels unsure of their abilities.... so if in any doubt, DON'T!

How it works, and why
The obvious thing to try is to just short out the channel pushbutton: this
doesn't work as the microprocessor is too clever for that! The switch-press
needs to be delayed for a few hundred milliseconds.

So, we looked at other ways to do it. We realised that the STANDBY LED came
on shortly after power-on, so tried to use this (via an opto-isolator) to
"press the button".

Whilst initial tests driving the opto's LED from a PSU showed that we could
indeed use the opto's transistor to "press the button", the STANDBY LED
comes on to early to make this idea work.

A little further investigation showed that the drive circuit for the LED
lends itself nicely to being delayed (using a capacitor).... with the result
that the STANDBY LED comes on sufficiently late to do what we want!

Existing Circuit

                 |             |
                +-+            |
           R159 | |            |
            4k7 | |            |
                +-+           e|
      +-------+  |       b | |/
  ----| R160  |--+---------|<   Q 155: pnp
      +-------+            |  \
        22k                   c|
                              | | R163
                              | |  470
                             ----- Standby
                              \ /   LED

Modified Circuit
                 |    | New    |
                +-+   | Cap    |
           R159 | | =====      |
            4k7 | |   |        |
                +-+   |       e|
      +-------+  |    |  b | |/
  ----| R160  |--+----+----|<   Q 155: pnp
      +-------+            |  \
        22k                   c+--------------+
                               |              |
                              +-+            +-+
                              | | R163       | | Res
                              | |  470       | |
                              +-+            +-+
                               |              |
                             ----- Standby  -----
                              \ /   LED      \ /  new: LED in
                              -v-            -v-  optoisolator
                               |              |
                               |              |

How to do the mod
1) Disconnect power, LNB input, RF in & out, scart & phonos.

2) Undo the three screws that hold the unit together (all recessed, near
   front). Remove the cover... it simply slides backwards, then lift off)

3) Remove the front panel: 3 clips at top, 2 underneath: gently prise these
   up, and the panel comes off.

4) Remove the control panel PCB: "hinge" the PCB downwards (yes, they are
   rather clever connectors!) then pull it outwards to disconnect the 2

5) Prepare your opto isolator: bend the pins outwards (ie flat), and cut off
   most of pins 3 and 6 (unused)

6) Solder the opto, pins 4 and 5 (transistor E and C) across the switch.
   Pin 5 (collector) should be connected to the pole of the switch that goes
   to connector CY152, and pin 4 to the pole that goes to connector CY151
   (on switch 4, this means that pin 5 goes to the BOTTOM pole of the

   NOTE: Before you do this, make sure that there will be enough PHYSICAL
   space when you re-install the board... we found switch 1 was too close to
   a power connector & fuse on the SRX200's main PCB.

7) Test this part of the mod: refit the contol panel PCB (reverse of
   removing it!), and plug in unit to mains: the SRX200 will go into
   standby. Drive the opto's LED at around 10ma (12v supply via 1k is fine:
   +ve to pin 1, -ve to pin 2): the relevant channel will now be selected.

   If this fails, (a) check the "orientation" of the opto's transistor; (b)
   I used an opto with 100% CTR.... if yours is lower, you may need to drive
   more current into the LED; (c) double check that the opto is alive, and
   not that one you meant to throw in the bin last week!!

   If that works, then remove the mains, unplug the board again, and

8) Connect from pin 2 of the opto (LED -ve) to 0v: you'll find a nice big
   lump of 0v where the metal shield of the remote control opto reciever is

9) Connect pin 1 of the opto (LED +ve) via a 1k resistor (don't forget to
   sleeve it!!) to the pad where R163 joins to 155 (it's over at the left
   hand end, next to the STANDBY LED). (Note: the PCB has a "lump of white"
   just nearby, to show where the plasic case touches the PCB.... keep the
   wire away from here)

10) Fit a 100uF, 10v capacitor across R159, -ve end to where the base of
    Q155 meets it.... again watch out that it's not going to foul the case.

11) Glue / tape down as you feel fit.

12) Refit the board, and power up the unit: after about half a second, the
    receiver should switch to the selected channel.

    If it doesn't, and the STANDBY LED is still on, try:

    a) increase the LED current (drop the 1k to 470R)
    b) using a meter, check orientations of capacitor & LED
    c) panic the presenters by running around shouting "Oh my God, I've
       blown up the satellite receiver"


(plb, rev 1, May 2000)  


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