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Spectrum +2 (Grey version) RGB SCART Question

endangermiceendangermice
edited April 2014 in Hardware
Hi Guys,

A quick question for you. As the new owner of a 128K Spectrum +2 (grey version), I'm using a cable from Retro Computer Shack which tries to use the CSync to drive the blanking video input on pin 16 of the SCART cable. As has been discussed before this works on some LCD TVs but not on others and unfortunately one of my TVs does not work with this configuration.

From doing a bit of reasearch it seems that the +2 can be reconfigured to provide +12v down pin 5 of the monitor port if LK1 is cut and LK3 is connected instead. If I was to perform the modification could I use this along with an inline resistor - around 450 ohms since SCART spects suggests the blanking signal draws around 20mA max to provide the necessary voltage (3V) to the blanking pin in order to switch the TV to RGB mode (obviously I'd need to modify the cable)?

In theory this seems to be possible but I'm not entirely sure where the 12v is coming from and whether attempting to use the output in this way would cause any damage to the Speccy.

Many thanks,

Damien
Post edited by endangermice on

Comments

  • LostLost
    edited April 2014
    endangermice wrote: »
    Hi Guys,

    A quick question for you. As the new owner of a 128K Spectrum +2 (grey version), I'm using a cable from Retro Computer Shack which tries to use the CSync to drive the blanking video input on pin 16 of the SCART cable. As has been discussed before this works on some LCD TVs but not on others and unfortunately one of my TVs does not work with this configuration.

    From doing a bit of reasearch it seems that the +2 can be reconfigured to provide +12v down pin 5 of the monitor port if LK1 is cut and LK3 is connected instead. If I was to perform the modification could I use this along with an inline resistor - around 450 ohms since SCART spects suggests the blanking signal draws around 20mA max to provide the necessary voltage (3V) to the blanking pin in order to switch the TV to RGB mode (obviously I'd need to modify the cable)?

    In theory this seems to be possible but I'm not entirely sure where the 12v is coming from and whether attempting to use the output in this way would cause any damage to the Speccy.

    Many thanks,

    Damien

    Think the recommend option is to use the serial port for the extra voltage.

    After some lengthy debate on here I think the fruitcake site is now bang up to date and recommends this approach.

    I used a AA battery which does well. Think Marc suggested that.
  • endangermiceendangermice
    edited April 2014
    Thanks for the info, I was hoping to be able to stick to the one cable but it sounds like that 12V out of the monitor socket might be inappropriate. I may mod the motherboard and take some power directly from elsewhere and feed it into pin 5, isolating pin 5 first of course by cutting the traces. I've already modified this board quite heavily so it will be another little addition :).
  • LostLost
    edited April 2014
    endangermice wrote: »
    Thanks for the info, I was hoping to be able to stick to the one cable but it sounds like that 12V out of the monitor socket might be inappropriate. I may mod the motherboard and take some power directly from elsewhere and feed it into pin 5, isolating pin 5 first of course by cutting the traces. I've already modified this board quite heavily so it will be another little addition :).

    Cool, post some pics when its finished, I'd like to see it.
  • guesserguesser
    edited April 2014
    endangermice wrote: »
    Hi Guys,

    From doing a bit of reasearch it seems that the +2 can be reconfigured to provide +12v down pin 5 of the monitor port if LK1 is cut and LK3 is connected instead. If I was to perform the modification could I use this along with an inline resistor - around 450 ohms since SCART spects suggests the blanking signal draws around 20mA max to provide the necessary voltage (3V) to the blanking pin in order to switch the TV to RGB mode (obviously I'd need to modify the cable)?

    In theory this seems to be possible but I'm not entirely sure where the 12v is coming from and whether attempting to use the output in this way would cause any damage to the Speccy.

    This is certainly the "right way to do it" if you're going to modify the main board. The resistor between 12v and LK3 is probably too high and will want changing if it's anything like the one in the +3 and friends though.
    Annoyingly the schematic I have here for the grey +2 doesn't have the value (or component number) of the resistor labelled.

    Edit: the service manual says R15, 1kohm so it will want lowering.
    My rubbish website including the redrawn Amstrad schematics and the Sinclair Wiki.
  • endangermiceendangermice
    edited April 2014
    Thanks guessed I've sorted it taking a slightly different approach though yours would be neater in theory and yes 1k is too high - something around 470 ought to do it. On investigation, the voltage available from the serial interface was 12v which to get down to about 3 would have required a quite hefty resistor and I was worried about the amount of heat I would have to dissipate (though at a 20mA load I'd expect this to be a non issue).

    However why get involved with 12v when there's a lovely clean 5v available from the regulator? So I tapped off there and ram it through a 100 ohm resistor (blanking input load 20mA). I disconnected LK1 which gives a direct input to pin 5 (schematics show with LK1 and 3 disconnected the pin is isolated) which was connected to the output of the resistor. Finally I modded the SCART cable to connect pin 5 Speccy end to pin 16 TV end. It now works perfectly and supplies the correct amount of power (tested using a 20mA led to be sure before plugging into the TV).

    I'll be updating my website www.endangermice.co.uk with details soon - I repair retro machines as a sideline and have a number of repair stories that need publishing!
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