Very dark video output with RGB to SCART on a Spectrum +2A

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homecomputerguy
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Joined: Sat May 23, 2020 7:41 pm

Very dark video output with RGB to SCART on a Spectrum +2A

Post by homecomputerguy »

I use a special RGB/Scart cable for the Spectrum +2A to connect the computer to an LCD TV. The TV supports RGB per Scart. But with the cable I only get a very dark (black) picture output and the text is difficult to recognize (see attached picture).
Does anyone know the problem and can help me?
I'm a Spectrum newby.

Thank you.
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ZXSpectrum+2_SCART_BlackVideoOutput.JPG

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1024MAK
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Re: Very dark video output with RGB to SCART on a Spectrum +2A

Post by 1024MAK »

The TV SCART input is in composite video mode, NOT RGB video mode. OR you are plugged into a SCART socket that does not support RGB video inputs.

Some TVs allow the user to select RGB mode via a on screen menu or via remote control buttons. But some only do so via the “automatic” function switching control inputs on the SCART socket. Although the computer has a +12V output, it is unable to deliver enough current to properly meet the requirements of the SCART function switching control inputs.

It is possible to wire up a battery for testing, then to either modify your computer, or to wire in a two extra wires from a 12V DC PSU to supply the required power.

Mark

homecomputerguy
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Re: Very dark video output with RGB to SCART on a Spectrum +2A

Post by homecomputerguy »

Hello Mark,
Thanks for the tip, that definitely got me ahead. My TV obviously only switches automatically between composite and RGB. But I found a Scart-Switch at home, where I can switch manually to RGB. With this switch it works fine. After that I did some more research and found the following information:
https://www.retrocomputershack.com/Tech ... index.html
With this instruction it should also be possible to mod my Spectrum +2 that way it should then work with my TV, right?

Kind regards
Juergen

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1024MAK
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Re: Very dark video output with RGB to SCART on a Spectrum +2A

Post by 1024MAK »

Yes, (but note that the ZX Spectrum +2 and the ZX Spectrum +2A are electrically two different machines, it’s important to be crystal clear with the model name).

I was not aware that Retro Computer Shack had published this modification (this modification has been discussed in Sinclair/Spectrum forums though).

By carrying out this modification, it should enable the SCART auto switching function to work correctly.

Also note that inside a ZX Spectrum +2A case, you may find one of two different PCBs, in most +2A machines, this is the board that is found in a +2B.

Quite why the Amstrad designer chose to use a 1kΩ resistor for the +12V feed in the ZX Spectrum +2, +2A, +2B and +3 machines, no one knows. It’s a unusually high value.

Mark

homecomputerguy
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Re: Very dark video output with RGB to SCART on a Spectrum +2A

Post by homecomputerguy »

Thank you for the hint. But it's really a +2A, I double checked it.
In the meantime I soldered a 1k-ohm resistor as picky pack to R44 and big surprise ...the video output now is perfect clear on my Panasonic LCD TV (see also attached pictures).
So again, thank you very much for your help and pointing me to the right direction.
And again a 80's home computer that lives on ;)
https://homecomputerguy.de
Attachments
sinclair_spectrum_plus2_02.JPG
sinclair_spectrum_plus2_04.jpg

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1024MAK
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Re: Very dark video output with RGB to SCART on a Spectrum +2A

Post by 1024MAK »

👍 :D

Mark

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Re: Very dark video output with RGB to SCART on a Spectrum +2A

Post by Moggy »

Nice one!

I just love it when another tooter is resurrected. :D

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