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RGB Scart cable done myself but...

Started by repetto74, 09:40, 02 November 17

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repetto74


Hi,

So I have built an RGB Scart cable for the CPC464 using the DIY tutorial here : http://www.cpcmania.com/Docs/Scart/Scart.htm[size=78%]


The TV is a 2004 CRT television with RGB Scart input (tested perfect with a Spectrum 128k +2 Grey) and the CPC video circuitry is OK as the seller showed me the display before sending me the machine.


Now the result on screen is a very weak picture, I have to adjust contrast and luminosity a lot to have a decent prompt. The image is also in black and white.  >:(
I DO NOT have connected the +5v input to the Scart cable because I though this was not necessary. Could you please confirm to me that this is the cause of a weak B&W picture with the CPC464? 
I have checked the wirings for continuity and all is fine. ::)


Thanks
Rick[/size]

SilverGreen93

Yes, you need to force the TV to RGB mode by applying 5V to pin 16 and maybe pin 8 (on my TV it's required), otherwise the TV will use only the Composite sync output which will give you a B&W picture.

repetto74

Hi Silvergreen93,


Ok noted thanks a lot  :doh: . I will add the DC plug for a 5V power input to the Scart then and see the result :-). I think a 180 ohm resistor is required also between the 5V and scart pin 16

khaz

#3
You can simply use CSYNC with a 100µF cap to generate a blanking signal.



http://www.cpcwiki.eu/index.php/TV_SCART_cable

I don't suppose there is a similar solution to generate the +5V/+12V needed for Status?



On most sets you don't need pin 8. It's a convenience pin that switches to the correct channel and on some sets even turns the TV on, then turn it off or changes back to the previous channel once you turn the device off. You can always use your remote to do that manually. (But it's a damn fine convenience to have. Only one switch to flick and the whole chain is up.)

repetto74

Hi Khaz,


So your wiring schematic here will output an RGB signal without the need of a 5V power supply to the scart ?
I have done all the wirings, some are slight different with this diagram but can redo that if I can get rid off the 5V extra cabling  :-X

khaz

Try it first with your own, then once you know it works you can try that one.

The wiring should be identical. You can use either CSYNC or LUMA as CSYNC, but with the schematic I linked you may have to use LUMA, as CSYNC may be too weak once used as BLANK.

repetto74

Hi Khaz,


I will try your cable design I think this will be less work than providing the 5V to the Scart. Appart from the Red/Blue/Green pinout all others are different but is should not be a problem to rework it. Also I have here some 100uF electrolytic caps so it should worth giving this a try first.


If that does not work I will redo the cable with 5V input.


Actually my cable pinout is as follow.


DIN        SCART
1  red          15
2  green       11
3  blue          7
4  sync         20
5  gnd           5
6  not used


No 5V provided and the cable gives a very weak B&W picture.
::)

repetto74

Luma is at DIN pin 6. So you suggest to wire the DIN pin 6 to the pin 16 of the scart + caps to pin 18 i/o wiring the DIN pin 4? What about the sync at DIN pin 4 then ? To the Pin 20 of scart ?

ZbyniuR

In my both TVs, was necessary to connect something (Luma or Synch) to pin 16 in Scart. Without this RGB doesn't work and picture was B&W straight from pin 20.

To have B&W you need only GND somewhere, and Luma or Synch to 20. But Luma to 20 is better because gave you proper B&W picture (little dark but) each color from 0 to 26 is little bit lighter, opposed to picture from Synch which look disaster with strange shades.

So Luma to 20, and Syntch to 16 with switch on pin 16 to make B&W picture for some slideshows. :)

I hear some bad TVs need some Volts for 16 or 8 pin to show any picture, or to show colors, I'm lucky I never needed.
In STARS, TREK is better than WARS.

repetto74

Hi Zbyniur,


Thanks for your explanation  8) . So if I want also colour what should I do ? (Because I would like to output RGB with colour of course) ;D

khaz

Just follow the instructions.

The picture I put in my post is the correct wiring.

repetto74


repetto74

#12

So I built the cable today  ;) .


First impression : it does work and the picture quality is wonderful in RGB  :o .
I have anyway a small issue. It was funny to see that the main yellow text colour was somehow turning to red or green or black and white (nice prompt mod by the way :D ) . I initially suspected the Scart lead being loose into the TV scart connector and checked also the wirings for any shortcut but then I found that moving the cable DIN connector will make the text colour to change. So now I need to figure out if the male DIN plug is the problem (one broken pin replaced time ago, pin number 4) or if this comes from the DIN socket of the CPC464. I may start by scrapping the cable DIN and replace it with a new one.
Anyway the cable works great ! ;)


Some pics here :

https://ibb.co/eFtUoG
https://ibb.co/mJV9oG
https://ibb.co/jmiUoG


On the second picture the text colour is green but it looks yellow also.

khaz

Pin 4 is not the problem. You'd have the old black and white picture if it wasn't working.

It's very likely that the socket solder points under the board broke. It's common for those 30 years old interfaces, especially if you keep fiddling with them. Applying fresh solder can do the trick but sometimes the whole track has been lifted and broke, and you need to repair it with a wire.

repetto74

Hi Khaz,


Yes this will be my next move tomorrow. I will open the CPC case and have a closer look at the DIN socket  ;)

repetto74

#15
And Yet Another CPC464 in fully working order! :D
I have unmounted the PCB board and redone the soldering on the DIN socket connections. This has definitely fixed the problem and now the CPC464 displays a nice and stable blue screen with yellow text.

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