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RGB Problem.

L

Leedelph

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Joined
Dec 12, 2024
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I have a BBC Model B working perfectly from the UHF output and also from the Video output but the RGB just gives a pure white screen as if all three color outputs are at maximum have you any thoughts on what might be causing it, thanks John.
 
Hi, thanks for your reply I am using the supplied 6 pin din to scart lead into a Samsung TV/Monitor but I have tried it on different tv with the same result.
 
I think I see where you’re going with this, when my Farther in law had this computer he used it with a Ferguson 14” crt, rgb monitor and it worked fine, but that was some time ago, the three tvs I tried are all flat screen, does that make a difference or is scart analog anyway.
 
Hi, quick question has anyone manage to run the model B to a flat screen TV via the RGB output socket.
 
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That is how I run my Beeb machine (it is a Master - but this should be the same as the Model 'B').

Is this a 'home made' cable or purchased. I am aware you say it is a 'supplied lead' - but you should be able to tell if it is home made still.

What pinout has been used? If you have a multimeter - can you check the pinout (from DIN to SCART) and compare them with the published schematics for this type of cable connection.

Dave
 
Hi Dave, Yes I have checked the pinout there's four leads connected apart from the Earths pin 7 blue, pin 11 green, and pin 15 red also pin 20 which is composite video.
I notice on the lead recommended by LambdaMikel, on eBay it says resisters are fitted to correct the levels, perhaps that will make the difference the eBay seller is away till January 2nd so I will order it then and see.
Thanks John.
 
My initial guess was that you were missing dropper resistors and trying to feed digital RGB into an analogue set. However, even with the colour drive levels being way too high for SCART (feeding 5V/TTL into 1Vpp?) I would assume you'd still see some black in the areas where it's not coloured and the black level is at 0V.

Although maybe the sync level is also at TTL?

You could probably check for the presence of resistors by using a multimeter set to resistance mode, if you have one. You'd measure from the "red" pin on the DIN to the "red" pin on SCART, for instance. Very low ohms (< 50) would likely indicate no resistor at all, and you'd be looking for something in the 200-300 range for "yes, has a resistor." Although high-quality cables should have a coupling cap in series, so this could throw things off quite a bit.

You can also open the SCART head, though it's sometimes glued shut on cables.
 
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Hi Thanks daver2, the lead I have connected to 6pin din to scart plug doesn't have any leads connected to pin 6 and 4 of the 6pin din plug, so I can't add RGB switching or sync resistors.
Reading the voltage at pin 6 and 4 of the RGB socket on the model B there is 4.9volts on pin 6 and 4volts on pin 4, so I guess the eBay lead will have the extra leads and resistors to switch the TV input to RGB.
I could always ask the eBay seller before I order it now I know what the problem is.
 
Hi, update I connected pin 16 in the scart socket to earth and now RGB picture is perfect.
Thanks everybody for your help.
 
Sorry wrong information I connected pin 16 to pin 20 composite video, thinking it was an earth.
pin 20 has 1.98volts cumming from the model B (4volts) when I unplug the scart end,should I connect these pins via a resistor ?
or leave it as you can see its working.
 

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You shouldn't be connecting SCART pins 16 and 20 together!

I have posted a link to the correct configuration of the required cable connections. Why not just follow that?

Dave
 
I don’t have any resistors so I was trying a few things to get it working till I get a professionally made lead from eBay I know you’re diagram is the right way to go but I don’t have the things needed to make one up.
 
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