Date   : Sun, 10 Dec 2006 14:02:20 -0000
From   : dominic@... (BRAHMS)
Subject: Chroma on BBC B

----- Original Message ----- 
From: "Alex Taylor" <astravan@...>
To: <bbc-micro@...>
Sent: Sunday, December 10, 2006 10:51 AM
Subject: Re: [BBC-Micro] Chroma on BBC B


> BRAHMS wrote:
>> A couple of questions about the chroma (on video out) for the BBC B
>>
>> 1) Is it possible to "improve"/adjust this easily - when enabled it
>> seems to break through quite badly (into the video) on a couple of the
>> beebs I have here. I'm guessing this is because either its not modulated
>> at the right frequency or the sidebands are not filtered out very well?
>> (sorry a little knowledge may prove dangerous here).
>>
>> 2) On some machines it drops in and out (on one removing the lid and
>> allowing air in improved matters)
>> This one particularly annoying as it happened during testing my new game
>> just as I was about to beat my girlfriends hi-score, I still managed but
>> only just, not easy when one has to remove the screws and lid whilst
>> still playing!!!
>
> If someone has an answer to this, I'd love to hear it too. There's a
> picture on my blog from when I connected a Beeb to an LCD projector and
> played Chuckie Egg with an 8' high picture :-)
>
> However the colour started to flicker after 10 minutes, the projector
> didn't like this and the picture started breaking up, eventually going
> blank. I didn't bother putting the Beeb back to monochrome composite
> video, because I wanted colour!
>
> I had to try a few to get this far, some of them wouldn't do this at all
> in colour, and it seems the rest started to fail once they'd warmed up.
>
>

So far, after downloading the service manual, some off list advice and some 
random poking about:

1) Adjusting VC1 (just South-West of the modulator) improves the initial 
flickering making hi-res modes far more "readable" - this drifts off after a 
while or putting the lid back on! gah!

2) Touching the top of the one of the many resistors near the modulator 
leads to a beautifully bright stable not flickering more colourful 
picture....I suppose I shall have to do some tracking down...

3) I'll also try fitting a heatsink to the video chip and see if that 
improved matters

Cheers

Dom