Colured text all modes (Yellow)

discuss both original and modern hardware for the bbc micro/electron
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Sniffer
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Colured text all modes (Yellow)

Post by Sniffer »

Is there a prime suspect for an Issue 3 board that produces yellow text, instead of white?
Kazzie
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Re: Colured text all modes (Yellow)

Post by Kazzie »

With additive colours (red, green blue), red plus green would give you yellow. Or to put it another way, the absence of blue would turn things yellow.

What video output are you using: RGB, Composite or UHF?
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
RiscPC 600 under repair
Acorn System 1 home-made replica
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Sniffer
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Re: Colured text all modes (Yellow)

Post by Sniffer »

RGB.

Composite didn't give me anything. Thought I'd worry about that later :)
Kazzie
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Re: Colured text all modes (Yellow)

Post by Kazzie »

That makes things a little easier.

(Do you have any test equipment to help you measure what's going on in your Beeb (multimeter, oscilloscope, etc)?)

Firstly, are you happy that your cable is in good condition? A continuity test from pin 3 of the RGB connector to the relevant pin on the other end would be a reasonable first check.

Is the problem exclusive to (the default) Mode 7? If it is, that indicates a problem with the SAA5050 Teletext generator (IC5) or its connection to the video chip (IC6). If not, you may want to try printing some text in various colours to confirm that the problem is an absence of the blue component, and not something else.

The only active part between the video chip's blue output (pin 10) and the RGB port is a BC239 transistor (Q5). It also has an 82 Ohm pull-down resistor (R120) to help make blacks dark: if this were shorted, then that wold suppress the blue colour in the RGB (and composite) outputs.

The blue output after the transistor is also used (via diode D20 and resistors R116 and R137) to generate the monochrome composite output. The signal before the transistor also goes to IC47 and Ic48 as part of the UHF circuitry. But from a glance at the circuit diagram I don't see any obvious way these could cause the blue component to disappear.
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
RiscPC 600 under repair
Acorn System 1 home-made replica
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Sniffer
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Joined: Tue Aug 11, 2015 10:30 pm
Location: Colwyn Bay, North Wales
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Re: Colured text all modes (Yellow)

Post by Sniffer »

Firstly, are you happy that your cable is in good condition?
Yep, cable is good but _not_ the scart connector :oops: Been using composite until I didn't get any output from this machine.

Odd thing is (or maybe not odd) that R118 which is a 3K9 is measuring at 280R.

Really sorry about this noise.
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