exxos's Atari, Amiga & retro forum

I've been working on some stuff, and I might need a little bit juice from the video port, I was thinking of either removing it (0R resistor aka 'wire') or adding a 120R in parallel. I've read (haven't checked the schematics) that an Amiga will output 100mA@12V by default. So the obvious choice would be "if it works in that world it ought to work in this", but is there really a need for the resistor at all?

Looking at the A500 schematics, there's no limiting resistor.. So, I ought to be able to draw like 50mA max without messing with audio (in STE, at least) and the diskdrive I recon. Anyone feels like they want to stop me?

If what I found there is reliable :

someone wrote:Note : all video (and control) signal lines see 75 Ohm impedance at the TV ends.

So in this case I guess that it would be ok-ish ?

EDIT : but there

input resistance # 10 kV
input capacitance # 2 nF
Output source resistance when contact 8
acts as an output:
300 V <= output source resistance <= 1kV

So in the end, I don't know.

My thinking is that I’ve never heard of an Amiga blowing up any monitors or TVs. So I’m probably ok, and on the other hand… I never use scart myself. :)

But I could just add one of my super switches (haven’t got a good name just yet) and make it configurable at cold boot… Allowing me to bypass the resistor or not.

Hm. Seems like I have too many projects to finish now. :/

You would have to look at some old CRT type TV stuff and see where that 12V goes. IIRC there is a 8V and 12V switching mode on it. But if older TV's had that signal feed directly into the base of a transistor, it would kill it. With so many TV's out there, can't assume all created equally.

I'd assume Atari added that resistor for a reason. So IMO, it's not a good idea to remove it.

The Peritelevision standard (EN 50049-1:1998) specifies that the source resistance of that signal shall be between 300 ohms and 1000 ohms.

Atari has 1.2 kohms. Maybe they wanted to make it extra safe, maybe older standard allowed for a higher resistance.

In any case, the standard mandates a limited current. Like Exxos, I would worry that some TV manufacturers took the standard seriously and rely on the current limiting resistor in the source. I, too, would advise against removing it.

400R in parallel it is. Might as well make it switchable. (And I’ll probably make a high power mode as well, 100ma or so.)