Transistor-based AV mod on Vader gone wrong

[ApolloBoy]

I recently tried out the simple transistor-based composite video mod on my Vader (which is basically the same as this mod, and I'm having some major problems with it. First off, the picture is fairly dark and it also has a severe color bleeding problem on certain TVs. What's especially strange is that I tried this exact same mod on my Jr. and my brother's 7800 and it worked perfectly fine on both.

 

I've tried everything I can think of to try and fix these issues, so I've replaced all the electrolytic caps in the system, moved the 5V supply for the mod from the RF modulator pin to the 5V pin on the TIA, and rebuilt the entire amp. I seem to be at a total loss here, any ideas?

[+batari]

I recently tried out the simple transistor-based composite video mod on my Vader (which is basically the same as this mod, and I'm having some major problems with it. First off, the picture is fairly dark and it also has a severe color bleeding problem on certain TVs. What's especially strange is that I tried this exact same mod on my Jr. and my brother's 7800 and it worked perfectly fine on both.

 

I've tried everything I can think of to try and fix these issues, so I've replaced all the electrolytic caps in the system, moved the 5V supply for the mod from the RF modulator pin to the 5V pin on the TIA, and rebuilt the entire amp. I seem to be at a total loss here, any ideas?

This is an example of "you get what you pay for."

 

None of these cheap mods produce anything close to a proper video signal. Most TVs are tolerant and that's why most work, but not all work on all consoles and all TVs.

 

And, the normal transistor mod I've seen has (IIRC) 5 resistors (they form a resistor ladder for D/A conversion of color signals, then combine with Luma and sync) while this just has two. If it works at all (and colors and luminance are actually correct - some cheap mods do not produce correct colors at all) then the mod must be relying on the D/A resistor ladder on the console itself.

 

Now here's the problem - the resistors in the resistor ladder vary between consoles and some use very large values. On these I would expect a very dark screen.

 

There probably isn't much you can do to fix it short of replacing the resistors on teh console itself with smaller values.

 

Or, use a quality mod and not one of these which are at best dirty hacks

[ApolloBoy]

Hm, that's what I suspected I guess. In that case I'll pull this out and replace it with your mod once I save up.