800XL and 1084S: Artifacting

[OX.]

OK.

 

• Download the UltraVideo XL 1.0 guide and follow the instructions. That brings the 800XL to the condition mine was in before I bought the 1084S (minus the chroma pick-off).
• For the chroma pick-off (currently not mentioned in the UltraVideo guide), instead of a 100 ohm resistor, use a 22uf capacitor with positive facing the chroma pin on the jack. Connect negative to the underside of the board where the left-hand leg of R68 used to be. This boosts the chroma signal to counteract the 1084's tendency to "compensate" for uneven chroma/luma levels (and so introducing artifacts). It also filters DC current from the chroma signal.
• Remove L5 and run a wire from where the front leg used to be to the front of C3 (improves power supply to the video circuit and can cure vertical banding).
• Lift the lower leg of Q3 and run a wire from the raised leg to the front of C3 (improves power supply to luma output transistor).
• (Optionally) replace U20 (CD4050BE) with an HC4050BE chip (improves character definition).

Better or worse, that's what my 800XL looks like now. It's picture still isn't as good as Preppie's, however, which has better vertical definition on the 80 column characters. Maybe this is down to my mods, or maybe it's the monitor. There is a feint vertical ghosting on the luma, which I'm uncertain how to fix. It's still great through the LCD, though.

 

OK FlashJazzCat I tried adding a 75ohm resistor + ferrite bead but all it did was knock the picture off completely. I also had to remove the wire from step 4 and put the base leg of Q3 back as that also knocked out the picture?? I now have a 100ohm resistor on the chroma line but I'm still getting bad vertical lines despite all these mods, it's a Rev D PAL board I'm using so don't know if that makes a difference, I think I read somewhere that you tried adding some diode to the chroma line - did this make any difference?

[flashjazzcat]

Yes the diode tended to help. Some pre-built cables had them built in but I decided to build it into the machine.

 

Have a look at the KJMann s-video mod as well. It picks chroma and luma from completely different points on the motherboard.

[OX.]

Yes the diode tended to help. Some pre-built cables had them built in but I decided to build it into the machine.

 

Have a look at the KJMann s-video mod as well. It picks chroma and luma from completely different points on the motherboard.

 

What diode did you use?

[flashjazzcat]

Nearly sure it was a 1N4148.

[OX.]

Nearly sure it was a 1N4148.

 

Thanks, I'll some ordered and give it a try.