"Tint issue" on modified Toshiba HX-10 (LVA510 related ?)

Mmmm... When reading this post, I have have the feeling I should try to put a resistor between the YUV output of the VDP and the little board that does the YUV->composite conversion (the one with the LVA510). If that's true, there would be a "voltage difference" between the TMS9928AN VDP I installed and the original one on the HX-10 ??

The numbers after "LVA510" are not part of the chip code. so the " LVA510-467" and "LVA510-497" chips will be the exact same thing. The different numbers simply mean the date the actual chips were manufacturated.

The reason for the hue differences are explained on the TMS99xx VDP datasheet. The 50hz version of the VDP is optimized for generating PAL video then it will output the color values with slightly different voltages than what the 60Hz chip would output. The LVA510 has mode select pins which are toggled to make the chip adapt to that variation. You're feeding it a NTSC-ish signal while the chip is set up for PALand that's why the hues are wrong.

So, to make it work correctly with the NTSC VDP, you connect pin 8 to GND (it's disconnected for PAL) and disconnect pin 10 (it's connected to pin 11 for PAL).

Let us know if it works for you.

"GREAT SUCCESS "

The composite output is perfect now, thanks a lot again guys !

@l_oliveira

Are the color values different, or just the color burst that is different between the TMS9x28 and TMS9x29? I ask because the table on page 2-70 of the TMS9918 datasheet shows only a difference in the color burst.

Por PAL-M machines this difference should also cause problems, shouldn't it? The Expert goes around by converting the YPrPb to RGB and then feed it to the awful MC1377, and this chip generates a new color burst. But how does the Hotbit solves this issue?

The way phase encoding works on PAL and NTSC are different. Because of that the encoding circuit has to be different too. You wouldn't need such difference at RGB because it is converted internally on the video encoder chip into Y/R-Y/B-Y which is then converted to Y/C to then finally become CVBS.

I'm not sure about the specifics but on the case of NTSC versus PAL the difference happens to be on how R-Y and B-Y are dealt with. On Expert, they ultimately want RGB to feed into the MC1377 (which isn't a bad encoder at all, just complicated to use with 12V voltage requirement and needing a lot of external parts) for then achieve the color video.

Because the TI VDP is made to output Y/R-Y/B-Y instead of RGB, it has some of the work of the video encoder already done (convert RGB into Y/R-Y/B-Y) and that's the reason why the chips are different. At the time these chips were designed RGB wasn't really the most common encoding. GI videochip from the INTL also use Y/R-Y/B-Y and I think National Semiconductor's Pong chipsets, again use that as video encoding.

Many years ago I converted a VG8235 to PAL-M and all I had to change was the clock crystal at the encoder board from PAL (4.3Mhz) to PAL-M (3.57Mhz) carrier frequency. Also patch the ROM so the VDP vertical frequency became 60Hz.
It only worked correctly because the encoder board has a circuit which converts RGB into Y/R-Y/B-Y from the RGB signal output by the V9938. That is fixed no matter if the VDP is working in 50 or 60Hz.

Hotbit has a LM1889 which is a combo of Y/R-Y/B-Y video encoder and RF modulator on a single chip.