Are we sure that VG-8235 is not upgradable to 2plus?

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By syn

Paragon (1915)

syn's picture

27-05-2014, 23:11

I dont mind LCD's, only things I worry about are input lag and color correctness. I really think they are a blessing, flat image and so much smaller in dept.

I dont mind MP3's either, since the difference between CD and say a 320KBS rip is really hard to hear.
Besides you would need expensive high end gear to notice the difference. I rather take the convinience of playlist and shuffle playing my entire collection over changing cd's every hour or less..

By yzi

Champion (441)

yzi's picture

28-05-2014, 16:00

If you think the added convenience of a smaller physical size is worth losing much of the beauty and smoothness of motion (without even going into things like historical accuracy or respecting the original artist's vision), that's one thing. But claiming that LCD is practically indistinguishable from CRT, is... a weird thing to say. Anyone can spot the difference, and when talking about smoothness of motion, the difference is actually even functional. Try reading scrolling text on an LCD, it's very difficult, whereas on CRT you can see all individual pixels as clearly as if the text was stationary. Well at least if it's a smooth scroller anyway. Maybe it's not a big thing on the MSX, where smooth scrolling was rare to begin with? Wink I have yet to see a C64 or Amiga scener who doesn't greatly prefer CRTs, because that's where you can see the original magic you fell in love with. Wink The smooth full framerate scrollers, copper bars, vectors, ...

By luppie

Paladin (854)

luppie's picture

28-05-2014, 17:17

Please on-topic, Are we sure that VG-8235 is not upgradable to 2plus?
And if it is possible, what is the best way to do it ?

By l_oliveira

Hero (520)

l_oliveira's picture

28-05-2014, 17:21

luppie wrote:

Please on-topic, Are we sure that VG-8235 is not upgradable to 2plus?
And if it is possible, what is the best way to do it ?

I don't think there's such a thing like "This VG-8235 cannot be upgraded to 2+".

That's a VG8020 and it's now a MSX2. I am 1000% sure I could make it into a MSX2+ if I wanted to.

The point is, you need to do different things based on the original design of the computer you're working with. I don't think this is something you can do with a simple "cake recipe". You just need to react to the problems as they show up. :)

By Omega

Master (196)

Omega's picture

17-06-2019, 01:12

Guys, I have the solution for converting an 8235 to v9958:

It was found as hand drawn comment in an old schematic, which was scanned and kindly provided by Hans Oranje.

Luppie is right. There are variations in the models around.. The VG-8235 video circuitry is, in my opinion, a mess. Especially on the /00 model, you'll see a yellow MSX logo turning white on power on, due to the poor RGB amp section.
Equally poor is the CSYNC section, which causes a voltage offset feedback into the V9958 that the chip can't handle.

Solution: Remove R40 (underside, smd, 820 ohm). Create a 'stack' (parallel) of one smd 100k resistor and one 220nf capacitor, size 0805 if I recall correctly. The value of the cap doesn't matter so much. I think I used 1uf. Solder it to the R40 pad connecting to vdp pin 6. Then put the original 820 ohm in series with those and use a wire to connect back to the other original pad of R40.

Presto: perfect MSX2+ video output on the 8235 (well perfect.. still the off white balance on power on happens).

By l_oliveira

Hero (520)

l_oliveira's picture

18-06-2019, 03:48

Omega, how about remove R40, Q2, R31and U13-4 altogether? That stuff is there because the V9938 sync output is a bit weird (due to being bidirectional) and that circuitry is not needed with the V9958. Maybe the HSYNC circuit could also be simplified.

By mohai

Paladin (774)

mohai's picture

18-06-2019, 19:53

Just a couple of tips:

There seem to be 2 issues/versions of the board in the 8235's as you may already know.
The older models, such as /00 and /19 (IIRC), have a "big" board with some discrete logic chips on it and are easy to upgrade: RAM, ROMs, etc.
The newer models /20 or /29 have a "small" board in it with all the discrete chips removed. It is very similar to 8245.
I think this model is harder to upgrade, as some extra signals have to be taken from the engine or the custom chip itself, BUT, if video circuitry is similar to the one in 8245, then the VDP change must be easy.
I have a 8245 with VDP changed, and I noticed no modification in video circuitry and video output is perfect.
I suggest to have a look at the upgrades on other Philips models and consider the changes needed for the 8235.

By Omega

Master (196)

Omega's picture

26-06-2019, 15:51

Leo - that could probably work, but time didn't permit me to do more experimentation.
And yes the above modification is probably only relevant for the /00 model!

By Jipe

Paragon (1365)

Jipe's picture

26-06-2019, 18:07

this schematic is for a VG8235 /00/02 or /19

By Jipe

Paragon (1365)

Jipe's picture

26-06-2019, 19:08

see the link for mod synchro for 2+ VDP
https://archive.org/stream/tablette3/Tablette%20MSX/tablette...

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