VG8235 and V9958 trouble

Supporter (11)

02-09-2020, 18:49

Hi guys, i'm attempting to convert a VG-8235 into a MSX2+ ; I replaced the rom and the V9958 VDP but i'm experiencing problems with the VDP:

I followed the upgrade described HERE because the nms8245 and the vg-8235 seems to be very similar.

But what I obtain on the display is the following:

any suggestions? thanks in advance :)

Por Fabf

Champion (266)

02-09-2020, 18:54

Hi
Check you data and adress Line.

Por galatronic

Supporter (11)

02-09-2020, 20:29

Fabf wrote:

Hi
Check you data and adress Line.

Can you explain me how to do, please? Thanks

Por Fabf

Champion (266)

02-09-2020, 21:31

Maybe you have cut one or more trace during dessolder.
With a multimeter you can check data from Z80 or buffers.
You can also check adress ans data to mémoires.
You need a schematic.

Por galatronic

Supporter (11)

02-09-2020, 23:14

Fabf wrote:

Maybe you have cut one or more trace during dessolder.
With a multimeter you can check data from Z80 or buffers.
You can also check adress ans data to mémoires.
You need a schematic.

I have a professional desoldering station, everything works fine if I put back the V9938

I think the upgrade is not complete, I read something elsewhere about remove some resistors or add some capacitors but it is not clear...

Por NYYRIKKI

Enlighted (6067)

03-09-2020, 00:52

galatronic wrote:

I think the upgrade is not complete, I read something elsewhere about remove some resistors or add some capacitors but it is not clear...

As your colors and sync seems just fine, I don't think the problem is there... I also tend to think that this is problem between VDP and VRAM... If you have used socket, I would start by cleaning the VDP legs.

Por Fabf

Champion (266)

03-09-2020, 06:43

Or a broken VDP ?

Por gdx

Enlighted (6212)

03-09-2020, 08:36

On other MSX2+ upgrades, pins 4, 20 and 27 are connected not the 1, 4 and 27.

https://hansotten.file-hunter.com/do-it-yourself/msx2-upgrades/

Por Fabf

Champion (266)

03-09-2020, 10:14

GDX is right pin26 (/WAIT) should not be grounded.

Por galatronic

Supporter (11)

03-09-2020, 11:34

DONE!!! I have connected the pins 1 to 4 to 27 , then pin 26 not connected, and pin 21 connected to pin 58 . the problem was oxydized pins, because I clean the pins and it begun to works fine. just a thing, the image seems to be a bit unstable, a bit of flickering, I think to need a capacitor somewhere ... any suggesions are always appreciated,

thank yuu all for your help guys, my VG8235 is an MSX2+ now

Por Fabf

Champion (266)

03-09-2020, 12:37

Pin26 should be in the socket.
If it doesn't fix flickering, you'll need to mod synchro.