V9958 black and white picture

Expert (73)

08-02-2023, 14:40

Hi everyone,

As can been seen in previous posts, i'm trying to convert my Philips NMS8250 to a MSX 2+. Bas Kornalijnslijper has send me a V9958 and i've hooked it up, but i'm getting a black and white picture. When i change color to 15,1,1 and then back to 15,4,4 i can see a little blue flash. This happen also when i power on the machine.

I've tested pin 22, 23 and 24 (R,G,B) with an oscilloscope and the signals look the same as with a V9938, so it looks that the chip is generating color.

To test the chip i'm using Philips NMS 8250 - V9958 Videochip.pdf as a guideline. The connections between pin 1,4, 27 and the connections between pin 21 and 58 are already there, so no need to connect these using a wire. The removal of the two resistors didn't change anything.

I''ve tested with a Philips MSX monitor and a Commodore 1084S and both results are the same. It looks to me, the color signal is being send, but the monitor can't hold it or something. Or could the V9958 be a NTSC version?

Is there anyone who regonizes this problem and has a solution?

Kind regards,
Dennis de Haan

By sdsnatcher73

Enlighted (4296)

08-02-2023, 14:48

How do you connect the MSX to the monitor? Are you using RGB (Scart)? Or are you using composite video? The video encoder may not be able to correctly create a PAL60 signal (meaning PAL at 60Hz) and/or the monitor does not understand the PAL60 signal.

Try issuing VDP(10)=2 command in BASIC to verify the hypothesis. This will switch the VDP to 50Hz.

By ddehaan

Expert (73)

08-02-2023, 15:11

I'm using RGB. The VDP command did not change anything.

I've just connected the monitor using scart and that gives a black background with yellow letters on color 15,4,4. And i just remembered that this was the reason i connected it to RGB, cause it always gave this problem on scart...

By ddehaan

Expert (73)

08-02-2023, 15:29

I've checked on a tv using scart and i have color! So the the "yellow color" problem is on the monitor side. The picture is not centered, but after changing the channel back and forth the signal locks on and i have a nice picture.

There's still something not quite right.

By ddehaan

Expert (73)

08-02-2023, 15:24

But it's strange to me that the RGB signal is not ok...

By Daemos

Prophet (2169)

08-02-2023, 17:08

Sync is not the same try 100nf in combo with 100ohm orso i can't remember the exact values but place those two parts in parralel between the sync pin and mobo and all your problems will be gone.

By ddehaan

Expert (73)

08-02-2023, 19:09

Thanks Daemos, you're reply got me searching for the values and i found this post:
https://www.msx.org/forum/msx-talk/hardware/nms8250-upgrade-...

It suggests to remove R357 and update the resistor to 10K and put a 100pF cap in parallel.

Looks that is the fix i need :)

By Daemos

Prophet (2169)

08-02-2023, 22:28

That is one of the approaches indeed. What I did along time ago was disconnect the sync pin and put a filter in between. I guess they where doing the same. The values look good so try those. What I did at the time was using a trimpot and test for the right value. Never had to look back. I believe I made post about that too on this forum but it was so long ago that it propably got burried deep in the past.

By ddehaan

Expert (73)

09-02-2023, 12:08

The R357 fix actualy made it worse... No picture on RGB and a scrolling picture on Scart.

By ddehaan

Expert (73)

09-02-2023, 14:23

I reverted my work on R357 and decided to fix my scart connector on the MSX VS0080 monitor i'm using. I now have a great picture on scart, but stil not on RGB. I Also tried the fix on de CSYNC signal, but it has the same result.

@Daemos, what max. value trimpot did you use? 10K?

Have to make a trip to the electronics store for some trimpods, i don't have them in stock

By Daemos

Prophet (2169)

10-02-2023, 08:11

I think I went for 1k orso. I really did it on feel. I wish i had photos. Used crocodile clamps and with the machine on tested it until it worked.
Start with 1k then use 10k then 100k etc. 100nf for the cap is a good start.