NMS8250 upgrade to MSX2+ and flickering

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By Fernando Martin...

Supporter (14)

Fernando Martin Morillo's picture

16-03-2016, 13:21

Hi:
So that: I have flickering on screen (not all of the time) since I've upgraded my NMS 8250 with the video chip V9958 and soldering some cables (wire from pin 4, 20 and 27 and also from pin 21 to 58) and desoldering two resistors (R166 and R167).

I used as heatsinks for the V9958 four small auto adhesive copper ones, instead of using thermal paste and the original aluminium heatsink.

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By Jipe

Paragon (1499)

Jipe's picture

16-03-2016, 14:22

the heatsinks is not present on real MSX2+ with V9958 !!

By maxis

Champion (512)

maxis's picture

16-03-2016, 14:35

Hello,

You can try to increase R357 from 4.7K to 10K and put 100pF capacitor in parallel to it.

Hope, this will help,

All the Best,

By maxis

Champion (512)

maxis's picture

16-03-2016, 14:40

Jipe wrote:

the heatsinks is not present on real MSX2+ with V9958 !!

IMHO, the heatsink might be needed since there are no bus termination series resistors on the VDP address bus in NMS8250.

Worst case - VDP99x8 in Yamaha YIS604 driving simultaneously 2 banks of 8x4164 DRAM chips. There it is really getting hot.

By Fernando Martin Morillo

Supporter (14)

Fernando Martin Morillo's picture

16-03-2016, 15:13

The flickering is that the screen "moves" 1mm from a side to another. But it doesn't seem to be related to heat, as it sometimes has it just booting or sometimes it doesn't have it once warm. Now for example it hasn't any, but it had an hour ago, maybe next boot it will have.

I am thinking now in cleaning with Isopropyl alcohol the soldering I made, because I hadn't then and I cleaned it with normal alcohol.

I'm using a scart to scart to connect the MSX to my led Tv. Is there any better option/better cable I should look?

By maxis

Champion (512)

maxis's picture

16-03-2016, 16:23

Fernando Martin Morillo wrote:

screen "moves" 1mm from a side to another

Hello,

Let's put your "flickering" definition aside. If the screen moves from left to the right 1mm, you have a sync problem. And since V9938 and V9958 are different in a way how the CSYNC drive is implemented, here is the question:

Have you increased the value of R357 from 4.7K to 10K? And have you connected the 100pF capacitor in parallel to it?

If you didn't, then your CSYNC signal has quite a bad slew rate, which will affect the TV sync and then your raster will wobble.

By Fernando Martin Morillo

Supporter (14)

Fernando Martin Morillo's picture

16-03-2016, 19:00

No, I haven't yet. I will order them and I will see if that fixes it. It will take 10 days or so but I will tell. Thanks a lot.

Edit: So the cap 100uF, 10v? I see it won't take so long to have them as I've ordered some caps to fix the FM Paq problem with that value, and I also have R166 and R167 that I desoldered and that are 10K

By maxis

Champion (512)

maxis's picture

16-03-2016, 20:38

Fernando Martin Morillo wrote:

So the cap 100uF, 10v?

Don't use 100uF, since it is 1000000 times bigger capacitance than 100pF! You will kill your VDP.

Try to look for a disk ceramic capacitor from the old radio or so. It will be marked as 101 underlined. You can use any capacitance between 100pF to 560pF. Check it with multimeter. If it shows infinite resistance, it will do the job.

By Fernando Martin Morillo

Supporter (14)

Fernando Martin Morillo's picture

17-03-2016, 01:58

Hi: I'm not good with the multimeter, so I need help to know what I'm soldering. I believe these caps -selecting 100pF- will be ok?

I think so. I will post anyway a photo before powering on the MSX. Thanks a lot for your help.

By RetroTechie

Paragon (1563)

RetroTechie's picture

17-03-2016, 04:20

Both V9938 and V9958 are pretty power-hungry for a passive cooled plastic part. So a heatsink makes sense in any case.

A reason for 'wobbly sync' might be if the +5V on pins 21 or 58 is poorly filtered. In which case it might help to insert some filtering components in the +5V line going to those pins. Like a miniature coil of a few dozen µH followed by a few hundred µF capacitor (or something along those lines - check some MSX2 -> MSX2+ descriptions online).

By maxis

Champion (512)

maxis's picture

17-03-2016, 10:45

Fernando Martin Morillo wrote:

these caps -selecting 100pF- will be ok?

I think so.

Hello,
Looking at the picture, these caps are 0.1uF or 100000pF. They are not OK. About the ceramic disc caps you can read here

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