I upgraded my 8250 with a V9958. Image gets heavily shifted to the left and blue signal is flaky... Any hint?

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Por friguron

Master (162)

Imagen del friguron

27-07-2019, 23:59

Hi all...

Yesterday after 3 months (!!) my new V9958 IC finally appeared into my mailbox, so I could start the VDP upgrade process of my NMS 8250, yay!! I want to do just this. No MSX2+ additional bios for the moment...

Well then, I started re-reading the MANY v9958 -> 8250 upgrade manuals I had already found (and read) months ago. Basically they all claim something like this:

1.- Let's propagate VDD (5V) from pin 58 to pin 21. (I'm getting 4.95 V, tested with my multimeter)
2.- Let's propagate GND from pin 1 to pins 4 and 27. (Sometimes I've seen some people propagating pin 20 instead of pin 1, which also tests as GND using a multimeter anyway)
3.- Let's Leave pin 26 floating (!?)...


Well... Not really, as after doing all my homework with the wire wrap (soldering and reconnecting the pins described above: https://vimeo.com/350510393 ), I get glitchy images. The main symptoms I get are basically these:

1.- Image is HEAVILY shifted to the left. And when I say heavily, I REALLY mean it, as "set adjust (7,0)" does virtually nothing (I still miss like 8-10 chars in width 80 screen 0) It's close to unusability :(
2.- Blue signal is somewhat flaky, specially when there is A LOT of blue signal all over the image (and particularly on the borders, unconfirmed). When this happens the WHOLE image blue signal gets off, and thus blue becomes black, white becomes yellow, etc... i.e. MSX2 boot logo appears as yellow, floating in a black background (it's logical as the whole blue signal has disappeared). In extreme cases, the whole image gets 100% black (f1 spirit 3d konami logo for example)

I "can survive" with these glitches (well not that much, sofarun left column is close to 100% hidden), but I'd like to know what additional tweaks I might perform to have the crystal clear solid sync stability of my V9938.

I have to say I socketed the VDP on the NMS 8250 mobo, so I can put my V9938 back and forth wihtout any problem. V9938 is still working amazingly.

What is the nice thing of my recently installed V9958? -> F1-Spirit 3D, Laydock, etc... (the typical V9958 Screen 11/12 stuff) all work perfectly, so we can deduct the V9958 is really working ok, even some parts of the sync signal are not. My main suspects are CSYNC and HSYNC (pins 6 and 5) of the VDP.

BTW, before writing all this asking for hints, I've also tried to do additional homework... Let's see:

Looking for info about why all this might be happening, I found a dark remark (I can't remember where... Here on msx.org forums also? probably yes... whatever) that said something like:
-You should somehow stabilyze sync signals from the VDP in case you get wobbly rippling artifacts...

Well, am I really getting wobbly sync image/signal? Not at all. I mean, my image is HEAVILY shifted to the left, but is rock solid, it doesn't show any sign of rippling interference. Still more, if I touch pin 5, 6 and parts of the upper analog board (in charge of dealing with sync signal), THEN I'm getting silly wobbling and flaky signal and image. But as soon as I stop probing these circuit points, image gets rock solid stable again.

BTW, I also raised one leg of R167 and R166 (to effectively disconnect them), as it seems to be standard advice and procedure when upgrading to a V9958 (no one ever explains why though, electronic gurus are so prone to convey that halo of mystery and wizardry...) Well, no change whatsoever (connected or disconeccted, that is)

Speaking of which, I can also remember a still darker remark (by I can't remember who either), claming also something like this:

1.- Look for R357 on the analog top board (4K7 ohm) and rise its value to 10K (Again no explanation about why...)

I said, "well, let's see if it does something", and I just added a 3K3 resistor in serial to get an 8K resistor.

Did the sync signal got better? Not really... Of course, it's not the recommended 10K but at least I suppose I might have get some image enhancement of some kind: I got nothing.

Additional issues I remember while dealing with all this:

-If I carefully probe with my multimeter on pin 5 (HSYNC) of the VDP, image gets ONE PIXEL to the left (and it's still 100% stable). Whenever I stop probing said pin 5, image gets 1 pixel to the right. It would be amazing something could be done nearby pin 5 (adding some R or C) to get more right offset to the screen... No idea (?)

-If I probe with my multimeter on pin 6 (CSYNC), now image gets randomly distorted, not just ripply sync, but just random sync.

-If I probe WITH MY FINGER on the middle part of the ---[__4K7__]---finger---[__3K3__]--- serial resistor I created (to raise R357 value from 4K7 to 8K), then image sync gets wavy and a "vertical slow wave" of GOOD SYNC appears (!!?). It's difficult to explain without a video though, but you get the idea... (Maybe I will upload a video if I have the time).

It's somehow like a demo-scene effect: A vertical sinus wave slowly distorts the image (in a good way!) going from top to bottom. When some selected parts of the image get the "good" wavy distortion and they get moved to the right (in fact, where they should really appear), they also regain the lost blue color (!).
So maybe there's some additional tweaking nearby R357 that still needs to be done? I wish I knew more electronics...

Whatever, this post became a LONG ramble a while ago.

Any additional hint I might be testing? Any 8250 V9958 upgrade experts ?

Greetings and thanks in advance.

NOTE: I'm using the SCART RGB connection against a fantastic Daewoo 14" nifty and cute crystal clear CRT.
I have no available RF (in case it might be interesting to test different connection methods)

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Por friguron

Master (162)

Imagen del friguron

28-07-2019, 12:49

Hi!, Problem solved (!!) <3

It REALLY was R357 resistor... I added an additional 1K8 resistor in serial so I transformed my 4K7 + 3K3 into a 10K3 R...
Result? crystal clear sync again. Now it REALLY works, perfect crystal clear image all over the place Smile

When I raised it to 8K and it didn't do anything, I discarded that way... But then I said, "WTF let's try pushing it to 10K". And it really did the trick. Now I'd like to know what's the electronic reason behind all this, but this might be probably out of scope for this topic.

BTW, in the case I'd like to put my V9938 back, are the new modifications (removing R167 and R166, rasing R357 to 10K) compatible with it?


Por friguron

Master (162)

Imagen del friguron

28-07-2019, 14:49

Meh... I claimed "victory" too early...

In the last 2-3 hours of normal usage, the original symptom has slowly started to reappear. Result? I'm at the beginning of my problems ... : (
And what is worse, with a 10K resistor instead of the 4K7 original R357 one... I'm sure raising R357 value did things in the direction of success, but still not enough. Any other hint?

Should I add some kind of additional passive circuit somewhere to stabilyze SYNC signals properly? (R's, C's, etc...)


Por Alexey

Guardian (2753)

Imagen del Alexey

28-07-2019, 19:02

Maybe this will help you:


The capacitors are 100nF. The computer is Yamaha YIS503III converted to MSX2+. The schematics could be easily found.

Por friguron

Master (162)

Imagen del friguron

28-07-2019, 20:22

Ok, somehow I understand you put some capacitors in parallel with the existing VDP Resistors... 100nF, seems fair enough... Typical lentil shaped ones, that's ok.
But then, I don't have this Yamaha (!!), how does it all translate onto my NMS Philips 8250 ? I somehow "feel" I need some stabilyzing capacitor somewhere, but no idea exactly where.

OTOH, after reviewing my 8250 official manuals, it seems that R357 resistor (the one I raised its value from 4K7 to 10K ohm), has nothing to do with my SCART connector, which is the one I'm using to connect to my CRT (or probably I don't know how to interpret my manual, I'm no super expert in electronics).

I mean, yes, CSYNC seems to be the input for R357, but then, its processed signal (through some transistor), seems to travel far away into the "RF" and or "Video out" areas of the analog PCB... Apparently this signal never reaches the SCART connector... But anyway, raising its value to 10K apparently did a bit of a thing... I'm clueless...


NOTE: If I understand you're suggesting me to check your Yamaha schematics to get ideas, I'll gladly do it, as soon as I get them somewhere Smile

EDIT: OTOH, Your video is FANTASTIC... It's a pity it's not at least subtitled into English. Otherwise it's really understandable.

Por friguron

Master (162)

Imagen del friguron

28-07-2019, 19:54

BTW, looking for many documents around, I also found this dark schematic for a v9958 upgrade for the 8280 (not the 8250 !!)


As far as I know, internally they're quite similar (if not exact clones) at least in some parts of the circuitry.
Whatever, according to that anynomous schematic for the 8280 -> V9958 upgrade, a completely different "scaffolding" of components is suggested to be added around the VDP.
I can understand some of them, but some others... not really.
Any expert in 8280's could certify if this additional circuitry depicted in the above .jpg schematic is relevant/useful/advisable for the 8250 ? If so, can someone decode it? (I have some problems with some parts)


Por Manuel

Ascended (16637)

Imagen del Manuel

28-07-2019, 20:18

I found https://www.hansoranje.nl/ which only says to connect pins 4 and 21 of the v9958 to 5V.
Bas Kornalijnlijpers page doesn't suggest any analogue circuitry changes as far as I can see. Oh, wait, there is a separate description for just the vdp change. See his PDF. It suggests to remove R166 and R167. The rest is some connections.

Por friguron

Master (162)

Imagen del friguron

28-07-2019, 20:28

Well, as I wrote on my initial post, that's what I did as "step 0", removing R166 and R167 (just disconnecting one leg of each R). In fact R166 and R167 have been disconnected for the last 2 days, as a first step for the whole modification.
But that did nothing, connected or disconnected, image doesn't change a bit no matter if they're connected or not.

What is funny is that even they're disconnected now, when I switch back to my V9938 it still works perfectly, so as far as it seems, they're somehow not needed whatsoever (?)...

Por tfh

Prophet (2240)

Imagen del tfh

28-07-2019, 20:34

Maybe you can find some additional information here: https://hansotten.file-hunter.com/do-it-yourself/msx2-upgrades/

Por sdsnatcher73

Paladin (937)

Imagen del sdsnatcher73

28-07-2019, 20:36

Have a look at the translated manual (original from Bas in Dutch) to perform a 2+ upgrade on a 8250.

Por friguron

Master (162)

Imagen del friguron

28-07-2019, 20:58

Well I have to say that I've already read AAAAAAALL the documents and webpages you're recommending me. I think I've performed the recommended steps in all of them, particularly the ones about rewiring the V9958 (ignoring rom upgrades).

I won't connect the wait signal (pin 26 (?)), as seems unnecesary, and my problems are of a different kind.

My first post in this thread is there to see if someone can check something I might have done wrong compared with the suggested documents you're sending me, and that I think I've already read (and re-read) many times... I can't find anything new leading to a better sync on them... I can't find anything wrong with my procedures either... Sad

My next movement should be adding some C around R357, as soon as I discover the exact procedure to do and where to connect it exactly.


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