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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By Philip

Champion (380)

Philip's picture

28-07-2019, 21:18

friguron wrote:

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 (?)...

These resistors are connected to pin 26 and 26 on the vdp and on the v9938 were used for a lightpan, which might explain why the v9938 works fine with these disconnected.

On my nms8250 I installed the v9958 like this:
Remove R166 and R167
Conect IC106 (V9958) pin 4 to IC120 (74LS374) pin 10 (gnd)
Connect IC106 (V9958) pin 21 to IC182 (74LS00) pin 14 (Vcc)
Connect IC106 (V9958) pin 27 to IC182 (74LS00) pin 7 (gnd)

I left pin 26 disconnected as well.

Picture is fine except for the jail bars which I understood is normal on a 8250

There is one thing in my notes which in the end I didn't need however it may work for you:
Place a capacitor 100nF between pin 21 and pin 20 (Analog GND).

By friguron

Master (188)

friguron's picture

17-08-2019, 11:35

Ok, everyone is giving their own recipe, probably being all of them good ones...

But somehow I still have some doubts. When propagating GND to some areas of the V9958 workzone, some people decide to get GND from pin 1, others from somewhere else, etc...
My doubt is: "are there better GND pins than other?"
With my V9958, I've got GND from pin 1 and then I propagated it into pins 4 and pin 20...

If I want to add the additional Capacitor (I already have) to stabylize pins 21 and 20 of my V9958 (5V and 0V), which GND should I use?
Theory says "any", but each document say different things... I think I'll join pins 1 and 21 (GND and VDD) with my Capacitor... Or should I join pins 20 and 21? (being them closer?)

Or it doesn't actually matter?


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