Video driver thoughts

[exxos]

I've lifted the Analouge VCC lines, wired to own 100uF ceramic, via a 10R resistor... Makes no odds.. Just makes the video a fraction darker if anything.

I've basically removed those 33R RGB resistors now, was to dark with them..

Also tried 47pF on the RGB lines, makes no odds either.

The jumper link doesn't seem to make any odds. Probably the voltage change is to small to visually see anything.

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[sandord]

Just wildly associating here but isn't this the kind of thing you can get with floating signals?

[exxos]

Just wildly associating here but isn't this the kind of thing you can get with floating signals?

That's what I was thinking originally, but Icky and myself having exactly the same problem means that are soldering can't be bad...

Everything is connected on the PCB to the DAC..2 pins are not connected, but they are not connected on the datasheet anyway.

But it seems like it's affecting all RGB channels.

I suppose other than visiting crazy town, I don't know what else to do...

Maybe the undershoot on the digital inputs from the shifter is causing the DAC internally to go negative voltage on the resistor ladder causing the odd colour changes..

[exxos]

I guess what I could do is wire the RGB lines all to 5V and see if I get a solid white image on the screen...

[exxos]

I guess what I could do is wire the RGB lines all to 5V and see if I get a solid white image on the screen...

Wired all RGB outputs (lifted shifter RGB pins out of the socket) and wired the PCB side all to 5V... totally black screen

[exxos]

Definitely all RGB lines are disconnected and wired to 5V..

I put a variable resistor so I could change the voltage from 0 to 5 V, at about 1.8V I almost get a image..

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But this makes no sense either.. Any higher or lower voltage and the screen is always black..I really don't get this...

The vertical lines might be damage to my machine, I connected the power supply backwards might have damaged something so I need to check on that next..

[exxos]

I also had the thought that maybe if the voltage was all high on the inputs the chip would turn to black... So I lifted the R2,G2,B2 lines, and of course the image went a lot darker, but the weirdness was all still there

I half wonder if these chips are faulty, but I have repaired the Falcon using one of these chips a few weeks ago and the video was fine.

I really am lost, it is looking like this idea might have to be abandoned as I really cannot think of why it would behave like this

[sandord]

What about comparing scope readings between the default ladder and the dac board? Sorry, I can't do it myself because my remake isn't ready to boot yet.

[sandord]

What about comparing scope readings between the default ladder and the dac board? Sorry, I can't do it myself because my remake isn't ready to boot yet.

Hmm, doesn't make much sense to do that I guess...

[stephen_usher]

Is it just me or does that look like an analogue issue on the output? Almost as if it's a "ringing" feedback?

The fact that it takes a while from the top of the screen for the white to fade up to white in the scan lines makes me think that there's a power issue of some kind where something has to charge up before "white" can be displayed, which takes a few scan lines to do.