From looking at a schematic of the CRT it looks like they have capacitors in series to cut the dc. Going to try today some caps in series to see what happens on the R/G/B channels. It may be that rather than lifting the signal out of the connector on the mother board I lift it somewhere out of the crt past its somewhat odd resistor and capacitor setup. I was wondering if they did this to make the whole lot more tolerant to noise and interference.?
On Mon, Nov 4, 2024 at 10:34?PM Kuba Sunderland-Ober via <kuba=
[email protected]> wrote:
I¡¯ve used EGA sources with VGA displays in a pinch with a passive ¡°DAC¡± with resistors and a black restorer using an RF Schottky. I don¡¯t think I understand what¡¯s so special about ¡°native EGA¡± monitors. Even back when I actually had an EGA in one of my PCs in the 80s there was a little homebrew dongle that adapted that output to VGA. Many hobbyists I knew had one. I don¡¯t have the schematics anymore but it was common and cheap to put together.?
Cheers, Kuba
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From the tomverbeure reference you provided,?
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"The voltage levels for the sync signals are a standard 5V TTL (good!), but the voltage levels for the analog signals are weird, with a baseline of -1.7V and a maximum value of 125mV (WTF?), or an amplitude of 1.825V."
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I guess this would explain why you got the display to sync but with the video all black. VGA appears to be 0 to 0.7V positive, ie composite without the sync.
So you'd need to shift the levels up 1.7V and halve them to get anything displayed.
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