Post by klyde on Aug 27, 2018 19:53:37 GMT
Bonjour à tous.
I have a very fussy LCD TV with regards to non-standard RGB-SCART signals generated by our old Ataris: ST, 2600 and 7800. Generally the picture is OK during 3 to 5 seconds after I power the computer or console up, then it bounces horizontally, and it's barely playable.
And yes, the culprit is my TV, not the computers or console, because when I plug another device on it, I have no troubles.
I can't have a CRT due to lack of free space, and my wife would not tolerate it.
So after I documented myself, I wanted to give a try to a CGA/EGA/RGBS to HDMI board. My idea was to integrate it into my Atari STf, since I don't want to have an external box or wires everywhere on my desk.
I ordered one on Aliexpress for less than 30.00€: aliexpress.com/item/New-Arrival-CGA-TO-HDMI-converter-RGBS-CGA-EGA-to-HDMI-converting-Board-PCB-for-HD/32837186422.html
The converter board
This board needs to be powered with 5V 2.0A, but the Atari PSU can't provide enough current, I then used a phone charger that meets these specs. I extracted the adaptor from its enclosure.
I crack opened the USB phone charger
The board also needs combined Horizontal and Vertical Synchronisation signals (HSync & VSync), but my Atari STf delivers separate HSync & VSync from video connector pin 9 & 12. You don't want to just tie both wires as it shorts the GLUE, but use a logic chip instead.
I designed a simple vero board with a 74LS86 and a decoupling capacitor. Some say 74HC00 works better instead, I bought one of each to compare:
The vero board with its 74LS86 seated
I soldered everything up temporarily for a test and powered it up:
(The 5V adaptor is wrapped into cardboard because it is soldered to the Atari PSU 240V)
Ready to be fired up!
At first I was disappointed: All I had was a black/flickering screen. I went into the board settings to tweak the parameters and... Voilà:
There came the (flickering) light!
But what you don't see on this picture is the screen flickering and going black for 5 seconds every 30 seconds or so...
I first tried the 74HC00 instead of the 74LS86, it seemed a bit better, but still unstable. I checked the HSync & VSync signals with an oscilloscope: the required 64µs and 20µs pulse at 5.0V were OK.
I went back to document myself about this kind of trouble and found that the board sync input have to be in a 3.3V range, not 5.0V! I then added a 470 Ohm resistor to the vero board sync output side, and here we go:
A perfectly stable picture!! I then loaded one of my all times favourite video game: Dungeon Master thanks to @fredm who sent me an original disk weeks ago:
Beware of mummies and giant worms!
My next step will be to integrate the board into the Atari, and install a female HDMI panel mount. And I will definitely buy another board to be integrated into my 7800.