Hi everyone!
First, i'd like to thank PVV and Fifan for such an awesome project!
(Oh, and sorry for posting this in English, I do understand some Russian but not enough to write what I'm about to)
I had some issues while getting the Galaksija to run. First, none of my monitors would sync. I have experimented with values of R12 and C3, and came to conclusion that either you can increase R12 to about 2k or you can change C3 to 22n to get stable image on screen.
After this problem was sorted, it looked like everything is working, but I couldn't get full initialization (either blank screen or READY without the ">" in the next line. BASIC programs, sort of, worked, but only in "minus" mode. In Plus mode, either text or graphics mode, everything was acting funny. Simple BASIC programs would not run in a loop. The screen editor would get confused at times. I beleive Pluto had similar issues. See video:
After reading trough this thread (again and again) i tried a similar approach - to lower the input voltage. When it was about 4 volts, I could get everything working. I even loaded games from the SD card, and while it seemed to work, I had some glitches on the screen in the graphical mode. See image:
At first I tried changing almost every TTL chip I could find a spare. Nothing. Since I suspected it is a memory problem, and I didn't have a second RAM chip, I tried it with a 6264 chip. It actually worked without any problems, on 5v power supply. After some interesting glitches on the screen (unfortunately, I don't have an image or video of that) I noticed that I got a "wrong" memory block displayed - on the screen I saw the content of the keyboard input area. I started to analyze the memory decoding circuit and decided to try something. After swapping the pullup resistors in the memory decoder to 1 kiloohm (namely, R35, R36, R37 and R48) all of my problems were gone! Everything was working fine - Regular (minus) mode, Plus mode, Graphics, everything! Video proof is here:
After checking the schematic, I should probably change R39 to 1 kiloohm too, but the SD card is working fine.
Oh, and one other thing. After removing C19 and C20 (and making a short across one of them) the picture on the screen is rock-solid. Of course, for better brightness, R9 must be replaced with a 1k resistor.
I also made a PS/2 keyboard adaptor, with an Arduino Nano and MT8816. After I get the first PCBs done and confirm everything is working, I'll share the design here, as a thank-you to everyone involved.
Again, a big thank you to everyone that is involved in this project!
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