Apple //e with PAL composite video displaying issue

[XKSTEVE]

I have been working on this //e for a while now and after a Tantalum blow itself apart and I sorted a faulty DRAM out plus the obligatory PSU repairs, I thought I had a good main board until that is I plugged it into composite video as per the attached photo.

It’s fine via both my VGA card and HDMI card (A2VGA and A2DVI) and that is how I would normally run it. But when on composite i get ’incorrect / unexpected characters’ .

I have swapped out the Video ROM - no change . The keyboard encoder must be ok as the display via VGA is fine. It plays games ok and the keyboard maps fine to games.

6502 tests ok as does the DRAM. (Apple //e test software and built in test)

The VGA card reads data from RAM via the 6502 then encodes via a RPi Pico2 so that’s another test it passes.

The video circuit seems ok but was difficult to test as the only schematics I had were for the NTSC boards not the international ones although I have found a PAL diagram now. By tests ok I mean I can see voltages and signals being passed to the socket... but then I would wouldn't I?

Anyone else ever seen this or better still know what to test/try? Could this be a faulty 74LS166 or 175?

Screen shots are whilst duel heading the //e .

 

[joevt]

The Video ROM is where the Apple II keeps the character bitmap data - the Character ROM?

Which screen shot is from the VGA card and which is from the composite output? You said the VGA was OK so the screenshot on the right must be from composite.

The built-in video is not getting the correct Character ROM data. Bit 6 of the character select address is stuck at 1.

hex1=$(printf '1234567890-=QWERTYUIOP[]ASDFGHJKL;'"'"'`\ZXCVBNM,./ ' | xxd -p -c 48)
hex2=$(printf 'qrstuvwxypm}QWERTYUIOP[]ASDFGHJKL{g`\ZXCVBNMlno`' | xxd -p -c 48)
printf "before: " ; sed -E 's/(..)/\1 /g' <<< "$hex1"
printf " after: " ; sed -E 's/(..)/\1 /g' <<< "$hex2"
for ((i=0;i<48;i++)); do
	changedbits=$((0x${hex1:$i*2:2} ^ 0x${hex2:$i*2:2}))
	before=$((0x${hex1:$i*2:2} & changedbits))
	after=$((0x${hex2:$i*2:2} & changedbits))
	printf "changed:%02x before:%02x after:%02x\n" "$changedbits" "$before" "$after"
done | sort -u

before: 31 32 33 34 35 36 37 38 39 30 2d 3d 51 57 45 52 54 59 55 49 4f 50 5b 5d 41 53 44 46 47 48 4a 4b 4c 3b 27 60 5c 5a 58 43 56 42 4e 4d 2c 2e 2f 20 
 after: 71 72 73 74 75 76 77 78 79 70 6d 7d 51 57 45 52 54 59 55 49 4f 50 5b 5d 41 53 44 46 47 48 4a 4b 4c 7b 67 60 5c 5a 58 43 56 42 4e 4d 6c 6e 6f 60 
changed:00 before:00 after:00
changed:40 before:00 after:40

If the Character ROM is 256*8 = 2K, then bit 6 corresponds to address line 9 because there are 3 line select address bits? I guess it depends on how the Character ROM is arranged. I haven't looked at the schematics or the Character ROM data.

 

[joevt]

The VGA card reads data from RAM via the 6502 then encodes via a RPi Pico2 so that’s another test it passes.

Does the VGA card actually read from RAM or does it mirror the RAM by capturing writes? Probably the latter since it would be competing with both the 6502 and the video circuitry otherwise.

 

[XKSTEVE]

Which screen shot is from the VGA card and which is from the composite output?

Hi, Thanks for your reply.

Yes the image on the right is from Composite out (IMG5682.jpeg)

Does the VGA card actually read from RAM or does it mirror the RAM by capturing writes? Probably the latter since it would be competing with both the 6502 and the video circuitry otherwise.

I think it is mirrored from what I read (can't recall the website - probable A2DVI GitHub)

Below is the schematic for the PAL video from the book "Understanding the //e" Which I don't lol.

From what you said could this be an issue with the chip base as the ROM is fine in my other 2 Apple //e machines? if not might it be a trace.?

Again going by what you mentioned, am I right to assume then that the rest of the video circuit is mainly to do with getting the actual image to the monitor and has nothing to do with character generation. Is that right?

Thanks again for your help so far, appreciated.