OSI 600D Superboard Build

[rubber_jonnie]

OK, so this is new:

IMG20240319130246.jpg

What I did was remove U61 (I was removing all IC's to see if any caused a significant change) and the screen changed to this.

I checked the video signal with and without U61 and could see a significant difference, here with it installed:

IMG20240319124004.jpg

And here without U61:

IMG20240319123816.jpg

The other thing I checked was the VSYNC & HSYNC signals at pins 12 & 13 respectively of U70, and both are reading 15.63KHz. Surely pin 12, which is VSYNC should be 50 or 60 Hz? Pin 4 of U65 is also reading 15.63Khz with U61 installed, so I am getting the wrong VSYNC it would seem.

[stephen_usher]

So, without that chip you see random characters but with it the scan lines are partly blanked. Strange. What is U61?

[rubber_jonnie]

So, without that chip you see random characters but with it the scan lines are partly blanked. Strange. What is U61?

Correct.

Well, according to the service manual & build manual, U61 is a 75163, but according to two separate separate schematics and the BoM from the build manual, it's a 74LS163, so we're in BOMARC territory again...

From the schematics, U61 creates a number of output frequencies, one of which is 60Hz at pin 11, which it then feeds to pin 1 of U65, and that does arrive on pin 1 as expected, and is also consistent with NTSC vertical sync.

VSYNC then comes out of pin 4 of U65, only I'm not getting 60Hz, I'm getting 15Khz. Pin 12 of U65 provides the 15Khz HSYNC, though there is no short between pin 4 or pin 12.

So it seems that I'm getting HSYNC twice, once at pin 4 and once at pin 12 (Where it should be). I lifted pin 4 of U65 so the 15KHz signal is only present where it should be and I get the same result as removing U61, so I think the problem is narrowed down to U65 right now.

At least assuming I should see 15.63Khz for HSYNC and 60Hz for VSYNC. I need to check the datasheet for the 123 and verify what it should be doing based on the components around it.

[stephen_usher]

At least assuming I should see 60Hz for HSYNC and 15.63Khz for VSYNC. I need to check the datasheet for the 123 and verify what it should be doing based on the components around it.

Surely that should be the other way around? VSYNC should be slower, once every frame, but HSYNC faster as it's every scan line? Assuming that it's no interlaced VSYNC would be 30Hz and HSYNC 60*scanlines Hz.

If interlaced then VSYNC every 60th of a second and HSYNC 60*scanlines/2 Hz.

[rubber_jonnie]

At least assuming I should see 60Hz for HSYNC and 15.63Khz for VSYNC. I need to check the datasheet for the 123 and verify what it should be doing based on the components around it.

Surely that should be the other way around? VSYNC should be slower, once every frame, but HSYNC faster as it's every scan line? Assuming that it's no interlaced VSYNC would be 30Hz and HSYNC 60*scanlines Hz.

If interlaced then VSYNC every 60th of a second and HSYNC 60*scanlines/2 Hz.

Typo on my part, I have corrected it :)

15.63Khz HSYNC 60Hz VSYNC.

The bottom line is that I see 15.63Khz for both VSYNC and HSYNC which can't be right. Removing U61 took away the 60Hz signal that goes to pin 1 of U65 where both SYNC signals come from. There is no short between the VSYNC and HSYNC pins of U65 BTW and lifting pin 4 of U65 has much the same effect as removing U61.

The 60Hz signal, C15, from U61 does arrive as expected at U65 pin 1, but leaves as 15.63Khz on pin 4...

[stephen_usher]

Can you get the machine to run fully other with the pin lifted? i.e. with everything else in place does it start up?

[rubber_jonnie]

Can you get the machine to run fully other with the pin lifted? i.e. with everything else in place does it start up?

No, it doesn't seem to. What I see is essentially the basic test that verifies the video circuit and with no CPU installed. Even adding the CPU and basic ROMs doesn't change things, so I need to get a stable video output so I can see what's going on before I can troubleshoot further

This basic test is part of the build and should show a screen of OSI char 255 or sometimes random characters.

What doesn't make sense is why I don't see a VSYNC of 60Hz at pin 4 of U65, so time to have a look at what the IC does. If I remove the CHARGEN ROM I don't get anything but a blank screen, but I do see some jail-bars, so that suggests that the CHARGEN works and so does the video RAM.

Annoyingly my mini TV now has a slightly collapsed picture, so I think I'll need to have a look at that too...

[rubber_jonnie]

Looking at the datsheet for the LS123 (U65) and the schematic, it uses just an external capacitor and resistor to determine the output, so I need to check those values are indeed correct because it looks like from what I can see that U65 converts the incoming 60Hz signal into a square wave at the same frequency, since this is what it does to the 15.63Khz signal in the 2nd part of U65.

I'm now wondering if I have a wrong capacitor here like I did in the clock circuit.

[rubber_jonnie]

OK, so we have progress. It was the capacitor, but not due to a wrong value, but incorrect location.

The component placement diagram I have shows this for C8:

Screenshot 2024-03-19 at 22.42.45.png

Based on the info I was given, it was my understanding that the cap should go in the red hole and the hole immediately to the right of it, turns out that the cap should go into the holes either side of the hole marked in red!!!

So I swapped it to this:

IMG20240319222739.jpg

And I now get a rock steady picture like this with no CPU fitted:

IMG20240319223254.jpg

Which is precisely what the build manual states I should get and proves the CHARGEN ROM and Video RAM is working as expected. U65 now shows 60Hz on pin 4, which is VSYNC. Picture is rock solid.

So I have to move onto the next issue now, which is when I plug in the CPU I just get onscreen garbage, with or without the BASIC ROMS.

I'll take the video circuit as a win for now, small steps :)

[stephen_usher]

:dualthumbup: :yay2: :cheer: