LC475 vertical lines in display issue

[Retronaut]

Hey, I have a LC475 and it has issue with its display. Predominantly it has vertical lines visible.
See this video here,

and here
https://www.youtube.com/watch?v=mxbaDJelooI&t=3000s
Where, not only can the lines be seen on the Mac desktop and windows, but also in Apple IIe graphics (IIe Card)

I understand that VRAM in these machines can be quirky, causing pixel corruption, and indeed I think the Apple logo, top left
as well as other graphics display this. Though.. that could be a specific colour band of the Apple, showing up the vertical line issue more clearly.
Maybe thats diagnostic?

I have taken the VRAM out and swapped the sticks around, exactly the same. Then took the VRAM from my Quadra 950 and tried all 4 sticks from it
in this machine. There was no difference.

I have cleaned both SIMM sockets and again no difference.

This week, I have a cap kit coming for both the PSU and the board. So that may spruce things up, but.. Im not expecting it to fix the display issues
Who knows it may, but I cant see why.

Does anyone have an similar experience here with an LC475 or indeed any other Mac of this era. Im wondering if there is a specific chip which may be at
fault.

My best guess right now is...
1. Broken pin on one of the two the sim sockets
2. Broken trace on one specific line or a cracked joint on one of the chips in this circuit
3. A flipflop or similar chip which is keeping the signal for a single bit high/low
4. The display chip has a fault (big pain)

Any pointers welcome. Also, is there a schematic available for the LC475? and ideally a Gerber of the board?

 

[MJ313]

Check this older post for the schematics-

LC475 / Quadra 605 Bad Caps Repair

Long time lurker; first post. Attempting to repair a bunch of bad vias on a LC475 / Quadra 605 motherboard and running into some problems because I don't have a good board to check against or any decent wiring diagrams... Board was owned by another person and they attempted a recap. I bought the...

68kmla.org

 

[zigzagjoe]

Are you certain that it's not an artifact of your display? You seem to have some pretty rough scaling going on.

Schematic attached, no gerbers are available for any apple boards unfortunately.

 

[Retronaut]

Yes, I'm sure, I have an apple RGB monitor, came with it, same issue, though it does look nicer. The rough scaling in the video is because of a VGA adapter + VGA to HMDI adapter needed to record the video.

 

[joevt]

What color modes does the LC 475 support? Does the yellow line frequency change when you switch between color modes B&W, 4 colors, 16 colors, 8 colors, Thousands of colors? How many pixels between incorrect lines in B&W mode?

 

[zefrenchtoon]

I had the same problem. Solved by removing vram simms, clean simms, spraying some contact cleaner on sill slots then insert simms back.

 

[Tchucu]

I had the same problem. Solved by removing vram simms, clean simms, spraying some contact cleaner on sill slots then insert simms back.

Yeah, if you read the original post, you'll see I have already cleaned the sockets/simms, several times, as well as trying completely different simms, and also swapping the simms around. No change

 

[Tchucu]

What color modes does the LC 475 support? Does the yellow line frequency change when you switch between color modes B&W, 4 colors, 16 colors, 8 colors, Thousands of colors? How many pixels between incorrect lines in B&W mode?

The LC has 16 colour mode and also monochrome, though I guess thats just a different palette. Same problem in each, though it manifests differently. I guess I need to make a note of each and do some analysis. It definately manifests in slightly different ways in each mode. I THINK in 16 colour mode for instance, the lines are deep purple, rather than being pale yellow.

 

[Retronaut]

BTW, posts above, from Tchu, are also me. Its my pre Retronaut account

 

[joevt]

The LC has 16 colour mode and also monochrome, though I guess thats just a different palette. Same problem in each, though it manifests differently. I guess I need to make a note of each and do some analysis. It definately manifests in slightly different ways in each mode. I THINK in 16 colour mode for instance, the lines are deep purple, rather than being pale yellow.

Only 16 colors and 16 grays? No 2 color mode? 4 color mode? 256 color mode? Thousands of colors mode? Millions o colors mode?
Maybe use the smallest resolution 512x384 or 640x480?

All the lines being the same color in 16 color mode means the bad data line bit number is a multiple of 4 at least. How many pixels between each line?

 

[Tchucu]

I'll try and take a look at all of this, this evening

 

[Retronaut]

Here is a video of the issue in action...

Dropbox

www.dropbox.com

It shows the vertical line in most of the display modes. Hope this helps diagnose the issue!

 

[joevt]

Here is a video of the issue in action...

Dropbox

www.dropbox.com

It shows the vertical line in most of the display modes. Hope this helps diagnose the issue!

Looks like the line appears every 32 bits.

32 pixels in black and white.
16 pixels in 4 color mode.
8 pixels in 16 color mode.
4 pixels in 256 color mode.

The line is difficult to see in higher color modes because each pixel is multiple bits and changing a single bit might not change the color significantly, depending on which bit is changed and the CLUT.

Does this Mac have a 32-bit bus somewhere related to VRAM? One of the bits is bad. The Macintosh LC 475 and Macintosh Quadra 605 Developer Note mentions a 32-bit pixel data bus between the VRAM and the Antelope IC. The problem would probably be there.
https://cdn.preterhuman.net/texts/computing/apple_hardware_devnotes/Mac LC 475-Quadra 605.pdf

It appears to be the 14th pixel in B&W mode (counting from the left). That would be data bit 18 (if data bit 31 is the left-most pixel and data bit 0 is the right-most pixel).

Did you reseat the VRAM? Does the problem occur without the Apple IIe card installed?

 

[Tchucu]

Thanks for doing this analysis
.
The VRAM has been re-seated, other VRAM chips also tested and the contacts cleaned over and over. No change. However, I have not removed the board to inspect it.
.
I have a re-cap kit, and plan to re-cap the board and the PSU and that seems to be a good point to also try and resolve this issue. I'll get the board out of the case ASAP and first see if there are any cracked solder joints, you never know... and if that proves to be not the case, then Ill have to inspect the traces and see whats going on with that signal eh?

 

[Tchucu]

I see this chip is also involved in video..

MEMCjr IC 2 The MEMCjr IC combines functions performed by several ICs in previous Macintosh designs.
The MEMCjr includes

• control and timing signals for the ROM, RAM, and VRAM
• control logic for system bus arbitration
• frame buffer controller for the video display

 

[Arbee]

Yes, MEMCjr has an integrated DAFB (video) chip.