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Mac LC and DB15 to VGA adapter problem...

MrFahrenheit

Well-known member
I use a few tools for testing my fixed Macs:

Norton Utilities 2.0 System Info benchmark

Apple MacCheck 1.3

NewerTech RAM Gauge (to test ram)

TechTool Pro

 

dragon_x

Well-known member
Yes!  Another one lives!
I am very happy for that :)

The last thing to do is whitening case, keyboard and mouse... hmm and find bigger capacity simms, cause I have only 4MB of RAM now.

I am looking for next project to bring to life second old Macintosh ... :D

 

dragon_x

Well-known member
Hi, I'm back after a long break. The struggle with LC continues. Computer is working, but as you can see in the pictures, the screen has vertical stripes and the image sometimes streaks. All capacitors have been replaced, the board has undergone a bath in IPA and cleaning. Two tracks were damaged cause capacitor leaked out. I have repaired it. Can I ask for advice on what else to check or do ?
 

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bdurbrow

Well-known member
I haven't been following the thread; but just jumping in at the end here and eye-balling it... I think...

I would suspect either bad VRAM, a fault in the data bus connecting to the VRAM, a bad buffer on the bus (if it even has one - I haven't looked at a schematic) or a bad video output chip.

Basically, what that vertical stripe pattern is indicating is that one of the bits in a byte (actually, it looks like a bit in two bytes, as it seems to be repeating every 16 pixels on the screen shot of black-and-white mode, and every 4 pixels in 16 color mode) of the video data is getting messed up. It could be that a whole column of the video memory chips is fried; or it could be something on the way to or from that video memory.
 

bdurbrow

Well-known member
Do you have an oscilloscope? If so, you might be able to trace the signal path and find a trace that's stuck low...
 

cheesestraws

Well-known member
Agreed, this definitely looks like either bad VRAM or something on the path to it.

I don't have access to any schematics for the LC but a couple of perhaps useful observations based on the LC II schematic:
  • The VRAM does have 16 data lines to it, at least if I'm reading this schematic right, so that would fit with the symptom you're seeing.
  • It looks like the buffers in front of video RAM are shared with "normal" DRAM on the LC II (p. 7 in the Bomarc PDF that's out there). Whether this is true on the original LC I'm not sure, but if so, it would mean that one or two of those failing would knacker main memory as well, which obviously isn't true for you or the machine wouldn't boot. So I'd probably look at the VRAM itself first, personally.
 

dragon_x

Well-known member
f... my both LC's have onboard VRAM ;( I have also IIvx ill check it if it has VRAM maybe it will works with my LC.
 
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