Macintosh LC MacsBug Video Issue

[danifunker]

Hi everyone! I need to run MacsBug on my Macintosh LC to gather some low-level hardware information, but when I try to run 6.2.2, I get a lot of strangeness on my screen.

My current setup:

Macintosh LC
FPU Card Installed
512KB VRAM
BlueSCSI (external)
MacOS 7.1.2

I tried in both 640x480 and 512x384.

I'm using a 10-DIP DB15 to VGA adapter and a RetroTink 4K, and everything seems to be displayed normally in all other screens other than MacsBug.

This machine was recently revived as well, everything seems to be working really well aside from this.

Am I doing something incorrect?

Here is a photo at 640x480. I've tried tweaking settings on the RetroTink 4kCE to no avail, I believe this might be a Mac LC issue or a software thing.

I've tried removing the mxbc resource from the preferences (Debugger Prefs, located inside of the System folder, beside MacsBug) but that doesn't seem to work.

This is my first time working with MacsBug, thanks so much!

 

[danifunker]

Looks like the issue has to do with the 512KB VRAM SIMM I bought from Jurassic Computing. When I put the original 256KB SIMM back, everything appears normally.

I'm going to reach out to them and see what they say.

 

[Siliconinsider]

There is a known issue with the TSSOP version of my VRAM, which uses NEC chips, and the Mac LC. A customer reported it to me several month ago and I have been able to replicate this on a LC that I have purchased since. Other macs are unaffected.

The glitch appear when a window is moved, and part of the display is replicated in the moved window.

I am not sure exactly why there is a glitch specifically with the LC, other than probably timing issues triggering a specific register in the VRAM.

The other 512k modules I make, with SOJ chips on a blue PCB, does work fine in the LC, and I use Toshiba or Samsung chips on those.

 

[danifunker]

Thanks for the quick and thorough response. what about the macsbug issue I found?

 

[Siliconinsider]

I do not think it relates to macsbug. It's an issue between the VRAM and the Mac LC. Glitches should occur within the finder when dragging windows around as well.