LC III, happy boing, no video output

[emuboy]

Hello!

For Marchintosh I managed to score an LCII and LCIII, while the II was a quick fix with new caps, this one it is definitely not, provided same cable,same PSU and same setup of the LCII i get an happy boing but no video, I get nothing at all, no h/v sync or colours, the scope show some faint noise, that's it, I checked the BOMAC schematics and I'm a bit confused by it, there doesn't seem to be anything that would keep the CLUT/DAC from operating and the system would do a sad mac noise if anything like that would happend, right?
Also, I have a db15 to VGA, it's a 2 row of dips, bought on ebay, i think is based on an opensource project, but if that works on the LCII shouldn't be a problem with a III, right?

If you have any idea, please let me know, I never owned an LC and I ususally repair compacts.

View attachment PXL_20260318_215230541.jpg

 

[dougg3]

I have an LC III that had this exact same symptom (chime and then nothing) and it ended up being a broken trace (well, a bad via) between the CPU and the RAM on the data bus. It was like the RAM test was failing so badly that the Chimes of Death couldn't play. If the adapter you're testing works with the LC II, I'd expect it to work with the LC III too.

The cap depicted here (shown with my tantalum replacement) tends to ruin the nearby vias with leakage. I've seen damage in this area on two LC IIIs.



What I would do is use the Bomarc schematics to trace out continuity between the RAM and CPU. I bet you'll find something broken. I'd put my money on one of the data lines or the address lines. My broken via was D13.

 

[emuboy]

Ok, I will search that area, but I thought that a good boing was a symptom of good ram?

I which it would crash, so I could drop into the debug serial test

 

[dougg3]

I’d normally agree with you, but I saw it with my own eyes on my LC III. Usually bad RAM results in a chime followed by the chimes of death.

 

[mikes-macs]

Probably a dumb question but did you remember to install a vram SIMM? Maybe swap out the one from the LC II. I started my performa once without any vram and it chimed and booted but I couldn’t see anything.

 

[Juror22]

Probably a dumb question but did you remember to install a vram SIMM? Maybe swap out the one from the LC II. I started my performa once without any vram and it chimed and booted but I couldn’t see anything.

You beat me to that suggestion by 2 mins!

 

[dougg3]

The LC III doesn't require a VRAM SIMM.

 

[mikes-macs]

Oh, I see. I guess it was a dumb question after all then.