iMac G3 (slot) mod - Video Connector

ym58

macrumors newbie
Original poster
Feb 18, 2014
11
1
Paris, France
Hi everyone,

I have been trying for the last few days to include a mini ITX-PC board in an iMac G3 slot-loading while keeping its PSU and CRT (sort of nostalgic 'vintage' thing , I guess ....).

I have made a short custom cable to internally connect the VGA output of the PC board to the HE20-pin video connector but it looks like I am still missing some pins.



Even when I short the +5V-trickle (T5V) to PFW, I cannot hear the "PONK" sound of the CRT charging up ... (needless to say that the CRT works quite right with the original iMac logic board).

Has anyone made such a mod and did you find out if pins 4, 5, 7, 8, 12, 19 and 20 have to be connected and where to ?

Thanks for any help you can share here or by PM.
ym58
 
Last edited:

ym58

macrumors newbie
Original poster
Feb 18, 2014
11
1
Paris, France
Hi everyone,

Unfortunately, I could not find any relevant info on the internet despite the numerous hours of google search that I made ... :-(

So I have decided to make a test board to parse every single signal that comes out of the mother board and connected to the video card.


What I found is that, besides the "classical" red/blue/green, hsync/vsync and ground signals, most of the other signals are NOT used BUT the two SDA and SDC signals (respectively pins 1 and 2).

When connected to an oscilloscope DURING THE BOOT, these two signals show that the motherboard raises them to +5V then real quick ( after a few 100th of msec ) applies a particular pattern sequence (logical states +5V and 0V) then finally leaves them to +5V all through out.
From this point on, the "PONK" of the CRT is heard, and everything works out right !

Then, you can even drop the two signals (SDA and SDC) to 0V, no matter what : the iMac is definitely ON and so is the CRT.

My deal now is to find how to replicate those TWO short sequences (they are both different, I mean SDA seq is not the same as SDC seq !) WITHOUT the main board.
And then, I should be ready to get rid of the iMac motherboard and install a PC board in the iMac G3 !

Any hint, idea, suggestion, reflection whatsover is VERY welcome ...
ym58