Slot Load G3 mod using CRT?

Posted by: bearsomg

Slot Load G3 mod using CRT? - 09/23/12 08:50 PM


A while back I bought an iMac G3 slot loading from a thrift sale because it was only $6, worked perfectly, and came with the keyboard and mouse.

I finally decided that I want to mod it and stick in an Intel mini-ITX board to replace the iMac's logic board, while still using the iMac's CRT as a monitor. I know this is possible because I have seen a few posts about people who have successfully installed Mac Mini hardware in these machines while still using the original CRT.

So far I have the case completely off, the logic board out, and I pulled off the video board to look and see if I could find the pins I needed to make a VGA adapter. I managed to find a pinmap for the HE20 connector and a VGA cable here:

Using that pinmap I have wired a VGA cable up to the HE20 video connector under the logic board. Since I read somewhere that the power board needs +5v on the PFW pin to turn on, I bridged it to the T5V pin. When I plug it in and bridge the pins, I can hear the CRT charging up which tells me that the system is powered on, however no video is displayed on the CRT. I am currently using my laptop to test it, with a resolution of 1024x768 at 75Hz, which I know is supported by the monitor because I used that resolution when the PPC logic board was still installed. Does anyone have any ideas? I'm pretty much exactly where this guy ended up:
Posted by: ym58

Re: Slot Load G3 mod using CRT? - 02/18/14 04:28 AM

Hello Bearsomg,
I have been trying the exact same mod as yours (to include a mini ITX-PC board in an iMac G3 slot-loading while keeping its PSU and CRT !) and I am now stuck exactly where you seemed to be stuck on Sept.2012 !
I have made a VGA cable to connect the HE20 connector to the VGA output of the PC board but it looks like I am still missing some pins.

Even when shorting +5V-trickle (T5V) to PFW, I cannot hear the "PONK" sound of the CRT charging up ....
Have you made yours working right finally ?
Could you finally find 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.