One day you're a hero
That should pretty much explain the strong motivation behind this project.
Many people were asking for it and many believed it is not possible. However now I hope that in a couple of years when we see a stream of ageing eMacs with fading CRTs we can hopefully secure better future for them.
First off, try not to kill yourself in the process. All the rest will be a bonus for both of us.
Secondly, it is a true DIY teamwork project, I share my findings and ideas with you for you to take them further and do your own share of head scratching.
And finally, all information refers to 800MHz and 1GHz ATI graphics eMacs. 700MHz and 800MHz GeForce2MX models look quite differenet. I did not have a chance to look at 1.25GHz eMac but I assume they must be quite similar to this one.
After this refreshingly novel disclaimer we shall proceed...
Logic board (LB) is what makes eMac a computer. We are going to keep it.
Analog board (AB) provides the power to all other circuits and contains high voltage part of CRT.
We are mostly interested in the voltages it provides to the logic board. They are...
+20V - down-converter power (via LB). Main source of power to all logic devices apart from CPU. Both drives - HDD and optical, USB, etc. are powered from here after converting to a suitable voltage.
+12V - CPU. This is for personal CPU use only and provides up to 3A of current converted further down to 1.55V at up to 20A.
+19V - Firewire bus power. It gets dropped to 18V by a linear regulator on LB and powers up external Firewire devices like portable hard drives or iSight.
+5V - trickle voltage. Provides initial power to startup circuits. This is quite a low current circuit.
Important fact is that those voltages are still there even when the eMac is off or asleep.
IVAD board inside a shiny can at the end of the CRT tube - low voltage part of the CRT. It presents itself to LB as a normal VESA analog VGA monitor but plays important part in power up process and provides control to AB.
Down-converter board (DCB) turns +20V power from AB into 12V, 5V, 3.3V and 2.5V that are supplied to LB, hard and optical drives via attached pigtail connectors.
You might want to replace internal CRT with a nice cheap 17" flat screen and keep eMac in its original case. Maybe add and extra internal hard drive or two. This will need solving two problems - mechanical, including enlarging the fascia opening and power-related. Yes, CRT can be removed and all high-voltage circuits disabled. However analog board uses CRT vertical sync signal to control its power supply so without it all the voltages tend to sag under load. It is solvable of course but I just did not look at it (yet?)
However what we are trying to do here is keep the logic board and throw away everything else.
It is possible but I suggest to keep the down-converter board. You can, of course, get 12V, 5V and 3.3V from a standard ATX power supply but then you need to generate 2.5V. Linear regulator should be enough as this is not a high current circuit. After all the trouble with breadboards you will wish you'd kept the down-converter. If you are trying to fit eMac into a slim case you can always turn DCB on its side and use ribbon cable to connect it to LB.
So the final decision is to keep the logic board and down-converter and power them up with ATX power supply from a PC. In fact, you can even use the older style AT power supply since all you need is 5V and 12V power.
Keep the DCB to save lots of time and frustration.
Use standard ATX power supply to power up everything. Unfortunately you cannot switch on the power supply from the eMac because eMac needs more then just trickle 5V to start up. I have not solved that one yet so for the moment power supply must be switched on manually (by grounding the green wire on its PC motherboard connector.) You only need two voltages: +5V and +12V. I strongly suggest to power hard and optical drives directly from the power supply, not via DCB.
DCB actually cuts off the drives' power when eMac goes to sleep but eMac spins drives down fisrt so even though their power will be on, they will be quietly asleep together with the rest of eMac.
We take an easy route here and use 12V to replace original 20V and 19V power. The result is that DCB cannot produce 12V out of 12V so the DCB output reaches only about 11V which may be too low for HDD or optical drive. But as I said above, get drives powered up by PSU directly to solve that.
Firewire power as well will end up around 11V. This is not an issue unless you have a specific device that expects more voltage. I have tested a couple of external Firewire 2.5" HDD drives and iSight and they were all happy to work on 11V. If you are really a perfectionist, get a power bricks from an old laptop with an output in 15..20V range and use it for Firewire power supply.
eMac Power Connector:
Here is how to connect it to ATX power supply. The easiest way is to use one of ATX PSU hard drive connectors.
Pin 12 (PRO) is the output that goes high when any of the voltages generated by the DCB exceeds its limit. It is supposed to shut down the main power supply. Pin 10 (DCO) turns DCB on when taken low.
eMac Video Connector:
Pins A and B are for power on button.
Connection to 15-pin VGA connector:
|3, 7, 8||5, 10||Ground|
Even though I provide connections for DDE data signals for monitor capabilities information eMac ignores them (at the time being) and uses standard eMac screen resolutions and refresh frequencies. IVAD board does have DDE information chip on it but I assume it is either not being used or resolution data is "stuck" in Open Firmware. Most monitors cannot support refresh frequencies that eMac screen uses (like 115Hz) so it is important to get prepared for possible screen lockout. I have created a bootable Firewire drive that has VNC server starting automatically so I can always boot from it and connect via VNC client over the network. The best would be to stick with 1280x960 72Hz resolution as it is low enough refresh rate for TFT monitors. In any case you want to invest in one of the utilities that allow you to set arbitrary resolution and refresh rates.
Video and power connections to eMac board can be made from hard drive flat cables - power from a standard and video from 2.5" drive one.
eMac will boot up with or without attached video connector so if you want, you can run it totally headless. Alternatively you can drive two external screens as pictured below. I tested it with two 19" TFT screens at 1280x1024 resolution each.
I am running it with only passive cooling for some time now and even though CPU gets quite hot under 100% load it is still stable. A gentle airflow over original heatsink or a quiet PC heatsink and a fan inside a case will be more then enough. G4 in eMac gives off only a fraction of heat of a typical Intel or AMD processor.
I took some pictures as I was taking eMac apart - you might find them useful. eMac works great as it is but I do not have any decent case to put it into yet. You are welcome to beat me to it!
Otherwise, have a good time!
© Leo Bodnar, originally published 28th February 2005. Home