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View Full Version : Very strange green halo on a successfully overclocked G4


gsl2
Apr 27, 2004, 01:31 PM
Hello All!

I am running Panther OS X 10.3.3 on an elderly Gigabit AGP G4 400 with 512 MB of RAM. While this machine was comfortably fast running OS 9, it became sluggish and unresponsive with OS X 10.2. Especially the Finder feels annoyingly slow. Panther made this a little better, but it still is too slow to be fun and is almost unusable. Going back to OS 9 is not an option for I love Panther for the UNIX and had so much trouble with OS 9 which is all gone now.
Therefore I started experimenting with overclocking and was very happily surprised when I successfully overclocked the CPU to 500 cycles just by removing one resistor from the board. The effect was astonishing, the responsivenes was back and the machine was usable again - stable at no noticeable temperature rise (fluke probe measures 26-28 C, same as before). It was fun working with it again but for a small but very annoying detail:
In black text over light background a few characters have a lime-green halo. Some kind of strange shadow that seems random and does not go away or move. It does this in OS X only. OS 9 is 100% OK.
I was hoping that this might be a minor problem which can be fixed by another tweak in HW or SW. It would be a great gift if a solution could be found that would enable me to keep on working with this babe for another year or two - now that I am starting enjoying her.
Any thoughts and suggestions would be greatly appreciated. Thanks and regards,
gsl.

Makosuke
Apr 27, 2004, 05:43 PM
Odd... here's an important question: when you take a screenshot, does the green halo show up in the image (if it does, attach one so we c'all see). That would point to a software problem, either unrelated to overclocking or due to some glitch introduced with the overclocked processor. If it doesn't "photograph", it's obviously hardware, and could be something as simple as a glitch that the overclocking introduced into your video card.

stoid
Apr 27, 2004, 05:48 PM
a halo?

*ring*

se..v...en......d...ay....s

*click*

cait-sith
Apr 27, 2004, 07:33 PM
your processor is now using more voltage than before.. (or is it driving more current? it's been a while)

in any event, there may be some stray electricity (or electromagnetism) now affecting your video hardware.

is it possible to resoldier the resistor and restore the machine to it's correct setting?

King Cobra
Apr 27, 2004, 07:40 PM
your processor is now using more voltage than before.. (or is it driving more current? it's been a while)

Boy, am I glad to have studied Ohm's Law in detail this semester:

V = IR

I = current = dQ/dt (Q = charge)
R = resistance

By removing a resistor, you don't change the voltage of a processor (or whatever). You change the current. As R goes down, I goes up to satisfy the same amount of voltage.

Although I can't explain how that induces the green glow.

haiggy
Apr 27, 2004, 08:54 PM
Wow, I am in grade nine and currently learning about that. (Ohm's Law)

Mav451
Apr 27, 2004, 08:56 PM
Haha u learn it once in Physics, or basic science. Then again in AP physics. Then again in College. Trust me, this won't be the first, and it won't be the last :(

gsl2
Apr 28, 2004, 03:29 AM
King Cobra and haiggy:
Thanks for your input. In this case the resistors are acting as registers (jumpers) setting the circuit to regulate the clock. Nothing to do with Ohms law. If you would like to read up on this, study the links below.

http://www.lowendmac.com/macdan/02/0624ek.html
http://www.macinfo.de/tuning/tuning-g4saw.html
http://www.xlr8yourmac.com/G4ZONE/sawtooth/SawtoothCPUdesign.html

cait-sith:
Since the voltage is set and fixed by another set of registers while the cpu is doing more cycles, it obviously is drawing more current. I have set the speed down to 450 MHz and the halo has disappeared. I miss the 50 MHz very much though.

Makosuke:
Very good point!!! I wish I had thought of capturing the green shadow before I downclocked the cpu. Now (oddly again) it won't boot into 500 any more but gives me a kernel panic. I may try to do it again on the weekend with more time and in peace.

Again, thank you for your time and thoughts. Even the childish ones I don't understand, stoid.