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Author Topic: iMac G4 700/800 bus overclock?  (Read 1944 times)

indibil

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iMac G4 700/800 bus overclock?
« on: December 18, 2023, 03:35:32 AM »

Hello, I have been searching on Google but a lot of information on the internet is being lost.

I have found some information on how to overclock the processor, but I have not found how to increase the voltage, nor if it is possible to increase the bus speed, because this iMac has a 100MHz bus but requires 133MHz RAM.

Does anyone have information about this? or where to find it? Are the schematics of these devices available in PDF? I have seen that the motherboards were planned to include L3 cache but their components were not assembled.

indibil

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Re: iMac G4 700/800 bus overclock?
« Reply #1 on: January 12, 2024, 03:59:11 AM »

It seems that the overclock or CPU swap posts are no longer of interest. Well, I'm here to tell you that it is possible to overclock the bus of an iMac G4 from 100MHz to 133MHz, but with exceptions and setbacks.

First detail, it can only be done on the 800MHz 17" GeForce4 (820-1386) models. Curiously, I have only found it explained in the schematics of the 700/800 15" GeForce2 (820-1275 or 820-1398) models, but none of those mount the components to adjust the bus, they have done it in a different way. Their locations are empty. I have not found the schematic of the 800MHz 17" GeForce4 model, but the design is almost identical, only the nomenclature of ALL the components changes, I had to compare.













Curiosity, the 700/800MHz GeForce2 models have the Vcore set to 1.65V, the 800 GeForce4 model has the Vcore set to 1.75V!!! Exaggerated!

For testing, I changed the CPU multiplier from x8 to x6 to maintain and compare the result at 800MHz.

To overclock the bus, following the instructions in the 820-1398 or 820-1257 schematic, you must remove R636 (100kohm) and add a 470ohm resistor to R637. But "translated" to the component coding of the 820-1386 model of the 800MHz-GF4, you have to remove R690 and add 470ohm in R691.









And I have verified it with a Geekbench



But I have noticed an inconvenience, if you look at these two photos, the amount of RAM has changed. With the OC bus, the RAM SO-DIMM slot has stopped working. I have tried three cards and none of them work on that slot. If I remember correctly, something similar happened with the Sawtooths with the bus overclocked to 120 or 133MHz, where one of the four slots stopped working.





I don't know if there was any solution, I haven't found how to solve it. Maybe someone can shed light.

UPDATE:

After trying a few more RAM DIMMs, I finally got the SO-DIMM slot recognized, without any other modifications. Perhaps compatibility becomes more delicate after the modification, perhaps that is why Apple left it at 100MHz. I don't know, but I confirm that both RAM slots work at 133MHz



p.s. the post in my blog:
https://rosysumenteinquieta.blogspot.com/2024/01/imac-g4-bus-overclock.html





« Last Edit: January 13, 2024, 02:56:31 AM by indibil »
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Protools5LEGuy

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Re: iMac G4 700/800 bus overclock?
« Reply #2 on: January 12, 2024, 05:49:26 AM »

It seems that the overclock or CPU swap posts are no longer of interest.

Ros, eso no es asi. Lo que pasa es que somos unos envidiosos y nos gustaria controlar como tu.
Cambiar de CPU es apasionante incluso oirlo o verlo. No nos dejes sin tus novedades que nos alegran el dia a unos cuantos.
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aBc

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Re: iMac G4 700/800 bus overclock?
« Reply #3 on: January 12, 2024, 08:38:32 AM »

Here’s a little reminder.
Indibil’s blog is located here: https://rosysumenteinquieta.blogspot.com/
It’s in Spanish so you may need to use Google Translate.
But, it’s most definitely worth your time and effort.

(Even if only for the "Panshovelution" engine build!) ALSO AMAZING WORK!

AND anyone having access to G4 Mac mini schematics, let him hear from you!
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indibil

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Re: iMac G4 700/800 bus overclock?
« Reply #4 on: January 12, 2024, 09:39:35 AM »

It seems that the overclock or CPU swap posts are no longer of interest.

Ros, eso no es asi. Lo que pasa es que somos unos envidiosos y nos gustaria controlar como tu.
Cambiar de CPU es apasionante incluso oirlo o verlo. No nos dejes sin tus novedades que nos alegran el dia a unos cuantos.

No es para tanto.  :)

I am no expert, I use very rudimentary equipment, and my experience is little. Changing CPUs is good for me, but you have already seen that I have damaged the VRAM chips of the Mini, those chips must be removed with a lower temperature and more time, or sudden changes will break them.

I need schematics and VRAMs :)

indibil

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Re: iMac G4 700/800 bus overclock?
« Reply #5 on: January 12, 2024, 11:40:02 PM »

EDIT:
I attach pages 6 and 8 of the schematic, where something is said about the terminations of each DIMM module. In the case of the iMac G4, the SO-DIMM is the first DIMM, the one that stopped working when it moved to 133MHz.

Can those values of 22 and 10ohm influence this?

I have read that the IC responsible for bus speed in the iMac G4 is the same as in the QS. In the QS it is possible to increase the bus speed up to 150MHz and the RAM slots continue to function. Maybe if we get the schematic of the QS we can compare how the RAM slots are configured
« Last Edit: January 13, 2024, 02:56:56 AM by indibil »
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indibil

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Re: iMac G4 700/800 bus overclock?
« Reply #6 on: January 13, 2024, 02:57:57 AM »

I have updated the initial post, it does seem that the SO-DIMM slot works, after trying other modules.
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