vCore adjustment on PowerBook G3

[Lycus]

Hello! I'm looking for any documents which will give me the needed information to adjust the vCore on my PowerBook G3, I replaced the CPU and it chimes, but the vCore is far too high for the CPU and I need to turn it down. Any help would be appreciated!

It's a Wallstreet BTW, but my guess is the adjustment resistors are on the CPU card itself otherwise it would be a much more labor intensive process to install any upgrade cards.

 

[Byrd]

Hi Lycus,

What did you overclock the WS CPU card too? Reducing the voltage on a card chat chimes won’t make it any more lively.

 

[3lectr1cPPC]

I believe this has something to do with a project he's talked about over on TinkerDifferent - he wants to solder in a PPC 604 and see what happens. My guess is that needs some vcore adjusting to have even a chance of doing anything.

 

[Lycus]

@3lectr1cPPC @Byrd I got the Mainstreet card in and it had a 750 instead of a 740! So I soldered a 750S on, and am overclocking it. The IBM PPC documents for what I installed say 2.0V or so, the Motorola ones (what was on there) is 2.6V.

 

[Lycus]

We figured it out.

R49 and R48 are in parallel and then R47 is in series with it. This makes up R2 on the formula for the vCore...

vCore = 1.23 * (1 + 10k/R2)

These here

Just copied from my TinkerDifferent post

Shout out to @Bolle for the help over in the TinkerDifferent Discord, and @dosdude1 for the help here!

 

[Bolle]

R49 and R48 are in parallel and then R47 is in series with it. This makes up R2 on the formula for the vCore...

vCore = 1.23 * (1 + 10k/R2)

Can confirm this is correct. There seem to be different revisions of the CPU card and the designators might be different. They were R64/R65 parallel and R63 in series with those two on my card but the location of the three resistors on the board is the same.
My card came with a 8.8k resistor on R49 and a 0 ohms jumper at R47 like in the image above resulting in 2.6V core power supply.
I removed both resistors and installed 10k ones in all three locations to give me 15k of total resistance for a ~2V core supply.

I soldered a 500MHz PPC750L to my card and replaced the cache chips with bigger and faster ones to go from 166MHz 512k to 225MHz 1MB of cache that runs fine at 250MHz.
For anyone who's wondering, the PLL config resistors on Wallstreet/PDQ CPU boards are located here:

(300MHz CPU card shown - 0111 results in 4.5 multiplicator)
Again the designators for those resistors differ with card revisions but the location stays the same.

The following table from the 740/750L datasheet applies:


End result: