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iMac G3 (Rev A -> C) G4 CPU Upgrade


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It's been a long time coming, But I finally gave this a stab today

 

I started by removing the old CPU from the card (anyone who dose BGA rework professionally might want to look away, my setup isn't exactly pretty....)

  

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A overview of the setup, I'm using a griddle as a preheater and a attan 858D as a top heater. It's cheap but it works

 

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A close up of the action

 

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Phew, Got the chip off without lifting any pads :)

 

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The "Victim" - One now unneeded Power PC 750

 

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The pads cleaned up nicely, Almost ready for a new CPU

 

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Not content with the slightly dodgy method of removing the old CPU, I select my "Weapon" for resoldering. 1600w of uncontrolled heat should do nicley

 

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New chip lined up, Also added a little shielding to stop the ram slot melting.

 

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One minute thirty seconds later we now have one Power PC 7410 firmly attached to the board.

 

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It lives!!! :)

 

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Mac OS 8.6 doesn't quite know what to make of the Chip, I guess nothing that supports 8.6 ever shipped with a Power PC 7410

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Mac OS 9.2.2 However knows exactly what it is :)

 

I still need to fiddle with the multiplier jumpers, the chip is a 500mhz part should I should be able to clock it up at least that fast.

 

Now, I ordered 3 chips, Assuming I would screw it up at least once.... What should be next to go under the knife for a G4 upgrade? :)

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Ooooo!! Once you perfect your technique, I'd be interested in possibly having you upgrade one of my CPU boards!

 

Alternatively, I can donate one from my Rev. A/B (not sure which, but it's definitely not a Rev. C or D) in the name of science, and if you're successful in upgrading it I'll pay you for your work and you can send it back.

 

Whatever you want to do....

 

c

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If your interested in one we could do a card swap, i'll upgrade on of my spares and swap if for your unupgraded one

 

It's actually a surprisingly simple process once you get the hang of it, I actually think BGA chips are quite a lot easier to solder then things like high pin count TQFP chips (But then I used to do this allot with xbox 360's so I do have a fair bit of experience with it)

I do need to see how it fares heat wise though, the 7410 is the lower power version of the 7400 iirc (at least it was the one apple used in there laptop's) So it should be ok, but the iMac dosen't have the biggest heatsink in the world

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Now, I ordered 3 chips, Assuming I would screw it up at least once.... What should be next to go under the knife for a G4 upgrade? :)

 

A G3 ZIF CPU card would be fun if possible, those can be throw in both G3s and ZIF carrier upgrades for double usefulness.

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If your interested in one we could do a card swap, i'll upgrade on of my spares and swap if for your unupgraded one

Sure! That can work.

 

I will try to dig it out. By chance, have you been able to up-clock the CPU board so the G4 chip runs at its rated speed of 500 MHz?

 

c

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Ahh, ok, I didn't know that. This may be of interest then: https://www.aliexpress.com/item/PPC740L-GB500A2-PPC740L/32321945368.htmlNot exactly cheap though

 

I have also tried clocking up the G4, with mixed results... 500mhz and 466mhz produced nothing, the iMac didn't boot at all. I then tried 400mhz (skipping 433mhz) which worked fine. There are three reasons I can think of the it didn't work, It may be the iMac simply won't support a CPU at that speed for some reason, it my be my soldering was a bit dodgy, or it could be that the chips I brought from china are not what they claim to be and are actually a slower version of the chip (Although I think this is unlikely). Either way I'll do some more experimenting over the next few days

Edited by max1zzz
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A PPC740 upgraded 6500 logic board could be an interesting upgrade path for a TAM. Comparing the datasheets for the 250-300MHz BGA 603e and the 500MHz 740L, I noticed that the 603e needs a VCore of 2.6v, vs 2.0v for the 740L. That would have to be addressed, as 2.6v is well beyond the maximum tolerance of the 740L.

 

As for the iMac upgrade, it appears the 7410 has a recommended VCore of 1.8v. Perhaps the VCore supply is too high for stable operation above 400MHz and a voltage regulation modification is necessary?

Edited by rsolberg
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Been a while but I thought the iMac processor card had cache chips on it. If so, wouldn't that be limiting the speed?

Depends on whether he's clocking it up by increasing the bus speed, or by increasing the multiplier.  The "proper" method in this case would be to keep the iMac's original bus speed and increase the G4's clock speed by increasing the clock multiplier.  With the original bus speed, the cache chips would see no difference.  I would not be surprised at all if the iMac has little or no head room for bus speed changes.

 

Very cool work, BTW.  Breathtaking.  Which is kind of weird to say about soldering,  but what the h---, it just is.

 

Hmmm, could one take an old G3 ZIF and replace the vanilla 750 with a 750FX or GX and remove the cache chips?  Probably not, because the L2 is internal on the FX and GX, so the pinouts are probably different.  It sure would be nice to increase the supply of 1 GHz G3 upgrades with L2 cache running at full CPU speed...  We could design new ZIF boards easily enough, but that darn pin grid array header was impossible to find the last time I looked for it.  OTH, we didn't have Ali Express back then.

 

Hmmm.  It might be worth comparing the pinout of the 750FX/GX to the pinout of the 603/740 though.   If a 740 would be an improvement, then a 750GX with 1 MB of internal L2 cache running at 1 GHz would be just the ticket.  20X bus multiplier for the win....

 

Voltage regulation is typically by an adjustable regulator whose level is set by two resistors.  Not always, but that's normal.  It shouldn't be too difficult to find the Vreg but it might not be on the CPU card.  It is not, in the Beige G3.

Edited by trag
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Answering my own question, the 750FX and 750GX have a different ball grid than the 740, so no chance of a swap there.  It also appears that IBM has gotten out of the PPC750 business and passed it along to Rochester.  Sold their big foundries (actually paid them to take them) to GlobalFoundries.  And their website pretends that they've never heard of PPC.  Irritating.

 

Also rereading the postings from Max1zzz it appears he was changing the bus multiplier.  Very puzzling that that didn't work, unless he got all the soldering perfect, except the pins that control the bus multiplier...   Is it possible that the G4 uses the bus multiplier pins in a different order or with different setting values than the G3 did?  So maybe when you think you're setting 466 it's actually 566 or some such?

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I'm overclocking it by changing the multiplier, so the cache should not be a problem

 

I have just had a look the the card and can't see anything that looks like a voltage regulator so it may be on the logicboard and not the card.

 

It's possible the multiplier pins could be different on the G4 but I suspect they are the same as 333mhz and 400mhz where the same positions for bot the G4 and G3.

 

Edit, Just tried 433mhz with no luck, So it looks like 400mhz is the max I can get the card to without other modifications

Edited by max1zzz
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I suspect the problem is the Cache RAM. According to the PPC750 datasheet the private cache bus can run at anywhere between a 1x and 3x divisor of the core CPU clock, and the choice of divisor is made in software, not hardware, when the CPU is initialized. I don't know what the iMac's initial setting was (it's probably documented somewhere) or if it's staticly set for all the speeds (233-333mhz) the system was originally sold at or if the firmware adjusts the divisor based on bus clock multiplier, but in either case my suspicion would be that when you clock the G4 higher than 400mhz the cache RAM bus is ending up running too fast.

 

But that's just a guess.

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It sounds like chasing down the Vreg is your best bet in this case. In the Beige G3 Apple used a voltage regulator module on a seperate plug-in card. It was a little complex because it received a signal from the ZIF CPU module telling it what voltage to set. The signal was just controlled by resistors installed or not installed on the ZIF module, so it's just a pins-tied-high/low kind of thing. I don't know what the iMac used though.

 

Okay, did a search on iMac overvoltage and found a thread on Macbidouille. Those whacky French guys found the voltage controlled by a cs5158 chip (16 pin SOIC) on the underside of the logic board. But, I think they were looking at a later dalmation iMac. The cs5158 is a programable voltage controller. Instead of the simple ratio between two resistors arrangement I mentioned earlier (older tech) this newer controller checks the bias on five pins (1, 2, 3, 4, and 6) and sets the voltage accordingly. I guess that gives one 32 settings. They found it fixed at 1.3V rather than taking the pin bias from the CPU card, although their multimeter probing may have been flawed.

Edited by trag
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