uncooking the sunflower seeds, first generation iMac G4 repair/CPU replacement

[LightBulbFun]

some of the long timers here may remember my post here

Show us your Chips!

this kids is your iMac G4 on drugs! :eek: I recently got ahold of 2 first gen iMac G4s one of them had no RAM but worked fine once RAM was installed :) , the second iMac G4 however did not work, (powered on but no life otherwise) it was also missing its bottom pan and 2 of the 4 chassis...

forums.macrumors.com

about an iMac G4 I had with a very dead CPU, now while this would normally be the end of the line for such a machine or its logic board, I did wonder if it could bre revived if I replaced the CPU and as those who have seen my previous threads know I have the gear to do that

and inspired by @dosdude1 most recent video

(which you can read the "backstory" to here https://forums.macrumors.com/threads/worlds-fastest-imac-g4-2-ghz-imac-g4-cpu-upgrade.2125765/ )

I finally got round to replacing it

I replaced the fried 800Mhz 7451 with a 1Ghz 7455 from a single 1Ghz MDD card that @d-oost kindly gave to me a while back specifically for this project

so while its technically an upgrade from its stock 7451 and it was more done to get the machine working, then as an upgrade, although I do plan to clock it to 1Ghz since it would be rude not to now

im pleased to report the swap worked, kinda, the machine POSTs and sometimes boots OS X but its very unstable etc

im still investigating these issues but it is rather complicated, in that there are quite a few variables at play more so then normal

i dont know for example what the rest of the board is like, for example it may have had other issues before it had its CPU fried?

but I am pretty pleased that it POST's etc it shows the PowerMac4,2 firmware and hardware is indeed compatible with the 7455 CPU, I had suspected it was because its firmware 4.3.5 is higher then say 4.2.8 the latest released for the PowerMac3,1(PowerMac G4 sawtooth) and I know the PM3,1 is compatible with the 7455

(also pleased it POSTed because this was a little bit of a slapdash job compared to my usual standards! when I go to clock the machine to 1Ghz I will reflow the CPU to be on the safe side)


now the fun one to try would be to install a 7457 CPU, but I know that one will require more work, specifically patching of the firmware which I cant do with a dead CPU and while it SHOULD be electrically compatible with the 7450/7451/7455, it is still a little unknown (the 7450-7455 are all more or less the same in regards to overall performance, the 7457 however has double the L2 cache from 256KB to 512KB, and of course can be clocked higher then a stock 700 or 800Mhz 7451 CPU!)

hence why I went with the relatively safe bet of the 7455

I do have a second identical iMac G4 that is fully functionally, I think i will look at upgrading that one to a 7457 CPU some day

(i got both of them together as a untested bundle, got lucky with one thats fully functional but not so "lucky" with the other! but it does mean I do have a working iMac G4 even if I cant get this one fully functional)

but for now I hope enjoy this iMac G4 CPU replacement

and of course heres a few pictures



first getting the donor CPU card on the board preheater and getting it up to temperature

then heat gunning the CPU off

then cleaning up and reballing the "new" CPU

then moving onto the iMac G4 board, on the preheater once more

old fried 7451 removed and pads cleaned up ready for the "new" 7455

and finally new CPU installed

heres a few shots from OpenFirmware and OS X

 

[RogerWilco6502]

Glad it worked for you! As I was watching dosdude's video I made a mental note to never do BGA soldering, as I could very easily mess it up badly

 

[Gamer9430]

It truly is fascinating to see how far our skillset and knowledge of these machines and their inner-workings have advanced as time has gone on. Stuff like this would have never been possible when I first got into the PPC scene back in 2014. The only person talking about doing insane stuff like @LightBulbFun and @dosdude1 are doing on a “normal” basis now was @rabidz7, and he never had the evidence to back up most of his claims of doing insane and far-fetched things with his PPC Macs. Just wild to think about how far two of the brightest minds on this forum have come when working on machines of various ages. I hear @dosdude1’s name get mentioned in a variety of places online (most recently on a Target Team Member discord server) thanks to his work on the macOS on Unsupported Macs applications year-after-year, and that’s just absolutely fascinating to me that normal people are hearing about the work being done here between LBF and dosdude. Kudos to both of you on your incredible efforts and developments with these beloved machines

 

[0248294]

"Gentlemen, we can rebuild him. We have the technology."

Glad to see my MDD CPU has been of use. It's not seen use since I traded that Macintosh SE for a 1.42GHz DP card with you, which it's still happily running with, so at least its old heart (or brain, if you insist) has been successfully transplanted into this poor old iMac. I think it deserves the slight upgrade after its hardship

Hopefully we can figure out what's bugging it, I'm sure it'll be fixed. We're already mostly booting, a big step up from what it did before, nothing.

 

[TiggrToo]

some of the long timers here may remember my post here

Show us your Chips!

this kids is your iMac G4 on drugs! :eek: I recently got ahold of 2 first gen iMac G4s one of them had no RAM but worked fine once RAM was installed :) , the second iMac G4 however did not work, (powered on but no life otherwise) it was also missing its bottom pan and 2 of the 4 chassis...

forums.macrumors.com

about an iMac G4 I had with a very dead CPU, now while this would normally be the end of the line for such a machine or its logic board, I did wonder if it could bre revived if I replaced the CPU and as those who have seen my previous threads know I have the gear to do that



and inspired by @dosdude1 most recent video

(which you can read the "backstory" to here https://forums.macrumors.com/threads/worlds-fastest-imac-g4-2-ghz-imac-g4-cpu-upgrade.2125765/ )

I finally got round to replacing it

I replaced the fried 800Mhz 7451 with a 1Ghz 7455 from a single 1Ghz MDD card that @d-oost kindly gave to me a while back specifically for this project

so while its technically an upgrade from its stock 7451 and it was more done to get the machine working, then as an upgrade, although I do plan to clock it to 1Ghz since it would be rude not to now

im pleased to report the swap worked, kinda, the machine POSTs and sometimes boots OS X but its very unstable etc

im still investigating these issues but it is rather complicated, in that there are quite a few variables at play more so then normal

i dont know for example what the rest of the board is like, for example it may have had other issues before it had its CPU fried?

but I am pretty pleased that it POST's etc it shows the PowerMac4,2 firmware and hardware is indeed compatible with the 7455 CPU, I had suspected it was because its firmware 4.3.5 is higher then say 4.2.8 the latest released for the PowerMac3,1(PowerMac G4 sawtooth) and I know the PM3,1 is compatible with the 7455

(also pleased it POSTed because this was a little bit of a slapdash job compared to my usual standards! when I go to clock the machine to 1Ghz I will reflow the CPU to be on the safe side)


now the fun one to try would be to install a 7457 CPU, but I know that one will require more work, specifically patching of the firmware which I cant do with a dead CPU and while it SHOULD be electrically compatible with the 7450/7451/7455, it is still a little unknown (the 7450-7455 are all more or less the same in regards to overall performance, the 7457 however has double the L2 cache from 256KB to 512KB, and of course can be clocked higher then a stock 700 or 800Mhz 7451 CPU!)

hence why I went with the relatively safe bet of the 7455

I do have a second identical iMac G4 that is fully functionally, I think i will look at upgrading that one to a 7457 CPU some day

(i got both of them together as a untested bundle, got lucky with one thats fully functional but not so "lucky" with the other! but it does mean I do have a working iMac G4 even if I cant get this one fully functional)

but for now I hope enjoy this iMac G4 CPU replacement

and of course heres a few pictures



first getting the donor CPU card on the board preheater and getting it up to temperature




then heat gunning the CPU off





then cleaning up and reballing the "new" CPU





then moving onto the iMac G4 board, on the preheater once more




old fried 7451 removed and pads cleaned up ready for the "new" 7455



and finally new CPU installed

heres a few shots from OpenFirmware and OS X

Personally I've zero interest in doing this myself, but I do so like watching others take on tasks like this.

Kudos to you and thanks for sharing!

 

[re4mat]

im pleased to report the swap worked, kinda, the machine POSTs and sometimes boots OS X but its very unstable etc

im still investigating these issues but it is rather complicated, in that there are quite a few variables at play more so then normal

I would bet money the issue is in the VCORE voltage that the iMac's logic board is set to. The 7451 on the 1st-gen iMac is set to 1.65V for VCORE, but the 7455 is a lower-voltage CPU. According to the datasheets, the 7451's VDD is 1.6 V ± 50 mV (hence 1.65V on the iMac's default setting), where the 7455's VDD is 1.3 V ± 50 mV. (See the table on page 12 of each processor's respective datasheets.) While the 7455's maximum VDD is 1.95V (see page 10), 1.65 is a pretty significant voltage bump over 1.3. If you look at page 22 in the schematics for the iMac (820-1257, which I know you have, since you gave me my copy! 😊), you'll find the resistor settings for setting VCORE/VDD. I bet if you set it for 1.3V or even 1.35V, your stability problems would go away.

(I'd have posted screengrabs of the tables from the datasheets, but apparently I don't have the privileges for posting images yet on this forum.)

 

[LightBulbFun]

I would bet money the issue is in the VCORE voltage that the iMac's logic board is set to. The 7451 on the 1st-gen iMac is set to 1.65V for VCORE, but the 7455 is a lower-voltage CPU. According to the datasheets, the 7451's VDD is 1.6 V ± 50 mV (hence 1.65V on the iMac's default setting), where the 7455's VDD is 1.3 V ± 50 mV. (See the table on page 12 of each processor's respective datasheets.) While the 7455's maximum VDD is 1.95V (see page 10), 1.65 is a pretty significant voltage bump over 1.3. If you look at page 22 in the schematics for the iMac (820-1257, which I know you have, since you gave me my copy! 😊), you'll find the resistor settings for setting VCORE/VDD. I bet if you set it for 1.3V or even 1.35V, your stability problems would go away.

(I'd have posted screengrabs of the tables from the datasheets, but apparently I don't have the privileges for posting images yet on this forum.)

ah yeah im well aware that its running outside of ideal specification (I always check the schematics and datasheets beforehand when doing such swaps to make sure I never exceed any voltage values etc)

tweaking the vCore has long been on the list to see if I can get it stable, but I am very limited for space etc so doing major soldering and disassembly work takes a lot of effort etc

im just pleased to see that it worked at all, eventually I will get round to seeing if I can get it stable (and running at 1Ghz since thats what the 7455 I installed is rated at so it would be rude not to try! )

however I do have a 2nd identical machine that is fully functional so I have less need to fix this one since if Need to test something PowerMac4,2 specific I have the fully functional machine

I appreciate the heads up none the less

 

[ervus]

I would bet money the issue is in the VCORE voltage that the iMac's logic board is set to.

+1 When you get around to it, I would suggest 1.35v and 1.2GHz. At those settings it would still run cooler than the original chip.

Bonus points for moving the L3 components from the MDD CPU over too...

 

[alectrona6400]

apologies for bumping an older thread, but i managed to score an 800mhz G4 iMac and i am looking to swap its 7451 for a 7455 and perhaps maybe get it to 1.2 @1.35v. would also be interesting if one could get L3 cache to work, as it does have those pads...

 

[alectrona6400]

Apologies for double posting, but ideas just keep racing to my head. Turns out I had a USB2 iMac board that was going to scrap, but I might try to get it to work with a 15" iMac. I think I do need to replace the neck to get all the connections in, however. Personally I would love to see a 15 inch iMac G4 with a nice set of specs...

 

[LightBulbFun]

apologies for bumping an older thread, but i managed to score an 800mhz G4 iMac and i am looking to swap its 7451 for a 7455 and perhaps maybe get it to 1.2 @1.35v. would also be interesting if one could get L3 cache to work, as it does have those pads...

to get L3 cache working, as well as the L3 cache chips you would also have to stuff the extra support components around it

thankfully the schematics for the PowerMac4,2 do fully document the L3 cache section and what components it uses so should be doable in theory

the early 7450/7451 based eMacs also have vacant L3 pads as well


and of course a 7457 upgrade would be possible, its what Daystar did back in the day for the PowerMac4,2 (tho make sure you patch the BootROM appropriately before doing so)

 

[alectrona6400]

to get L3 cache working, as well as the L3 cache chips you would also have to stuff the extra support components around it

thankfully the schematics for the PowerMac4,2 do fully document the L3 cache section and what components it uses so should be doable in theory

the early 7450/7451 based eMacs also have vacant L3 pads as well


and of course a 7457 upgrade would be possible, its what Daystar did back in the day for the PowerMac4,2 (tho make sure you patch the BootROM appropriately before doing so)

Well for PowerMac4,2 (aka Tessera, the internal codename for this specific iMac) I would like to use a 7455 to maintain easy OS 9 support. I should take a look at the documentation for the L3 too... maybe I could get some of the easy components installed, like capacitors and whatnot. There is a 7457 on eBay for about 40 but the 7455 is much cheaper and in my opinion, somewhat more practical.

 

[alectrona6400]

Apologies for double posting, but ideas just keep racing to my head. Turns out I had a USB2 iMac board that was going to scrap, but I might try to get it to work with a 15" iMac. I think I do need to replace the neck to get all the connections in, however. Personally I would love to see a 15 inch iMac G4 with a nice set of specs...

Turns out that's not possible; the Horizon board swaps the I/O "clusters" and changes the location of the PRAM battery and a screw hole. I wish I still had the rest of my USB2 parts, even if they were in terrible shape. Apparently you need a new motherboard tray, neck assembly, power supply, 2 separate IDE cables, and maybe even a different screen. Yeah, this isn't going to work. Tessera motherboards are an almost completely different internal design.

 

[alectrona6400]

Well, I have made an interesting discovery with overclocking the existing 7451 processor. The 700MHz iMac G4 board (820-1527-06) can only go up to 800MHz without crashing. The 800MHz board (820-1527-A) seems to go up to 900MHz no problem. What's weird is that the 2002 eMac can easily reach 900MHz from 700MHz, but the 700MHz board I have only got up to 800MHz. Perhaps it could just be a lower bin compared to other chips and maybe even other systems. I'd also like to add the 700MHz board came from a system earlier in production, with some parts originally dating back to December 2001. No, it was not previously opened...

 

[indibil]

to get L3 cache working, as well as the L3 cache chips you would also have to stuff the extra support components around it

thankfully the schematics for the PowerMac4,2 do fully document the L3 cache section and what components it uses so should be doable in theory

the early 7450/7451 based eMacs also have vacant L3 pads as well


and of course a 7457 upgrade would be possible, its what Daystar did back in the day for the PowerMac4,2 (tho make sure you patch the BootROM appropriately before doing so)

Hello! Yesterday, looking for photos of how to overclock the first iMac G4, the same question arose when I saw the pads to add cache and their components.

Do you have the schematics? I might get one of these iMacs in a few weeks and I'd like to give it a try.

Will an EEPROM also be needed to adjust the speed ratio of the L3? Would the same dump of a dual MDD 1.42 work?

I don't have any MC7457 but I can have some MC7455 at 1GHz.

 

[LightBulbFun]

if you google the board number 820-1257 you can find schematics on the first page of google these days

as for the EEPROM, I dont know, most AIO macs like tend to have L2/L3 cache settings in firmware rather then a seperate SPD ROM, but I have not studied that part of the schematics in detail to see whats what on the iMac G4

also as an aside 820-1381 and 820-1389, also seem to be PowerMac4,2 boards, not sure how they differ against the 820-1257 mind, but worth noting, especially as there seem to be some slightly later schematics floating around (DVT vs PVT etc)


will be fun to see someone finally populate the L3 cache section, combine with a fast clocking 7455 or 757 and a healthy 2MB of L3, that would be a speedy machine, on par with a single CPU high speed QS or MDD

especially if you can bump the FSB clock speed (the schematics have strap settings from 100 to 150)

 

[indibil]

Thank you!! I have already found the schematic!! it will not be easy. Yes, I have seen how to raise the bus to 133MHz, it would only be necessary to remove one resistor and add another, but it needs to be stable, in addition to lowering the CPU multiplier.

I repeat, it is not going to be an easy task to add the L3 cache, especially if modifications have to be made at the firmware level for it to be recognized. Is that possible with any Gigadesigns/Powerlogix patcher?

 

[indibil]

I've been researching the Vcore of both the 7451 and the 7455 and they both run around 1.6V, that shouldn't be the problem.

I have even looked for factory settings on the MDDs for 1.25GHz and 1.42GHz and working at 1.65V should not be a problem. I have downloaded a few datasheets to see differences and they all work around 1.6V. Your iMac @ 1GHz should not fail from excessive voltage.

 

[indibil]

I finally got the iMac G4 800, it's model 820-1398. I could only get the schematic of 820-1257 for free but the values do not match. The Vcore resistors measure 5k and those values are not in the schematic, and the bus speed resistors R636 and R637 are empty. Does anyone have my schematic?

 

[indibil]

It seems that the post is not of much interest, so I am not going to go into too much detail.

In my blog I have reported my SWAP on an iMac G4 800 that works stable at 1.3GHz:

http://rosysumenteinquieta.blogspot.com/2023/12/imac-g4-800-cpu-swap.html

Merry christmas.

 

[LightBulbFun]

It seems that the post is not of much interest, so I am not going to go into too much detail.

In my blog I have reported my SWAP on an iMac G4 800 that works stable at 1.3GHz:

http://rosysumenteinquieta.blogspot.com/2023/12/imac-g4-800-cpu-swap.html

Merry christmas.

sweet glad to see such a swap worked for you


it will be interesting to hear how the L3 cache upgrade goes

 

[indibil]

Hello. Today I finished making the changes to add L3 cache to my iMac G4 800@1.3GHz. I have removed the NOL3_Cache components and added the necessary ones, and the 2MB of cache, from the XSERVER CPU that the G4 1.3GHz donated.

The iMac boots perfectly but does not recognize the L3 cache. As I was told recently, in these AIO devices they must adjust the ratio and configuration of the L3 cache in the BOOT ROM and not in an EPROM, is that true?

I have tried to run the GigaDesigns patcher, which includes a cache enabler, but it gives me the following error:

It seems that the firmware does not contain such information. Do you know if it could be activated via terminal? or with some other patcher? @dosdude1 can you help me?

 

[LightBulbFun]

Awesome Stuff! glad to see it all still POST's with the L3 cache and its components in place, thats a good start

have you checked under the "Diagnostics" tab in system profiler to see if your getting any POST messages relating to Cache now?


otherwise yeah AFAIK on AIO machines like this external cache is configured by/in the BootROM itself (on Apple CPU cards its in the form of a SPD EEPROM)


as for enabling the cache, have you tried Sonnets Cache enabler?

I find when I use it in Leopard (on my G3 beige to enable the cache of any 7400/7410 I chuck in there) you dont see the newly enabled cache in any system profiler or application report, but its easy enough to run some benchmarks and see if it is or is not being enabled

Sonnet Support

 

[indibil]

Awesome Stuff! glad to see it all still POST's with the L3 cache and its components in place, thats a good start

have you checked under the "Diagnostics" tab in system profiler to see if your getting any POST messages relating to Cache now?


otherwise yeah AFAIK on AIO machines like this external cache is configured by/in the BootROM itself (on Apple CPU cards its in the form of a SPD EEPROM)


as for enabling the cache, have you tried Sonnets Cache enabler?

I find when I use it in Leopard (on my G3 beige to enable the cache of any 7400/7410 I chuck in there) you dont see the newly enabled cache in any system profiler or application report, but its easy enough to run some benchmarks and see if it is or is not being enabled

Sonnet Support

Thanks for answering.

As you will see in the attached photos, the diagnosis appears correct (the date is wrong because the battery has been removed).

I have applied the Sonnet Cache Enabler and no information appears in the System Profiler, so I have run GeekBench and the result has been the same as without L3 cache mounted, and it is not recognized either.

The message that must be overcome is the GigaDesigns patcher error, something must be added to the ROM so that it recognizes it.

 

[LightBulbFun]

Thanks for answering.

As you will see in the attached photos, the diagnosis appears correct (the date is wrong because the battery has been removed).

I have applied the Sonnet Cache Enabler and no information appears in the System Profiler, so I have run GeekBench and the result has been the same as without L3 cache mounted, and it is not recognized either.

View attachment 2340144 View attachment 2340145

The message that must be overcome is the GigaDesigns patcher error, something must be added to the ROM so that it recognizes it.

it might be worth your while PM'ing @dosdude1 directly or such


the CPU firmware patchers, mostly all they do is remove the Unknown CPU check, they dont explicitly enable any *L3* cache, since even without any BootROM support MacOS just reads the L3 cache info off the SPD data (in the case of 7457 cards etc)


just as a sanity check, does the cache enabler do anything in Tiger?



also if in OS9 you install Sonnets crescendo software do you see any L3 cache pop up in Metronome? (I think 3.1 is the last OS9 crescendo software)