Revisiting the G3 iMac

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Joined: Sep 23 2005
Posts: 316

So; after seeing my last project my coworker asked me if there was anything that he could do with his old G3 Tray load iMac that he had sitting in his basement.

He said that it would be nice to be able to use something more modern on it; even if it was slow. And that his ultimate intent was to put it at the end of his bar in his basement and have it appear as one of those touch screen gaming machines; but actually control some light automation, etc...

Well; that all sounds pretty neat - but I'll have to follow up with you guys if he actually does any of that. Smile

I told him that I could handle a bit of the updating of the hardware for him by covering the standard ground of RAM upgrades, Hard drive upgrades, OS upgrades and Overclocking.

Those of us who have been around as long as I have have probably seen all of this stuff before - but here I'll try to put it all together in one post.

He came back from lunch and dropped the machine on my desk. A nice, Lime Tray load iMac; and CLEAN too - like it had been hidden in the closet since it was purchased.

I took it home and fired it up to find a nice little time capsule. Revision B iMac, G3 333MHz, 64MB of RAM and a 6gig hard drive running System 8.6!!!!!!

So first things first - lets shut it down and get to overclocking the CPU. He was lucky for having a 333MHz CPU; every one of those that i have ever worked with has ALWAYS successfully overclocked to 400MHz

We begin by removing the logic board "sled" from the machine. To do this, undo the one phillips head screw on the bottom of the iMac and pop off the bottom panel. Once the bottom panel has been removed you will see two phillips head screws along the back edge of the logic board sled. Remove those. Then there are three cables connected to the logic board assembly. Use your screwdriver to loosen the video cable; and just unclip the other two by hand. Then you can slide out the trayand you are left with this:

to get to the CPU card lift off the metal "screen" covering it

lift up the retaining clip:


and GENTLY lift out the CPU card:



The CPU card also holds the system RAM. some of these iMacs shipped with only 32MB of RAM. The STATED physical RAM limitation is 256MB; achieved by using two 128MB RAM chips. However; a FEW mythological units WILL support 512MB of RAM if you get lucky. (Supposedly the 256MB modules will be recognized at their full capacity if they are CL2)

The Tray Load iMacs require old style low density (more chips on the ram module) PC100 RAM. High density RAM will only be recognized at half of its capacity. (i.e. a 128mb high density RAM module will show up as a 64mb module)

Now; back to the task of overclocking. The CPU speed is set by the Processor clock PLL; which is configured by the placement of the resistors R117, R118, R119, R120, R121, R122, R123 and R124 - located on the back of the CPU module


Below is a table showing what CPU speed the various resistor placements will result in:

CPUClock
PLLConfig
R117
R118
R119
R120
R121

R122
R123
R124
Note

233MHz
x3.5
x
S

S
S
S
x
x
x
Rev.A & B

266MHz
x4
x
S
x
S
S
x
S

x
Rev.C

300MHz
x4.5
S
S
S
x

x
x
x
S

333MHz
x5
S

S
x
S
x
x
S
x
Rev.D

366MHz
x5.5
S
x
x
S
x
S

S
x
DANGER!

400MHz
x6
S
x
S

S
x
S
x
x
DANGER!

433MHz
x6.5

S
x
S
x
x
S
x
S
DANGER!

466MHz
x7
x
S
x
x
S
x

S
S
DANGER!

500MHz
x7.5
S
x
x

x
x
S
S
S
DANGER!

533MHz
x8

x
x
S
S
S
S
x
x
DANGER!

S:Mount Resister
x:Remove Resister

Obviously; this table is NOT confirmation that the little iMacs will run stable; or even at ALL at some of these speeds - and making these modifications should be done at your own risk. I personally have never had success at anything over 400MHz.

So; since this machine was already a 333MHz model; that means that I needed to move two resistors - i needed to move R118 over to R119; and R123 over to R122 to achieve 400MHz

Its quite simple actually; just unsolder the resistors from their current position; and then resolder them back into the appropriate place. The only challenge is their size (however; this size seems like a luxury after having done a few G4 Mac Mini overclocks!!)

For size reference; that is one of the resistors on the table surface - between the penny and the screwdriver tip:


After moving the resistors around; I threw the machine back together temporarily and fired up System 8.6; opened the System Profiler to confirm my work was successful:


I also checked the iMacs firmware revision at this point - in order to install OSX a tray load iMac MUST have iMac Firmware Update 1.2 - or very bad things will happen!

As this machine was already at Firmware 1.2 - I set about to swap out the hard drive and install the software.

I happened to find an old; unused 20GB drive in my parts drawer - so into the machine it went!! Another note to remember is that tray load iMacs are on the very edge of the "Old World" Macs; and as such The System software MUST be within the first 8 gigs of the drive - if the system sftware happens to stray elsewhere on the drive OSX may not start at all; and you'll have problems with classic as well. (the LARGEST physical drive that a tray load iMac will support is 128 gig)

To avoid this problem it is advised to slice your hard drive in to two partition - and to be even safer - make the first partition SMALLER than 8 gigs - in this case I made the first partition 7 gigs; and the second partition for the remainder of the drive. Also in the installer make sure to check the "Install system 9 drivers" or whatever the tick box says on the partitioning screen. If you neglect to do this you wont be able to boot to classic - or install classic support under OSX.

Now my intention was to install Tiger (OS X 10.4) onto this system - the only trick to that is that Tiger is not officially supported on the Tray Load G3 iMacs; and the installer will halt and not let you install Tiger on this machine; the additional caveat is that Tiger 10.4 came on DVD (a 4 cd installer set was available through special order direct from Apple; but it is not common to stumble across in the wild) - For these reasons 10.3 Panther is typically recomennded for Tray load iMacs.

Although there are a few ways to get Tiger onto an unsupported Macintosh. One way; which is useful if you have the Tiger CD's is through the use of a nice piece of software called XPostFacto - which will bypass the installer checks and let you install.

The quicker; easier method which I went with was to take the bare drive and install it into my heavily modified G4 Sawtooth tower (equipped with a Dual Layer DVD burner) and install straight off of the retail Tiger DVD with no issues. Smile

So; after the install process there I dropped the drive back into the iMac and fired it up!!


Also; I had dug through my RAM stash and was able to turn up a 64MB stick and a 128MB stick. Laughing out loud

Then I copied the entire contents of his old drive into the large "Data" partition on his new drive through the use of a USB bridge that I picked up from NewEgg.com



Then I moved along and hooked up a DVD drive o install Classic (System 9.2.2) and any other ancillary software that I was going to put on the system


Proceeded to run system update to patch the system up to 10.4.11 and proceeded to bask in the glory of my efforts:

Ancient Appleworks in Tiger:

Bugdom anyone? (runs like POOP - not playable - under Classic inside of OSX - you can only do so much with 6MB of VRAM)

And; so there it is!! ...Its still pretty slow; but its workable for web browsing; and listening to iTunes, etc.

Sorry for yet another long post; but I think that alot of this information was slipping away into the "Forgotten Realms"

hope someone gets something from this!

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Offline
Joined: Sep 23 2005
Posts: 316
messed up table

I tried to edit the post; but I don't have any edit buttons.

I tried to post an HTML table; and I guess that the boards here don't support that - so here is how the above table should have appeared:

EDIT: Strange; I get edit buttons for my post replies; but not for the topic parent. Sad

Hawaii Cruiser's picture
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Joined: Jan 20 2005
Posts: 1434
whatcha gonna be cooking?

With the CRT iMacs being already famous for heat problems, overclocking sounds like it might turn it into a nice big toaster for as long as it survives. But nice work. Have you considered doing paintings on the head of a needle? They might be bigger than that resister.

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Joined: Sep 23 2005
Posts: 316
tray load iMacs have internal

tray load iMacs have internal fans (its the slot loads that don't)

..And I have done this mod on at least 6 machines in the past.. and EVERY ONE OF THEM is still running JUST FINE. the earliest one I did about 6 years ago - so; yeah - I dont think anyone has to worry about it "turning into a nice toaster if it survives" Laughing out loud

Hawaii Cruiser's picture
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Joined: Jan 20 2005
Posts: 1434
That's interesting. I would t

That's interesting. I would think the combination of a 7200rpm drive and overclocking would tax that fan beyond its ability. Does the mod make it noisier?

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Joined: Sep 23 2005
Posts: 316
noise

...It's not any noticeably noisier as far as I can tell; in fact - it seems a bit quieter - since it hasn't got a ten+ year old hard drive making the creaking, groaning sounds that it used to make anymore. Smile

doogie1303's picture
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Joined: Dec 19 2003
Posts: 14
Anyone tried overclocking FSB on these machines?

I was wondering has anyone ever tried overclocking the FSB (Front Side Bus) on the original tray loading imacs? I did this on a dual USB ibook G3 500Mhz a few years ago at that gave a noticeable performace boost in the system. The original FSB was 66Mhz and I sucessfully changed it to 100Mhz. Since these same machines also have a 66Mhz FSB, I was wondering if it was also possible to do this to these machines.

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Macbook Core 2 Duo Poly White (wife's machine)
Ibook G3 800Mhz (baked and functioning again!)
Macintosh SE (3 in three machine, currently in a box somewhere in my garage)

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Joined: Feb 2 2017
Posts: 5
Re: Revisiting the G3 iMac

Sorry for the thread necromancy but I just did this and it seems to work fine at 400MHz. Having some trouble getting it to work with an 120GB SSD but the overclocking seems to have worked. FSB overclocking would be interesting to try but the MPC-107 is only 66MHz and it has no heatsink and barely any room for one. Now only if I could find some RAM in the pile that worked with it.

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Joined: Feb 2 2017
Posts: 5
Re: Revisiting the G3 iMac

I was experiencing lockups copying files with the SSD, upgrading to OS 9.2.2 seems to have fixed that. So now using 120GB Sandisk SSD on a SATA to IDE adapter.

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Joined: Feb 2 2017
Posts: 5
Re: Revisiting the G3 iMac

One last thing I did was buy some RAM I wasn't sure about what it would support as there were various reports about what it would support 128MB/256MB per socket. Since it was so cheap I bought a 512MB SO-DIMM which only 256MB was recognized but it does work. And added a 128MB stick to the other slot.