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Lab Notes: Yosemite CPU Speed Settings
Motherboard Jumper Block Information
Published: 1/14/99, Last Updated: 3/16/99
    Disclaimer/Warning: This information is for reference only - not an endorsement of overclocking. Modifications like this will void your warranty and may result in failures and/or data loss. We do not recommend you overclock your system. You assume all risk of any modifications you perform to your computer. Remember that electronics often fail early in their life cycle (called infant failures)- it's wise to wait to verify all is well for an extended period before performing any modifications to your system.

    To see the Overclocking results from Beige and B&W G3 owners - go to the CPU Upgrade/OC reports database search page and select either Apple B&W G3 or Apple Beige G3, then select 'Apple' as the upgrade manufacturer for Overclocking reports - or select a CPU Upgrade brand to see reports on G3 and G4 CPU Upgrades. (For full reviews of CPU Upgrades w/performance tests/comparisons I've done see the CPU Upgrade reviews page.)

    Note: DO NOT run an overclocked ZIF with the case open. After 5 minutes or so I see overheating errors this way even when the speed is reliable with the case closed. With the case closed the internal fan helps cool the heatsink, but even though reported temps (which are not accurate) was only 31C - I saw errors in the Finder when operated with the case open. Note the later revision B&W G3s have a taller heatsink on the CPU. (If you remove the heatsink Clip, make sure you oriented it correctly when reinstalling it - the Bend (pressure point) must be over the CPU side of teh ZIF.)


As reported last week on the main Yosemite page, the new G3 systems have a jumper block on the motherboard for configuring speed settings similar to the original G3 models (settings shown in our Lab Notes article published last year).

Once my Yosemite arrived I began to explore the possibilities and several readers have also contributed their settings/results from experiments with the new model. This page is not to be construed as a recommendation to overclock your system. Each system and component (CPU, ram, etc.) may exhibit different results from trying to run beyond the rated speeds. Just lifting the evidence tape over the jumper block voids your warranty - the tape cannot be reused without visible indications it has been removed.

The new Yosemite motherboard speeds are controlled by jumper settings that determine the bus speed (default 100MHz) and CPU to Bus speed ratio (multiplier that determines the CPU speed) as well as the PCI Bus speed (not covered in this initial report).

The picture below [I'll have a clearer one soon] shows the location of the jumper block (evidence/warranty void tape removed). The picture shows the original jumper block cap sitting beside the pins which have individual jumpers placed on them for experimenting/verifying other speed settings. The blue plastic cap contains prepositioned shunts (a tiny two pin shorted jumper) that are configured for the various G3 speed models. Preconfigured jumper block caps for each model are used to speed the assembly process and eliminate the chance of human error in the settings.

modified jumper block
Modified Jumper Block


Yosemite Jumper Block Settings:

CPU to Bus Speed Ratio Settings Matix:
For reference only (some speeds far too high for current CPUs)

Tests on the new Yosemite show that the following speed settings are possible (obviously some speeds are far beyond the capability of current CPUs). The vertical "Mulitiple" column shows the Bus to CPU ratio setting and the row shows which of the first 4 jumper positions are populated with jumpers. Note: Position 1 is the farthest position away from the edge of the motherboard - or the top of the jumper strip. This is noted on the blue stock jumper cap with a 1 notation.

   CPU Speeds Listed Assume 100MHz (default) Bus Speed

    (S = Jumper present or "Set", " " = no jumper)
          Multiple     | 4 | 3 | 2 | 1 |
          3x   (300MHz)|   | S | S | S |
          3.5x (350MHz)|   |   |   | S |
          4x   (400MHz)|   | S |   | S |
          4.5x (450MHz)| S |   |   |   |
          5x   (500Mhz)|   | S |   |   |
          5.5x (550MHz)|   | S | S |   |
          6x   (600Mhz)|   |   | S |   |
          6.5x (650Mhz)| S |   | S |   |
          7x   (700Mhz)| S | S |   | S |

(Jumpers are not to be changed in the other positions, they control the PCI & bus speed (more on these later when I get a full data sheet on the controller).

Bus Speed Settings: [I do not recommend changing the 100MHz bus speed, benefits will likely be minimal at best and you're only adding more potential problems to the mix. As I suspected this also changes the PCI Bus speed (see below). If the bus speed is changed then the CPU speeds in the table above will not be accurate since bus speed x Ratio = CPU Speed. And as shown in the Yosemite Performance page, bus speed does not seem to be factor in most applications.]

  • 6,7,9 = 66MHz
  • 5,6,7,8,9 = 90MHz
  • 5,6,7,8 = 94.5MHz
  • 5,6,8,9 = 100MHz
  • 5,6,8 = 116.3MHz [unstable in 1 test system - 38.75Mhz PCI bus]
  • 5,6,9,10 = 120MHz [stable in 1 test system - 30MHz PCI bus]

Update: A reader sends a followup to the above settings

" i'm running at 94.5MHz * 4.5 because otherwise i had to attach a fan. (which i don't like) what i was searching for was 105/70/35MHz, but no luck.

at the 120 setting, pci runs at 30MHz (that's why it is stable, contrary to the 116,3 setting, which seems to run the pci at 38.75MHz) "

Complete List of Populated Jumpers (bus speed left at 100MHz):

Jumpers are not changed in positions 5,6,8 and 9. using the above table to set the proper ratio, the complete populated settings of the entire jumper strip are:

  • For 350MHz: 1, 5, 6, 8, 9

  • For 400MHz: 1,3, 5, 6, 8 , 9

  • For 450MHz: 4, 5 , 6, 8, 9

The diagram below shows the configuration for the 400MHz G3 system (w/default 100MHz Bus Speed):

400Mhz configuration

Initial Experiment Results:

  • G3/350 (400MHz Tests): 400MHz ran fine all night. Max CPU temp was shown as 31 degrees C as noted below:

    400OC temp
  • G3/400 (450MHz Tests): A reader initally reported a failure to boot at 450MHz (see below) but later reported success after adding a fan cooled heatsink. Since failures to boot are not normally temperature related [it could be too fast a speed for the cache as cache speed is a ratio of CPU speed or just too high a freq. for the CPU to function]. PowerLogix's cache control software (see the G3-ZONE OC page) worked here to reduce my cache speed but it does not directly list the new G3 models yet so I hesitate to suggest others use it.

    I'm not confident that the fan/heatsink made a difference since (it's not that much instaneous thermal efficiency increase) but I'll report the longer term findings here. Update: Another 400Mhz owner reports he's running at 450Mhz, CPU temp 19 degrees C (after a few minutes - he will report stabilized temp after running apps). Most 450Mhz reports so far say the temp is 31-35C appx after several hours. Note that running the Yosemite with case open will increase the temperature of the CPU and video card (I saw a 8C increase with the case door open at 400MHz).

I used Seagate Hard Drive jumpers (spares I had) but readers say Radio Shack part number #276-1512a also fit.

Reader Comments: (most recent first)

Feedback on Success/Failures:

[Update: 3/16/99] To see the most current Overclocking results from other B&W G3 owners - go to the CPU Upgrade/OC's reports database and select Apple B&W G3, then select 'Apple' as the upgrade manufacturer.

350 @400Mhz: Several readers reported success. This one initially reported all was well (and 19C) but reported with the case open temps eventually climbed to 27C (still lower than my 31C case closed) which caused a crash:

" Yup. It went to 27c before a crash with the case open. Case closed back to 19c in a hurry. It crashed on the Umax power saver control panel. 1st I have seen of that. It does seem to hang on Appletalk networking to my C660, but that is not completely clear. As this is my 1st PPC machine, I am not sure the hangs are unique.
[later he said] I am staying at 400. I had crashes/hangs at stock 350 which are not figured out. Networking to my C660 via Ethernet/Appletalk seems to be a no-no for any length of time. The Umax power saver panel shuts off the light on my new scanner (1220U), but nothing else. I was surfing on another machines SINCE I CAN'T FIND A FORKING MODEM FOR THE DAMN THING [Amen brother, I called Bestdata on their USB Mac modem advertised at MacConnection and got a voicemail, still no reply days later -Mike] and went back to find it bombed on that util while the case was open.

I am confident that chipping up to 400 makes no difference and am staying there.

My 350 @400mhz stays at 31C (case closed) with no reliabilty problems so far but it's not networked. No errors of any kind in several days of tests on and off. But as they say 'your mileage may vary' as any overclocking is risky and each CPU batch may have a different amount of 'headroom' (CPUs are normally lot sampled and tested over the entire operating temperature range (hi/low) and also at the range of min/max voltages.

G3/400 owner wrote:

" I've verified 300, 350, and 400MHz settings. 450 (or what i guess is 450) doesn't boot. My yosemite 400 running @ 450 jumps into openfirmware saying default catch or rom checksum-failure... :-(

Strange thing: Norton 4 Sysinfo says that my old 333MHz G3 accelerated to 366MHz is faster (than yosemite 400) in most cpu-tests! Overall-result is SLIGHTLY faster. don't know if I can trust Norton.

[He later wrote:]
After attaching a heatsink with fan, i can definitely say 4,5,6,8,9 are the settings for 450MHz! pretty fast...

He later answered my questions on the CPU temp and Fan/heatsink used:

" What's the CPU temperature?

15 degrees celsius! [???] see the attachment (PL speedmeter says the same) it's not rising since my first mail of success. Reliable until now @450MHz...

450Mhz Temp

[I didn't believe the reading of 15C so I asked for a Speedmeter reading] Update: 19C was reported by another owner with no Fan on the heatsink, temp started at 15C he said but rose to 19C almost immediately. More later on max temp he sees after hours of use.

Speed meter 450 reading

Did it run too hot or unreliably otherwise? (without fan)

without fan/heatsink it jumped into openfirmware or did stay black...

What Heatsink/fan did you use?

pentium-style, nothing special, beside that it's mountable with the apple-clip. oh, and i can't close the door at the moment, because the heatsink and fan together are too high :-( "

Another 400MHz owner replies that he's also running at 450MHz:

" Mike, I just read on your side about over clocking (I did it with my last system too), and tried on mine (Yos G3/400) 450 Mhz. It does work so far. Processor score is now at 1479 and Disk is at 1417. "

Failures: [most recent entries first]

A 400MHz owner reports 450MHz was not reliable:

" Hi Mike. Just wanted to let you know that I overclocked my 400Mhz G3 Blue to 450 Mhz using your jumper suggestions.

While it runs for a while, as soon as you do any FPU or video intensive work, such as Graphing calculator 2.1 or Nanosaur 1.1, the system freezes. So it's 400 Mhz for now. "

A 350MHz owner and previous G3/233 Overclocker (to 300MHz) writes he did not have success:

" mike,
after seeing your explanations i immediately tried to overclock my 350 yosemite to 400... without success so far, get an immediate boot failure rom checksum failure.
-- bernie (overcklocked a 233 to 300 with no problems...)

My MacBench 5 scores and application tests for the 350 running at 400Mhz are listed on my new Yosemite Performance page. For game tests/comparisons see the Yosemite Gaming Performance page (includes tips and updates on beta driver performance).

Other Yosemite/G3 Related Links:

For reviews and tips on CPU Upgrades for other Mac models, or Graphics card, SCSI/Disk or other performance information and daily news, see the main site page at

This page will be updated with additional information in the future. If you have comments or input on the new Yosemite G3's please contact me.

Back to G3-ZONE for more Apple G3 Info

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