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You are here: MacNN Forums > Hardware - Troubleshooting and Discussion > Mac Desktops > Powermac QS Dual 1 GHz Frequent kernal panics - Fauly logic board or faulty L3 cache?

Powermac QS Dual 1 GHz Frequent kernal panics - Fauly logic board or faulty L3 cache?
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Mar 12, 2010, 07:18 AM
My G4 Quicksilver Dual 1 GHz has been experiencing frequent kernal panics for some time. These result in a system wide freeze often accompanied by the screen darkening with the "You need to restart your computer" message.

On first booting the computer it can take five or six attempts before I can get in as it will often kernal panic during the startup sequence and once in it will continue to kernal panic usually every few minutes! Gradually the kernal panics reduce in frequency to the point where the machine remains relatively stable for four to five hours then the cycle starts to repeat.

It was originally suggested to me that I might have a faulty RAM module as bad (or mismatched) RAM can produce the symptoms described and to run the 'Rember' RAM check utility. I ran Rember three times and each time it would fail on the 'Sequential Block' test, although not at the same point.

I took the machine to my nearest Mac service centre and explained the symptoms and that it was likely (but not definite) I had bad RAM given the symptoms and the fact Rember had failed. They replaced the two Apple store bought 512 MB RAM modules with brand new modules (The original factory installed 512 MB RAM was untouched). However the symptoms persisted.

Recently I took the machine back to the service centre and explained the frequent kernal panics were still occuring despite the RAM being replaced and I instructed them to track down the cause since it was clearly not RAM related. This time they said they used Apple's own diagnostic tools and they informed me I had a faulty logic board which they could replace for $1,000, which is substantially more than the machine is worth!

Luckily I have a non functioning (dead fan) Quicksilver Dual 800 MHz which I could take the Rev 'A' logic board from and have put in the Quicksilver Dual 1 GHz which has a Rev 'B' logic board. I understand the only difference between the Rev 'A' and 'B' boards is that 'B' supports hard drives larger than 137 GB, which is of no matter to me.

To complicate matters I have noticed 'System Profiler' no longer reports the level 3 cache in the Quicksilver Dual 1 GHz. It should say:

L3 Cache (per CPU): 2 MB

Also the 'Power On Self-Test' reports:

Result: Failed
Failure type: External cache

I did a search on 'external cache failure' and found the following reply to a post from an owner of a Quicksilver Dual 1 GHz:

External cache failed; can I live without it?


It probably refers to the 2 MB level 3 cache. This takes the form of two memory chips, one per processor, on the CPU daughter card (but external to the CPU itself). I guess it would technically be possible to replace just the chips, although realistically replacing the whole CPU daughter card is probably the only solution.

The level 3 cache helps improve performance and was only available on the mid and high end Quicksilver models, the base models lack it entirely. It sits on the processor side of the system bus and helps to compensate for it's relatively slow speed and high latency. Some applications such as PhotoShop show significant performance gains if the level 3 cache is present, others less so.

I guess the danger is it's failed the POST test but is still being used, which may lead to some instability. Certain versions of Apple's CHUD tools allow you to temporarily disable it, the idea being software developers can test the performance of their software with and without the cache.

Going by this reply does an external cache failure mean both caches have failed or that just one needs to have failed for an external cache failure to be reported? Is there a utility that will specifically test level 3 cache as 'TechTool Deluxe' doesn't test beyond level 2 cache?

Am I correct in assuming that if either or both level 3 caches have failed (I assume that if only one has failed the other would be shared between the two processors?) that this would be the same as having level 3 cache disabled? In other words the computer would take a performance hit (depending on the applications running).

My questions is would a level 3 cache failure account for the frequent kernal panics? As I say I have only just noticed the issue with the level 3 cache and so it may have failed quite sometime ago, perhaps even when the kernal panics first started to occur.

I also found another post relating to an external cache failure and kernal panics which is again with a Quicksilver Dual 1 GHz (could this be a pattern?):

External cache failure & kernal panics

Hi all

Using a Quicksilver G4 Dual 1 Ghz running 10.4.11

Powe On Self-Test:

Result: Failed
Failure type: External cache

and since recently, have kernal panics constantly on bootup.

and the reply:

Ouch. That's a pretty serious hardware fault.

The "external cache" referred to is probably the level 3 cache between the CPU and memory. If this is failing, you will have an extremely unreliable computer, with crashes and data corruption.

Depending on the exact problem, it will basically mean the CPU is sometimes reading junk out of memory instead of what is actually there. As the fault is intermittent it may be heat related - you may find that the computer starts crashing or corrupting data when it is trying to work hard, or on particularly warm days (or it might only happen when it is cold). It could also be localised to one particular part of the cache, so it might only happen when that part of the cache is occupied.

The level 3 cache is on the processor card, not the logic board. The self-test might be fooled into thinking the fault is in the L3 cache when it is actually on the logic board, but I doubt it.

It might be feasible to modify the system in some way so that it disables the L3 cache at startup. If so, your computer may then work reliably, but it will be a lot slower.

The Mac service centre says I have a faulty logic board, but could they be wrong or only half right? Could it have been a level 3 cache failure all along causing the frequent kernal panics or worse a logic board fault combined with a level 3 cache failure! One would hope Apple's diagnostic tools and procedures would have also picked up a problem with level 3 cache?

Assuming a worst case scenario, that both the logic board and CPU daughter card need replacing (double the repair cost?) can I also take the CPU daughter card from a non functioning (dead PSU) FireWire 800 Dual 1.42 GHz to have put in the Quicksilver Dual 1 GHz?
( Last edited by DarkAvenger; Mar 15, 2010 at 05:12 AM. Reason: Minor correction)
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Mar 18, 2010, 10:32 AM
Logic Board - $62 w/ shipping
Apple PowerMac G4 Quicksilver Logic board DUAL 1 GHz - eBay (item 370297584840 end time Mar-26-10 19:10:04 PDT)

Dual 1 Ghz CPU - $135
DUAL 1GHz Apple G4 PowerMac QuickSilver Processor CPU - eBay (item 160410338472 end time Apr-03-10 10:58:22 PDT)

Buy the Logic Board first, get a neighborhood kid to install it, and see if it works.

If that doesn't work, start trolling on eBay and Craigslist for a complete system. You could probably get a functional quicksilver for $2-300.

Hope that helps.
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Mar 18, 2010, 03:50 PM
My suggestion, put the original logic board, the 800Mhz CPU and original RAM (including the apple purchased RAM) back together and test it, if kernel panics stop, it's most likely the 1Ghz CPU.
chris v
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Mar 22, 2010, 10:25 PM
My money's on the CPU. I had one of these. The CPU failed twice under warranty (first replacement was bad -- didn't make it out of the repair shop), then the third one finally died about 6 months ago.

If you look on Ebay, there's 500 million logic boards, and hardly any CPU's for these that have been parted out.

Why don't you get a new power supply for the dual 1.42 instead? That's a much better machine, and PSU's are cheaper than CPU's for Quicksilvers, and the used CPU is likely to fail not too long after you get it, since these seem to be all going bye-bye one after the other, now.

Edit: At any rate, it's going to cost 3/4's of what a functioning machine would cost, for parts -- in fact, I have seen whole dual 1.0's going for less than just the CPU. If you want a to buy a functioning used G4, keep your eye on Craigslist and be patient for a few days. Something will pop up there.

When a true genius appears in the world you may know him by this sign, that the dunces are all in confederacy against him. -- Jonathan Swift.
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Mar 23, 2010, 12:20 PM
Wow, weird. I had the exact same failure on my DP 1GHz. I managed to get a replacement CPU off Craigslist for $40.
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