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themacmeister's picture
Joined: 2009 Oct 26
Power Mac G5 problems... MARGINAL RAM?!

I got a Power Mac G5 aluminium tower to repair. It is the original Dual 2.0GHz G5 from 2003.

No matter what combination of 12 matching pairs of RAM I use (PC3200), I am still getting the 3-flash - pause - three flash code on startup (no chime). This, I discovered means MARGINAL/BAD RAM. All this RAM came from working PCs, so there is nothing wrong with it at all.

I have managed to get a chime, and start booting the computer about 5 times out of 250. I have reached the desktop once, and machine froze when I got info on the hard drive (capacity -- free space --).

I believe this is the power supply failing, or a faulty board, as the OS was crashing mid start-up, or even before. The freezing is also a worry.

Apart from this obvious problem of not getting past the chime on startup, I don't know where to turn. My PC RAM runs the entire gamut of DDR400, CL2 to CL3. I don't believe there is anything wrong with the RAM whatsoever. I am reticent to spend $195 on 2GB RAM (you read that right!!!). hell, I could spend $195 and get 8 or 16GB of DDR3 1666MHz SDRAM (or even SO-DIMMs) at that price... sheesh

If anyone knows of a way to hook up a PC AT/ATX power supply to the Power Mac G5, I am all ears. The power supply is sealed in the bottom of the case... could be interesting getting to it.

thanks for any advice or help!

PS. I tried bad (PC2100) RAM, and only one chip, and I get the BAD/NO RAM single flash, so I think the auto detect is working OK.

Just before it fails, you can sometimes hear a faint POP, like it wants to start playing the chime, but fails just a moment before then.

PPS. Could this be a failing capacitor? Weird symptom?!

PPPS. the RAM inside to begin with looked very dicey old PC RAM anyways. Mix of plain and ECC. yuurch.


bertyboy's picture
Joined: 2009 Jun 14

you still have ECC RAM in the G5 ? Not a good idea. AFAIK ECC is only suitable for the Mac Pro's.

DDR 400, CL3 or faster, Non-ECC, Unbuffered, 2.6V.

When I'm buying 2nd hand for my PMG5, it's always Crucial / Micron branded, maybe Hynix if it's cheap. But I don't buy unless it's clearly non-ecc, unbuffered.
I would expect to pay about $24 per GB 2nd hand in the UK, it's about $35 per GB in NZ. I can order it new from in the US for about $40 per GB. All prices stated here are NZ$.

Now the bad new, when I've used marginal RAM, it usually gives me KP's. No freezes, no strange sounds, etc. But I'm hoping that this is because you've been trying ECC RAM.

Joined: 2010 Oct 3

from what i understand ECC rams shouldn't be a problem, providing all the ram is ECC, from what i understand mixing it with Non-ECC ram will cause problems. It is possible to run the g5 from a atx psu, but you'll need a powerful one (600w-1kw from what i remember), also i don't think any graphics cards that need power from the extra tab on the board (to power ADC displays) will work (due to the 28v line not being present on a axt psu) i will do some digging and see if i can find the guide to do it

Mulder's picture
Joined: 2012 Jan 29

I don't claim to be an expert, but I doubt the RAM is the problem. If this is a replacement G5 for your previous one that started behaving badly, was it working perfectly fine before you bought it from the prior owner? If not, that may be the reason they were willing to sell it.

But if that's the case, is there an independent computer repair shop near you that could diagnose the problem for you for free, or a small fee? If so, that would save you a lot of time looking for a problem that might not be there; it could be something entirely different. Or, you could go back to the original owner and ask them what's wrong with it.

bertyboy's picture
Joined: 2009 Jun 14

from what i understand ECC rams shouldn't be a problem

Again, AFAIK, the use of ECC was "optional", but ONLY in the last model PMG5, the one with a dual-core CPU that used DDR2 RAM.

I had read elsewhere, Apple Support Communities or MacRumors forums that you couldn't use it at all on a dual processor G5. Maybe it worked on Uniprocessor G5's but I don't know.

Just checked the OWC PMG5 Memory page, it talks ablout only UN-REGISTERED ECC RAM being optional for the last model PMG5. I still believe that it can't be used in dual-processor models.

themacmeister's picture
Joined: 2009 Oct 26

Only two of my 24 sticks of compatible RAM is ECC. All the rest is standard.

When you say CL3, is CL2/2.5 faster than CL3 (I don't know how that works).

If that is the case, 22 sticks of my RAM are perfectly compatible Sad

Watching the start button when the machine actually runs, it is different from when it fails. It fades in when it fails, and is stronger/brighter, and instant on when it works? I think it is a power board/power supply issue. I think of the board isn't getting enough voltage to the RAM slots at startup, it may be failing the memory check due to that?

Also, the battery may well be dead... dunno what that has to do with startup RAM checks tho?

The very last time I had it "running", I got the infamous grey overlay "you need to hold the power button for 5 seconds" screen, with corruptions in the graphics. It has to be the power.

Anyway, I am going to clean everything today, unplug and reseat every available cable, and see if anything improves...

Cheers for the help!

PS. can you mix different RAM CL3/CL2.5, and expect it all to run at 2.5 or 3.0 (whichever is slower).

PPS. The two sticks I have in at the moment are non-matching CL values. :0

bertyboy's picture
Joined: 2009 Jun 14

AFAIK, you can mix the latency (CL) in sticks.

And yes, CL2.5 is slightly "faster" than CL3. It's not really any faster (see below) and i'm sure that your Mac will be fine with CL3.

The CL rating is just a measure of how long the RAM takes to retrieve requested bits back to the bus. So your (and my) DDR-400 is actually a 200MHz bus, but the double data rate or width (DDR) gives it the 400 in DDR-400.
So the 200MHz equates to a 5ns cycle time (against 70ns in my old Mac IIci) and a CL of 2.5 means it takes 2.5x this period, or 12.5ns to retrieve bits requested.
So you can probably already see that even if it's CL 2.5 the bits retrieved won't actually be picked up until the next bus cycle, at 15ns, making it no faster than CL3.
Well that's the theory anyway, marginal CL3 RAM may be slightly too slow and it'll operate like CL4 memory.

Memory for current Macs and PCs is not really significantly faster, it has higher data width, like DDR3, and a much faster bus speed, like 800MHz, but the CL is up at 8, 9 or 10 so it still takes 8-10ns to get requested bits, and OK you get 4x-5x as many bits in the same time.

themacmeister's picture
Joined: 2009 Oct 26

I gave it a thorough cleaning, and tried it with new RAM and PRAM Battery... still no go.

Shame Sad

MikeTomTom's picture
Joined: 2009 Dec 7

If you type "Dual 2.0GHz G5 problems failing" into Google (without the quotes), you get a lot of hits. Mostly for logic board and PSU issues, but a large number of logic board failures. The issues with this Mac might be more serious than RAM suitability.