How do i unlock 4th core on Phenom II x3 720?

I'm not sure you can. It takes a special bios, and I believe a board from Biostar. Not all boards will allow this.

it would also depends on if that 4th core is really defective (hence it will never work), or if it was working, and just disabled to turn it into an X3.

Yeah, but where is the fun in that?

hmm, dunno if it's worth it, would my computer boot IF the 4th core was indeed defective? like, as in, the other 3 cores still run with the half-a$$ed core?

I think it's more dependent on your CPU. Some people are suggesting that fully functional 4-core phenom IIs were intentionally disabled to meet launch and could explained why initial responses were very positive. I don't know if that's true or not but it would be great if we could collect a comprehensive data base of build dates to find "stable" 4th core batches.

I was one of the lucky ones to get a stable 4-core 720x3. I had all four cores up to 3.4 stable at one point with voltage adjustment but found the increase noise of my CPU fan a little too loud for my taste. Less Noise > Marginal CPU gain.

BTW, I'm using a Asrock 790GX motherboard and it works great.

X4 920 is not BE, and it's AM2+.

The AM3 versions should come out this April, i believe.

I've got a Biostar TA790GX Motherboard and a Phenom 2 X3 720 BE CPU. I had no asperations of trying to get a fourth core from the three core processor but after reading the article on Tom's Hardware concerning their attempts at unlocking the fourth core with limited success I decided to enable the ACC in my bios and instantly after rebooting I came up with what appears to be a stable fourth core. I ran Cinebench for establishing a bench mark between the two cores (three and four) and here are the results:

Three Cores:
Rendering (Single CPU): 3226 CB-CPU
Rendering (Multiple CPU): 8582 CB-CPU

Four Cores:
Rendering (Single CPU): 3177 CB-CPU
Rendering (Multiple CPU): 11533 CB-CPU

The change appears to be completely stable at this point but could someone suggest some other benchmark tests that would push the limits of the current four cores to check on it's future stability? I've never been one in the past to run benchmark tests on my new builds. Based on the article I'm not sure if the change from the three to four core processor provides me any added benefits but I'm tempted to leave as four if there isn't any stability problems. What do you think?

Thanks jjbtexas for the suggestion. I came across Prime95 (Win 64 bit) this morning and ran it for about 9 hours with the new unlocked CPU passing every test so I guess that puts a dent in the comments I read about the X3's being built from defective X4's.

they don't, all of them are slightly defective, except one batch which was to meet demand.

and don't forget the 720 is in a fairly high demand due to being better and cheaper than the e8400.

it was in late february/early march so I doubt you have one

how exactly do you clear the CMOS and restore default BIOS?

???
Cheating at what? was there a point to your post? It does not make any sense!

anybody respond on how to restore CMOS and BIOS in case it was unsuccessful?

are the 720s the only ones that can be unlocked? I have a 710...

go into bios press Ctrl + F1 go into overclocking tweak at top you will see ACC( advance clock calibration set to hybrid and all cores.you will have 4 cores on reboot...

I thought you set them to "auto"? Are u sure you have the same mobo as me? (GIGABYTE GA-MA790FXT-UD5P)

oh wait, you have the UD4P one, but they both should be similar, i guess... anyway...

What clock speed and voltage was the tri-core at before you enabled the ACC option? I want something to start off from, since i have it at 3.2 Ghz at stock voltage, do i have to up the voltage to unlock it?

trust me, the 720 is very cool, especially when overclocked, even on the stock cooler.

mine's at 3.4 Ghz, with a 1.350 voltage, which is only a tick above normal. I'll say 3.4 Ghz is where it is at, you can go higher, but i'll recommend a better cooler. For stock, it's 30C at idle and 45C load (playing FSX, the most CPU intensive game i have)

but still, i'm on the fence about unlocking it, even though i bought it when it first came out (higher chance of success), but still... you never know, and taking apart the case and hitting the CMOS switch is a PAIN IN THE ASS if that's the worst-case scenario.

Couldnt unlock my 4th core .. oh well i wasnt expecting it to when i bought the tricore. Still have my 710 clocked at 3.5 with 1.35 for a while but have it at 3.2 just to be safe.

I've used All cores and Auto.It actually works better on Auto more stable at high oc.On ALL CORES you can adjust the % of oc on each individual core. Wind down the 4th core if unstable..As for the bro with the 720 that crashes on boot with 4th core activated try cranking your vcore voltage up to 1.45 -1.5 and wind down your ram e.g if its 1600hz knock it back to 1333 if its 1333 knock it down to 1066 and make sure your ram voltage is set to ram specs not board specs.Same with your ram timings.At the moment ive got my ram set at 1.8 volts. 1333 hz 7,7,7,24.Corsair XMS3 1600 hz. Works best for gaming....Change your bios to F4 or f4g .Some bios don't work i've been told.I have tried these 2 and they work.

Have to say i was pretty sceptical about this but....It works!
I have the Gigabyte MA790GP-UD4H and using the f3 bios.
I set the ACC to hybrid and auto and when i rebooted it now says that its a phenom x4 20, now none of the temp monitor programs that i have will show a valid temp but i definately have four cores working and seemingly stable at 3.5gig (just changing cpu multi to 17.5) and no other changes at all!

Umm.. 790GX is not a chipset, its the name of the northbridge / SB750 is what you need (ACC) in order to unlock and Biostars are not the best for unlocking.. To my knowledge they are low end boards.. The top unlockers are Gigabyte UD3P's/Asus M4A79/MSI 790FX-GD70....

I have the Gigabyte MA790GP-UD4H with the SB750 and i just didnt get lucky unless there are other things i can try. I havent looked at the manufacturing date of cpu so i dont know if its one of the few or not. But i have seen a few people with my board that have been able to unlock 4th core. I just dont think there is anything else i can try. I have the F3a bios. Is there a wasy i can knock it back to the F2 bios? I've seen people have alot of luck with that version.

thanks OvrClkr. I will check for a newer bios. I think im just out of luck. Im sitting at 3.6 stable on 3 cores so im not too dissapointed right now.

That is all i do is gaming pretty much. A little bit of home movie making but not much else. Thank you . This is my first build so im pretty happy. I just found out that the EPP on my OCZ DDRII 800 memory lowered my timings to 4-4-3-15 and boosted it up to 412Mhz with no errors with 100% coverage on memtest too. Woohoo

Thanks again. I will change it right now. Would 4 more gigs do me any good ? I was thinking about getting 2 more sticks. Or should i consider 1066 ram ?

I do have Vista 64. And i can 4 more gigs of the same ram for 40 bucks. And a future build is definitely coming so i think i will take your advice. I have very much enjoyed the experience and am looking forward to building another. And everyone on Toms have been great with me from the begininng. I had some pretty noob questions when i started. Thank you for your input.