Abit KA7-100

[dood]

This is the worst board I've had to work on yet... this actually has a MOSFET that got hot enough that it half-desoldered and slid down the board a little. It's a Fairchild FDB7030L, which I ordered a replacement for through Mouser. It has tons of dead caps on it.... I think I'll let the pictures do the talking (see my next post)

[dood]

Re: Abit KA7-100

Okay, here's the pics so far:

The board as I received it:





this one almost looked moldy! :






Mostly decapped... all the bad ones are gone at least:



Two questions. One, how do I remove all the electrolyte residue from the board, safely? Can I use topcat's washing procedure? Would the fact taht I have a Culligan salt system leave salt deposits on the board afterward? We have high iron content water...

Two, How do I remove the MOSFET from the board? The legs are desoldering, but the chip itself is somehow attached to the board. Do I pry it off, or heat it with a blowdryer, or what?

Thanks,

dood

[Rainbow]

Re: Abit KA7-100

I clean the boards using rag and alcohol.
Desoldering the MOSFET requires a lot of heat - I use 75W soldering gun to heat the tab. It takes pretty long but finally, I can lift it.
Also check the other MOSFETs if they're not shorted (they appear OK but you never know...)
I'd change the smaller caps too (like the one near the chipset) as they appear to be the same crap as the bigger ones.

[dood]

Re: Abit KA7-100

Yes, the smaller caps are going to be replaced... I'm going to recap most of it first, though, and make sure it works. Then continue on to the other smaller caps. I'm assuming that I should just check continuity across the two posts of each mosfet? Should they be open, or a specific resistance?

BTW- I went with 70% alcohol and q-tips to remove the crud... worked great!

[dood]

Re: Abit KA7-100

Okay, i measured open across all of them except for the one that fried, which I measure 2ohms resistance on. I'm guessing it's shot, then!

[dood]

Re: Abit KA7-100

Err, not good.... the pad came up with the one leg of the mosfet, and then I couldn't get the bit underneath to come off the bottom, so it pretty much destroyed that (I'm assuming it's just grounding?)

Here's a pic:

[Rainbow]

Re: Abit KA7-100

Damage to the board under a fried MOSFET happens often - I have a board that has the copper plate from under one MOSFET almost completely missing - it came off with it (I'll probably need to get a copper square somewhere to solder it there). Yours it not that bad, you only need to be careful when soldering new MOSFET in.
The broken pad can be fixed using a wire - find where it goes (to one pin of the small 8-pin chip - MOSFET driver) and a piece of wire will do it.

[Topcat]

Re: Abit KA7-100

KA7's = indestructable. After recapping, I've never had one not work. I think I have pics of a KA7 in as bad of shape on the badcaps.net homepage.

If you're going to use the board, rig a fan to blow on the caps. They get extremely hot otherwise, and that will take its toll even on high-end replacement caps.

[dood]

Re: Abit KA7-100

The board as it sits now while I wait for the replacement MOSFET:

[Topcat]

Re: Abit KA7-100

Recapping the KA7 with 10mm caps is not fun..... There's an easier way using 10mm caps though. I've done a lot of them like this before I was able to locate 8mm replacements.

http://root.kabalsrealm.com/index.php?page=ka7_p3

[linuxguru]

Re: Abit KA7-100

YMMV, but you can use 63:37 low-melting solder when replacing the FET - it will flow easier and allow easier replacement in the future. The downside is that it may also desolder itself easily if high temperatures are encountered again.

[Topcat]

Re: Abit KA7-100



That's what a KA7 looks like redone in Rubies

[dood]

Re: Abit KA7-100

Topcat- those are 8mm replacements there, right? I couldn't find any 8mm with suitable specs. How did you get the 10mm to fit in there so nice on the other page? I'm having a hell of a time with these... I'd like to make it look neater, but I can't seem to be able to do it.

[Topcat]

Re: Abit KA7-100

The secret to packing the 10mm caps in there is to set orientation off one cap in one of the end positions of the CPU slot. I always start with the one closest to the AGP, because its easy to eyeball it without other obstacles in the way. Solder the negative lead, making sure the cap's base is as close to the board as it can possibly get, angled away from the CPU slot, just slightly. The set the angle you want (CPU clearance is critical with these fat babies), then solder the positive lead. Then, for every cap in the line, use the angle of the first one for orientation. They are packed in there tight, and I mean TIGHT, but it just takes a little patience, but it'll come out perfect every time.

The Rubies I used in the other board are all 1500uF 10v MBZ's. I even replaced the 2200uF caps with 1500uF caps by the FET's. This caused absolutely no issues, and I've done it dozens of times on dozens of KA7's.

Did you check the FET that desoldered with a diode tester? Most likely it was still good, it just got hot and desoldered itself from the pad.

[kc8adu]

Re: Abit KA7-100

i always replace any "sliders"i find.
even if they arent shorted i find they have been wounded.high on res and such.
dont worry about the solder type though.
if it gets hot enough to slide again you have worse problems anyway....

[dood]

Re: Abit KA7-100

I'm going to try to clean up the current cap install if I can. I don't like the looks of the board when there's caps leaning all over the place. I prefer to have nice, neat rows, which I think shows that I care about what I'm doing.

How should I test the MOSFET? I have a multimeter, that appears to be able to test diodes. When I measure across the two legs I get 104... what does that number mean? I get 680 when I test one of the mosfets still on the board, but that could be sending current through other devices on the board as well, right? I'm testing on the legs of the MOSFET itself, btw...

[Rainbow]

Re: Abit KA7-100

-------
|   |
-------
 | | |
 G D S

G = Gate, D = Drain, S = Source
If it's out of circuit, resistance between G and other two should be very high - diode test shouldn't show anything. If it shows, the MOSFET is bad.

[dood]

Re: Abit KA7-100

The actual resistance between Gate and Source is 104 ohms (what the diode test was telling me as well), between gate and drain is 5 ohms, between source and drain is 99ohms. I don't know that this tells me anything, but I'm replacing it regardless.

[Rainbow]

Re: Abit KA7-100

It's shorted so it must be replaced.

[dood]

Re: Abit KA7-100

Here's an interesting size comparison with this motherboard... New caps vs old caps:


From left to right - New Panasonic 1000uf 10v, old JPCON 1000uf 10v, new Panasonic 1500uf 6.3v, old JPCON 1500uf 6.3v, New Nichicon 2200uf 6.3v, old JPCON 2200uf 6.3v

Now, there's nothing wrong with 8mm caps, but typically they'll be taller than their equivalent spec'ed 10mm counterparts. Here, the 10mm's are bigger in every respect!