Nearly saw my 503+ go up in smoke

Discussion in 'FIC' started by Kyle, Jan 22, 2006.

  1. Kyle

    Kyle Guest

    This group is so dead I thought I'd share a saga about my old hardware
    reliable 503+ system.

    I had some problems with my 503+ system that uses a k6-3+ at 550Mhz. I
    thought it was the PS since the PS fan was locked up, so swapped in
    another PS, which in fact was a better quality supply made by Delta.
    Oddly, it was several days after installing the new PS that the system
    crashed/locked, I reset it and walked away to the next room over to
    work at another computer. The printer connected to the 503+ was not
    accessible over the network, so I walked back into the room where the
    503+ system lives, and omg, the smell of something burning permeated
    the air.

    I killed power and opened the case and did a sniff test, found the CPU
    to smell the worst, so I touched the HSF, and it was hotter than a
    firecracker. Further analysis revealed the Vcore PS circuit had 2
    bulging electrolytic caps. OH NO me sez, thinking the CPU has just
    fried. I cycled power just long enuf to get a BIOS boot screen, and
    to my amazement, the system tried to run. I quickly power down, and
    pulled the CPU, and started checking some voltages. Seems the CPU was
    getting 3.8v, that's right, 3.8v, only 1.6 volts above the max spec
    for the CPU. I surmise that the bad fan in the old PS may have
    caused overheating in the box, and closer inspection revealed the
    Vcore switch mode power supply circuit was having problems.

    I did a little circuit probing, and replaced the swollen caps, and a
    voltage test revealed the CPU socket was still getting 3.8v still (had
    the CPU out of the socket during these tests just in case). The
    signal from the PWM controller should have the MOSFET turned off, but
    it was conducting lots of current to the CPU socket. Sure looked like
    the MOSFET in the Vcore circuit had died (it's drain-source impedance
    was quite low with no power on the system). I searched the net for
    info on the device that failed, it is an n-channel enhanced mode
    MOSFET made by Hitachi. I found the data sheet on the 8-pin Vcore PWM
    controller chip (it sits right by the MOSFET device), and a typical
    circuit schematic was provided by the manufacturer in its data sheet
    detailing how such a CPU PS circuit should be built using a MOSFET
    device. A brief survey of some old boards I keep around to cabbage
    parts out of yielded a good quality Philips MOSFET device of similar
    specs (I have to complement Abit in their AX7 design, I've stolen all
    the PS caps out of this old socket7 board, and now I'm stealing the
    CPU PS components and all were good name brand stuff). Philips has a
    nice web site for obtaining data sheets on their semiconductor
    components, I tip my hat to their IT department.

    The worst part was getting the old MOSFET off the circuit board, some
    trace damage occurred since the tab/body of the MOSFET device is
    soldered to a large area copper trace on the mobo, and it's darn
    difficult to get a enuf heat into such areas to reflow the solder and
    separate the device from the mobo without special tools. Fortunately,
    enuf of the copper pad remained (after I tore most of the copper off
    beneath the MOSFET) to build up some solder onto for soldering to the
    replacement MOSFET body tab, which was not actually designed for
    surface mount, but bah who cares, a little jerry-rigging, lead
    bending, and solder built up on the tab of the replacement device and
    I had it attached in place. With fingers crossed, I hit the power
    switch (had pulled all boards out of the mobo, and pulled the CPU) and
    all was right with the world again as I measured 2.1v in the CPU
    socket (I use 2.1v with my k63+, seems to be what it likes). I then
    developed a big smile on my face cuz things are back where they should
    be, then I drop the CPU and the boards back in, hook up all the
    cables, and the old workhorse is back alive again.

    I do recall a dead HS fan on an old AMD233 k6 system where I burned HS
    fin stripes in my finger when I touched the HS, and it appears a k63+
    can also take some serious overvoltage and heat abuse without failing.
    It's so satisfying to fix a problem like this w/o even buying a single
    component!
     
    Kyle, Jan 22, 2006
    #1
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  2. Hey, Kyle.

    Wow. That is quite impressive. Congratulations on a job well done!
    Your diagnostic and cannibalism skills are to be commended. I thought
    it was too funny reading you rip apart that Abit AX7 (Athlon XP
    motherboard) for parts so that you could save the 503+. The graphic
    detail reminds me of some of the stories my plastic surgeon friend tells
    me about his work. Not for the squeamish.

    By the way, I looked at the "Mobile AMD-K6®-III+ Processor Data Sheet"
    (
    http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/23535.pdf
    ) and saw what you mean about the absolute max Vcore of 2.2 V. I also
    saw an absolute max Vio of 3.6 V, and an absolute max Vpin of (Vio + 0.4
    V) and <= 3.8 V. The Vpin is the voltage on any I/O pin. It looks like
    even with this highest absolute maximum, you were right there. Yikes!!!!!

    You are right. It has been dead here for a while. I guess most people
    have moved on to more interactive forums. Even so, that was quite an
    entertaining read. Thanks!

    --Alex
     
    Alex Zorrilla, Jan 23, 2006
    #2
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  3. Kyle

    Kyle Guest

    | Hey, Kyle.
    |
    | Wow. That is quite impressive. Congratulations on a job well done!
    | Your diagnostic and cannibalism skills are to be commended. I
    thought
    | it was too funny reading you rip apart that Abit AX7 (Athlon XP
    | motherboard) for parts so that you could save the 503+. The graphic
    | detail reminds me of some of the stories my plastic surgeon friend
    tells
    | me about his work. Not for the squeamish.
    |
    | By the way, I looked at the "Mobile AMD-K6®-III+ Processor Data
    Sheet"
    | (
    |
    http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/23535.pdf
    | ) and saw what you mean about the absolute max Vcore of 2.2 V. I
    also
    | saw an absolute max Vio of 3.6 V, and an absolute max Vpin of (Vio +
    0.4
    | V) and <= 3.8 V. The Vpin is the voltage on any I/O pin. It looks
    like
    | even with this highest absolute maximum, you were right there.
    Yikes!!!!!
    |
    | You are right. It has been dead here for a while. I guess most
    people
    | have moved on to more interactive forums. Even so, that was quite
    an
    | entertaining read. Thanks!
    |
    | --Alex
    |
    |
    |
    |
    |

    Hi Alex:

    Glad someone got some reading pleasure out of my long-winded story. I
    actually figured the bad part was no more than a $2 part if I had to
    buy one, but it was more fun to find a decent replacement in some old
    stock.

    The real reason for even bothering with the system is it will probably
    become the first puter for my daughter, who is nearing 2 years old.
    She already has the "grab the mouse" impulse (hehe) so I can tell
    she's got some "geek" in her blood, lmao. Just yesterday I let her
    sit in my lap and watch me play a bit of Counterstrike (which is
    probably not a good thing) and she kept shouting "more more more",
    hahahaha. Then she grabs the mouse and gives the scroll wheel a dozen
    or so turns and proceeds to pound on the keyboard. What a little
    monkey, she does everything we do. Here's a pic for your enjoyment:

    http://mywebpages.comcast.net/kylesb/abby-xmas2005.jpg

    One small note, the AX7 was a socket 7 mobo that supported up to 233
    MHz AMD and Pentium CPUs, in fact, the board lost it's floppy
    controller at some point, so I put retired it to the spare parts
    stack. It's not a socket A board. The AN7 is a socket A Abit board,
    IIRC.

    Cya!
     
    Kyle, Jan 24, 2006
    #3
  4. Kyle

    Roger Hunt Guest

    (snip)

    !
    Your daughter will be overclocking it before long - and a toddlers
    fingers should be small enough to get at those jumpers with ease!

    Best wishes
     
    Roger Hunt, Jan 24, 2006
    #4
  5. Heh, playing Counterstrike with your daughter reminds me of one of my
    friends. His daughter was probably about 4 years old, when one day at
    nursery school, one of her classmates said something about being scared
    of monsters. My friend's daughter told him, "Don't worry. If the
    monster comes to get you, you can always kill it!"

    I am not sure the teacher was too happy about that. Bah! What does she
    know? Video games empower young minds to conquer their fears and the
    unknown! (That's my story, and I'm sticking to it.)

    Oh, yeah. My friend is a psychiatrist.
     
    Alex Zorrilla, Jan 24, 2006
    #5
  6. Kyle

    farmuse Guest

    back in 98 this newsgroup was crazy, and very helpful when folks
    were not flaming one another. it was my first taste of what a real
    newsgroup was like, and I guess you are right forums have taken over.

    good to see some folks still posting, keep it up
     
    farmuse, Jan 25, 2006
    #6
  7. I have to second the "wow". I once found a Socket 7 mobo where a
    bulging cap caused a MOSFET to get hot enough to turn purplish brown
    and short its gate. But after replacing those parts, the new MOSFET
    (taken from a PSU so bad that PC Power & Cooling used to use it as an
    example of a bad PSU) ran at over 100C, even with a 2" x 4" chunk of
    aluminum clamped to it with a clothespin. When I later gained access
    to a scope I found that the Rockwell PWM chip that drove the MOSFET
    had also been damaged, making its output pulses a couple of volts too
    low. I was able to fix that by putting an open collecor TTL driver
    between them.

    I needed a 50W iron to melt the solder on the MOSFET tab, and even then
    I had to add fresh solder to make it flow freely. It seems that mobo
    manufacturers use solder with a higher melting point.
     
    larry moe 'n curly, Jan 27, 2006
    #7
  8. Kyle

    Kyle Guest

    | I have to second the "wow". I once found a Socket 7 mobo where a
    | bulging cap caused a MOSFET to get hot enough to turn purplish brown
    | and short its gate. But after replacing those parts, the new MOSFET
    | (taken from a PSU so bad that PC Power & Cooling used to use it as
    an
    | example of a bad PSU) ran at over 100C, even with a 2" x 4" chunk of
    | aluminum clamped to it with a clothespin. When I later gained
    access
    | to a scope I found that the Rockwell PWM chip that drove the MOSFET
    | had also been damaged, making its output pulses a couple of volts
    too
    | low. I was able to fix that by putting an open collecor TTL driver
    | between them.
    |
    | I needed a 50W iron to melt the solder on the MOSFET tab, and even
    then
    | I had to add fresh solder to make it flow freely. It seems that
    mobo
    | manufacturers use solder with a higher melting point.
    |

    Aha, your story speak of a higher suffering to restore the mobo to
    operational status.

    I agree, I think the foreign solder is not the typical 60/40 lead/tin
    mix solder, it reflows poorly. I could have borrowed the soldering
    iron from hell (my neighbor owns it) with 400w power (he used it for
    some massive purpose years ago) but opted to test my skills with a
    lesser powered iron (think mine is 25w, it's a dual power jobbie 10
    and 25). I should have tried adding solder to assist in the reflow,
    failed to think of that in time before the damage was done.

    What strikes me as scary is that the mobo traces are the only heatsink
    for these surface mounted MOSFETs. The mobo that I cabbaged the
    replacement MOSFET off of used a small stand-up HS with the power
    devices in the Vcore and I/O voltage switch mode PS circuits. Guess
    the need to cut costs to the bare minimum has prompted the designers
    to take all kinds of shortcuts. I considered using the HS, but
    figured there might be a height problem (the device is in-line with an
    ISA slot) and if the old/dead MOSFET had no HS, then maybe the
    replacement is OK w/o a HS. I tested the temp of the replacement
    MOSFET by touching the device, and remarkably, it is barely warm to
    the touch.
     
    Kyle, Jan 27, 2006
    #8
  9. I haven't noticed much relationship between MOSFET temps and heatsinks,
    maybe because MOSFETs come with different resistance ratings. For
    example, my ECS K7VTA3 v. 8 with 1.6 GHz Duron has noticeably warmer
    MOSFETs (no heatsinks) than my Asus/Asrock K7VT4A Pro does (also no
    heatsinks), even though the latter's are in much smaller packages.
    OTOH the Asrock has high quality Chemicon capacitors in the CPU voltage
    regulator instead of the OSTs found in the ECS.
     
    larry moe 'n curly, Jan 27, 2006
    #9
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