Nearly saw my 503+ go up in smoke

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!

--
Best regards,
Kyle

Alex Zorrilla wrote:
> Hey, Kyle.
>
> Wow. That is quite impressive. Congratulations on a job well done!

snip
>
> 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
>
>
>
>
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