Weird and Persistant problems of my desktop frying keyboards

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I put together my computer around 2005-6 ish, and I've been having
this persistant problem for the last few years. Every once in a while,
my computer would just decide to fry a keyboard. The keys would
progressively, groups by groups stop functioning. E.g. first B and N
would stop one day, then a few more the next.

I've only originally used keyboards that plug into the I/O slots, and
over the years, I've gone through 4 or 5, yet on other computers, they
last for way longer. Yes I've plugged in the keyboards in other
computers and they still do not work.

Recently I've switched to a USB keyboard, and thougth my problems were
solved, whatever these problems may be. But I was wrong... as
yesterday, I discovered the colon would not work yet the semi colon
worked. Also the @ symbol does not work while the number 2 worked. My
shift key is functioning properly.

I'm on a Asus A8N SLI Motherboard with AMD Athlon 3700+ CPU, Nvidia
GeForce 8600GTS video card, 2 gigs of ddr2 ram etc, the normal stuff.
I got the motherboard for the good reviews back when I was looking.

This problem isn't too large, as keyboards are rather cheap. But its
REALLY perplexing. If anyone has any guesses, I'd be interested to
hear them.

Re: Weird and Persistant problems of my desktop frying keyboards wrote:
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Is there any chance that spillage is the culprit?  I once had a
girlfriend that would spill her wine into my keyboard, and rather than
appear like a sloppy tart, she would carefully wipe the keys clean.
Unfortunately, the rest of the liquid would eat away at the delicate
electrical traces that make up the circuitry of the keyboard.

Although your description does not really match my suggested failure, as
the shifted version of the same key works fine, it would explain the
persistence of the problem across multiple keyboards and ports.

It would be interesting to see if your problem goes away if you boot
from a linux livecd, such as ubuntu.  If it does, then we can be
reasonably sure you're suffering some sort of software problem, as
opposed to hardware failure.

Weird, though.

Re: Weird and Persistant problems of my desktop frying keyboards

Grinder wrote:
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The fact that keys are failing "groups by groups" certainly suggests that
some external factor, possibly liquid in nature, is involved. The influence
that the computer has over the keyboard isn't that specific -- it  could
certainly fry the KB or mess up overall communications but getting down the
key level is a bit beyond its reach. If it was some defect in the BIOS
software that handles the keyboard I don't see it healing itself suddenly
when the KB is replaced. If it was some higher-level function such as the
accessibility options in the OS the same argument applies.

The best bet would be to disassemble and autopsy the bad keyboards. After
all they have already been declared dead...

John McGaw
[Knoxville, TN, USA]

Re: Weird and Persistant problems of my desktop frying keyboards wrote:
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Keyboards have the keys arranged in a matrix. The chip driving
the key array has horizontal and vertical lines. One set are
drivers, the other receivers. Together, they look for key closures.
Since there isn't a wire or wire pair per key, then the keyboard has
some limits as to how many keys it can detect simultaneously. The
scanning method saves pins on the keyboard chip - so it is a cost
shaving feature for the chip manufacturer, and doesn't represent
the best possible design for the end consumer. It is good enough,
as it were.

This document shows an early example of a home brew keyboard (see page 10).
The schematic shows the scanning matrix, and if you had this information
for a particular keyboard, the clusters of keys that are failing would make more
sense. Keyboard controllers come with various "N x M" numbers of
horizontal and vertical wires, so unfortunately you cannot analyse
your keyboard problems using the schematic of this one. One keyboard
might be 17 x 6, another 12 x 10 or whatever. The keyboard
controllers are not all designed the same.

On the MOS controllers of that era, the scan matrix is relatively
high impedance, and conductive foreign substances can cause
phantom characters to be detected, or cause the thing to give
up entirely.

To figure out why the keyboards are failing, you'd have to consider
all the possible ways electricity could "get at" the thing. For example,
if the +5V or +5VSB feeding the keyboard is on the high side, that
may accelerate the failure of the keyboard controller. If the condition
existed on the USB port though, you'd think other USB devices would
get killed too.

The PS/2 ports are fed by their own Polyfuse. Each two USB interfaces
are fed by their own common Polyfuse. This is where I'd connect my multimeter,
for a quick determination of whether there is a problem with the voltage
used to feed the ports. On my Asus motherboards, the Polyfuses may
have a green body color, and have a dimple on each end. But they're
not necessarily going to be that easy to find. You can also check
the connectors themselves, but that is a bit tougher to get at.

In this example picture here, if you look in the upper right hand
corner, there is a component with "P" and "110" printed on it. It
is right next to the keyboard stack, and that is likely to be where
the +5V feeds the PS/2 connectors. That would be an example of a
Polyfuse. There are three more Polyfuses near the green LED on
that motherboard, and those fuses might be for the three blue
2x5 USB headers. Polyfuses are resettable fuses that aren't supposed
to need maintenance - the fuse closes again after it has a chance
to cool off.

One thing you won't be able to see with a multimeter, is if
there are any transients on the power. To do a complete
check, would require an oscilloscope. And that falls outside
the budget range of most home hobbyists.

You could always take one of the dead keyboards apart, for
a visual inspection, and see if there is anything
physically wrong in there.


Re: Weird and Persistant problems of my desktop frying keyboards

On Tue, 13 Jan 2009 12:50:56 -0800 (PST),

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Since it is the same system, we need to consider whether it
is the system or the location.  For example, suppose you had
a cat with a weak bladder that often relieved itself on the
keyboard, or a spouse with a habit of knocking over a cup of
tea into the keyboard every once in a while, or you are in a
very humid or especially an area near the sea where there is
saltly air, these things can corrode the traces.

As another person suggested, an autopsy might help.  On the
typical keyboard you have black or gold colored traces and
contacts on a plastic sheet, corrosion would be visually
obvious from residue, while a power problem frying the
traces would cause discoloration that looks like heat

If your system uses the PSU 5VSB rail to power the USB
and/or PS2 ports, it may be weak and fluctuating out of spec
causing damage.  If your mainboard has 5V/5VSB jumpers on it
to select which if these two power rails is used for PS2
and/or USB, try jumpering them to 5V instead of 5VSB.  Their
location may be listed in the mainboar manual, or they're
comonly found near the port they effect.  They will always
have a jumper on two of the three pins present to complete
the circuit, but be careful about indiscriminately changing
jumpers if you have not positively identified them.

If your keyboards are flimsy and you often use them off of a
solid, flat surface, it could be that their frame flexes too
much and the intermittent contact is between the small
internal circuit board contacts and the contacts on the
plastic sheets.  In that case, try vigorously rubbing both
with a dry paper towel for a very mildly abrasive cleaning,
and ensure that the screws holding the keyboard together are
fully tightened.  

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