[SGVLUG] OT: No negative voltage (PC problems fixed)

[David Lawyer]

I wrote this about a week ago and wasn't sure if anyone would be
interested so I thought about deleting it.  But here it is!

I had some PC hardware problems that had me stumped and I was about
to ask for help from this list.  But they are now fixed and I think
they were all to a loose power connection at the pin which supplies
-12v DC to the motherboard.

These problems started when I switched PCs and also installed a
cannibalized power supply (known to work OK).  One was that my dumb
terminal on my serial port got a parity error at roughly every 20,000
bits sent but it was erratic and not based on counting bits.  Another
was that I had no sound and another was that a different serial port
(a PCI bus one) that had worked before, wouldn't send anything.

I started to wonder if all these problems could all be due to the
power supply.  Since the CPU and the hard drives worked fine (as did
the video card and modem) it seems that the power supply must be
working.  But working at all voltages?  Now the serial transmission
uses +-12v, the CPU uses +5v (reduced on the MB perhaps), the HD +5
and +12.  Suspect are the negative voltages.  But which negative
voltage was it?  I sent a file to the working serial port and looked
at the transmitted wave at 9,600 bps with an analog voltmeter.  -12v
is a 1 bit and +12v is a 0 bit.  So I should see a vibrating needle at
about 0 volts average.  But it was about +5.  Then I looked at what
should have been the -12v idle signal with the meter and it was +1v.
In other words my port was sending out perhaps +1v instead of -12v for
a 1 bit.  But it still worked for my dumb terminal which is (or was)
guaranteed to accept any voltage pulse between -25v and +0.75v as a 1
bit.  And any pulse between +2.25v and +25v is a 0 bit.  So +1v is in
the dead zone but closer to a 1 bit.  So this explains the few random
parity errors.  It's amazing that something that in error sends +1v
when it should be sending -12v actually works (even with the wrong
polarity). 

I pulled out the power supply plug from the motherboard which has the
-12v pin, lubed the pins with TV tuner lube, reinserted it, and
measured -12v.  Then I fired up the computer and looked at the
transmission of a file out the serial port and the meter needle
vibrated near 0 volts like it should.  The other two problems were
fixed too.  Had I put the plug in wrong originally?  No, since it has
grooves so it can only go in one way and also it has the "power good"
signal pin on it which the motherboard needs to function.

So I think the problem was due to a bad connection (corrosion) at the
-12v pin.  I tried to search the Internet for others with this problem
and only found a complaint by someone who bought a new power supply
but can make no use of the negative voltage sources since his computer
has no use for negative voltage.  Could it be that most modern
computers don't need any negative power supply voltages unless they
have serial ports?

			David Lawyer