Re: Safely swap monitors?



jakdedert wrote:
>
> "James Sweet" <jamessweet@xxxxxxxxxxx> wrote in message
> news:DXnze.5161$VN3.1101@xxxxxxxxxxx
> >
> > >
> > > Why is a monitor less likely to damage a computer? If the monitor
> > > proves bad, as indicated by the open on a pin that should be grounded,
> > > won't I still have to connect a good monitor to the neighbors' computer?
> > >
> >
> > It's not, it's more likely as it contains much higher voltages right on
> the
> > same logic board as the low voltage stuff, but the chance of damage is so
> > remote it's hardly worth mentioning, you're more likely to damage one or
> the
> > other by dropping them.
> >
> > > What might be wrong with a computer's video card that would damage a
> > > monitor? I could check the video outputs with a scope. Would that
> > > assure me that it was safe to plug in my monitor?
> > >
> >
> > Nothing, not on a reasonably modern monitor anyway. Some of the early ones
> > would fry if fed an invalid sync signal, anything made in the last decade
> at
> > least will just shut down.
> >
> > > I thought I'd connect a good monitor and see if the computer worked.
> > > What's the point of disconnecting everything in the computer without
> > > first knowing that anything is wrong?
> >
> > There isn't one, though if it doesn't boot you should disconnect anything
> > non essential and see if that allows it to boot.
> >
> My point. There's already evidently more things wrong than just video, as
> evidenced by the OP's description of the audio. At the very least, pull
> that sound card which is squealing (unless it's integrated into the
> motherboard).
>
> Lightning's a funny thing. I had a system which got hit. The only thing
> that went out was the onboard IDE/floppy controller...in the days when
> onboard controllers were rare (Pentium 60). Disabling it in BIOS allowed
> the system to live out it's life in harmony; with the addition of controller
> card.
>
> I never fully trusted the machine, however, and thereafter relegated it to
> non-essential duties.
>
> I had another machine which fried in the same incident. About all that I
> could salvage from that one was the hard drive and case/ps. It even blew
> the fairly espensive speakers which were connected, as well as the parallel
> port in the printer.
>
> My son brought a machine back to me, one which I had originally built from
> all new components, as a graduation present. That one was totally
> unsalvagable. I mean that *nothing* was usable except the case. The ps
> blew, all the memory, motherboard, CPU, hard drive, all outboard cards
> (sound, video, modem etc)...a complete loss.
>
> Lat week's strike (detailed in this forum) only took out one of the many
> computer-related devices in the household (laser
> printer/fax/copier/scanner); while destroying or disabling many, many pieces
> of telephone and home entertainment devices.
>
> If the OP can get it to boot by simply replacing the monitor, fine. If not;
> what I said.....
>
> jak
> >

Taking the gist of your advice that a monitor *could* damage a computer
and visa versa, I checked resistance with DMM on the monitor cable and
found an open on the ground for the green signal. So there is something
wrong with the monitor, although it seems possible that it was working
with the green shield grounded only at the computer end.

The next step would have been to check the connector live with a scope,
but I would have needed a half-inch of tubing to keep my probe on each
pin.

The computer output was easier for a probe, being holes. None of the 15
holes carried any signal I saw. Two sat at 5V. The rest were at
ground. With a DMM, one of the signal holes (horizontal sync, I think)
showed zero ohms, but that could be normal.

Satisfied that nothing looked dangerous to my monitor, I hooked it up
and, as expected, got only the amber standby light.

I imagine a problem elsewhere could put the video card to sleep. Does a
video card sleep with two outputs at 5V? If not, the card must be damaged.

I don't want to try their monitor on my computer until I scope the
monitor pins for anything that looks threatening. If their monitor
works and it's normal for two conductors on a PC video output to sit at
5 Volts, I suppose the next thing is to see if their computer will run
with everything possible unplugged, as you recommended.
.

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