Neighbors have a computer that won't work after a lightning strike. With the video cable connected, the monitor stays black and in the standby mode. There are howls from the speakers, like feedback. With the cable disconnected, the monitor will briefly show a notice that it's not connected. There are no howls from the speakers although the mic cable remains connected.
With the power off, I checked the pins of the monitor cable to see that the grounding pins were grounded and the signal pins were not. One grounding pin is open, so it seems certain that the monitor is damaged.
The next step would be to try another monitor to see if there's other damage to the system. In case the computer's video card is damaged, can a monitor be damaged by plugging in to it?
'Probably' not, but why not try plugging their monitor into your computer? That's extremely unlikely to cause any damage. Then, I'd take *everything* out of the neighbor's computer, except the vid card...unplug memory, all drives including floppy...just leave in the CPU and vid card.
See if it boots up and asks for memory. If so, install that. Keep going--adding one device at a time--until you find a problem device...skip that one on the next go-round, until you've ID'd any and all such.
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?
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?
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?
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.
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.
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.....
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.
Exactly why I just 'shotgun' them whenever I suspect lightning problems. 'Something' is damaged...often many things. It's a lot quicker (of course, I have monitors, RAM, video cards etc, spilling out of every corner) to just see what 'does' work as opposed to trying to track down 'the' problem with a lot of diagnostics. It takes five minutes to pull everything out. It takes another minute to see if it will boot in that configuration, and if it won't, there's not much use in going any further unless one has the necessary spares to replace MB/CPU. If the computer's more than a couple of years old, it's cheaper to replace; unless you're doing it as an 'excercise'.....
In your case, it would be simple to google 'VGA pinout' in order to see what signals should be on each pin, if you want to diagnose at that level.
Which may be what 75 ohms looks like depending on the range the meter is set on. I suspect there's visible damage inside the monitor, I'd open it up and look at the board where the cable plugs in. Lightning is interesting stuff, we don't get much of it out here though.
It won't ask for memory. You have to have memory in the pc for it to do anything but beep. That's all you'll get without memory BEEEP BEEEP BEEEP.... If you take everything out except the processor it should beep..(including memory and vid card) Beeping a good sign and and means the bios is working.
I got 75 on the R, G, and B signal pins. Without a schematic, I wondered if the sync pin was fed by a transformer, in which case zero ohms could have been correct.
When I checked the monitor pins, a scope showed several problems. A local drugstore chain has refurbished computers and monitors similar to theirs for $250. Why fix it?
My own computer is a Mac. I think with a Mac, the display comes up before it checks the memory. On a PC, would bad memory keep the monitor light from turning green?
On the PC, a Compaq Presario 1710 I think, all ports except the modem are on the motherboard. Would I need the hard drive to get the monitor to light up?
Offhand, I remember I could unplug the floppy drive and the CD drive.
It can, it really depends on the fault though. If the RAM is bad in a location that prevents the POST from running then the display adapter will never be initialized. Seems I've had Macs with bad enough RAM that they wouldn't show anything on the screen either but I haven't had a ton of exposure to the recent ones.
Well, a man more familiar with PCs got the display going by removing the CPU and plugging it back in. I suppose in each case where I'd found an open ground on the computer or monitor, its counterpart supplied the necessary ground. He found it still needs a new motherboard.
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