monitor goes black

Usually, everything works fine.  Occasionally  (twice this week already) my monitor goes black.  The computer is still running fine, I just can't see anything.  I replaced the monitor hoping to solve the problem, but no luck.

When this happens, I disconnect from power, then reboot.  At re-boot, I make it past the memory test OK, but before it can finish booting up, before testing extended memory, all goes black.  Boot up continues, I just can't see it.

After several re-tries, it will finally boot up complete with display.

Today all ran well for 16 hours, then, while doing a backup -- black screen appeared.  Earlier this week it happened when I touched the spacebar to terminate the screen saver (Razzle Dazzle).

This is a 486 dos set up, about 6 years old, used as a cash register.  

I had a pretty old RGB monitor; replaced it with a new thin flat screen Samsung.

Any ideas?  If you're getting ready to tell me to buy a new computer, I worry that my DOS cash register program won't work under windows.  It's a perfectly excellent program that I don't want to give up.  Windows seems so touchy; DOS has been rock solid for our store situation.


Thank You all for your ideas, I'll just have to wait for the problem to happen again to try your remedies.

One more symptom I just remembered:  One day, twice in a row, as it was booting up, it reported that the memory test failed.  At the third try to boot up, all went well and has been OK for a week.  Was that just a quirk or maybe related to my blackout problem?
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First, it may be a video card, cable or a motherboard. If it's a motherboard or a video card you can no longer get as a direct replacement, you'd be better off to go to another computer. Unlike what you said, this is very appropriate and easily possible.

There is no reason why you cannot use a replacement computer, but continue to use DOS. You'd need to simply verify that the I/O devices the old computer used can connect - this class of computer used ISA almost exclusively for add-on cards.

here's how:

Verify the type of I/O card used in the computer, if any. If you can't get a similar card for this part, you might have a problem.

Verify the level of DOS you are using, and either use your  old install disks or buy new ones at the level you were at (DOS 6.3 or whatever).

Utilize a parallel-port -based backup tape unit to copy all your data off your hard drive as an image,, rather than trying to reuse the old drive. This allows you to set up a newer machine and test it before you transition.

Acquire a newer computer (almost anything would be an improvement) - I'd recommend a good pentium or K6 motherboard with PCI as well as ISA (if your previous boards need it). All video cards work the same way with standard VGA modes, so everything will display the same way. Install the interface boards (if any) for the cash drawer(?)

Use the DOS startup disks and perform a FDISK and then a FORMAT of the C: drive on the NEW computer. Load the  DOS, restart then load the backup/recover software. Recover the backup to the new drive. Restart, and it should work the same way, but faster.

I'd love to hear how this all works out.

With the same behavior on two monitors, I would tend to suspect something with the video card. The wide variation in times between occurances leads me to think of something mechanical:

* The socket the monitor cable plugs into tends to get a lot of stress over the years. One pin's connection to the internal cable may be marginal.  Next time this happens, fiddle around with the monitor cable where it plugs into the video card - usually firmly pushing it up or down will get the signal back to the monitor if this is the problem.

* Video card itself has become ever so slightly unseated or one of the slot's contacts has become worn.  In cases like this, slight environmental changes can make and/or break a marginal connection. Pull the video card out and re-seat it.

* The video card itself may be failing.  Alas, they don't last forever.  As dust and crud builds up on cards, they run hotter and the chips die sooner.  Try a different video card.

This also leads to the possibility of the MB going belly up.  I'd take the box apart, pull all the cards (MB. too), clean everything, then put it all back together before trying another MB.

For your sake, I hope it's just the monitor connector on the video card.
If you can't play with this one and fix it, you may be able to buy an old computer from your local clone builder.  Many of them have a consignment area where they have older parts.
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You sure its not just your computer going to sleep? Perhaps your computer is just having sleep issues?
In DOS, on an old 486?  I don't believe that is too likely.  You can look in the BIOS and see if there are and sleep mode settings.
Verify power supply cable (110v or 220v)...unplug and plug ...
WhitleyAuthor Commented:
I chose to grade this answer as an A because you told me step by step what to do, giving me confidence that I really could get the job done myself.

 Cookre probably deserves some credit, too, because it could be that the only thing wrong with the old computer was bad contacts at the monitor cable and/or the video card.  I opted to go for total replacement of the computer.

So, here's how it went:  Fortunately, the local community college just had an obsolete equipment sale.  I bought a "lot" consisting of 2 complete systems(pentium), both with DOS 6.22 loaded.  I don't have a tape backup, so I just used floppies to copy every file off of my old computer, then onto the "new" ones.  I then copied the autoexec.bat and config.sys to the "new" computers.  Presto, Chango, I'm back in business, with a backup system to boot!  You were right, it sure works a lot faster now.  

Thanks, Bonnie
No  problem. Glad you got it done... I REALLY should have mentioned autoexec and config.sys!! really important files!

Good luck!

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