• Please review our updated Terms and Rules here

IBM 5151 Monitor, bright spot when turning off

pcdata76

pcdata76

Experienced Member
Joined
Apr 9, 2015
Messages
125
Location
Germany, Munich
Last week i picked up an IBM 5151 together with a clone XT computer. I'm new to pre-386 computers and their accessories and didnt used a 5151 monitor till now.

Monitor is functioning without problem, image is clear and crisp, but when i turned off the computer, monitor turns off with a very bright spot around the middle part of the screen. Is it normal for these monitors or something needs repair? (aged capacitors in monitor, or something bad on clone MDA card maybe?) In color monitors there is a circuit which turns the electron guns off just before turning the deflection off during power loss to prevent phosphor burn caused by bright spot at the middle of the screen, but i have no idea such a measure is built on these monitors too. Before starting to troubleshooting, i wanted to ask.

I recorded a video, http://vid1376.photobucket.com/albums/ah14/pcdata76/KARISIK/CAM00794_zps3gfn7lmu.mp4
 
If you have the 5151 power cord plugged into the rear of the 5150 power supply.......turning off the 5150 power switch turns
off the 5150 and 5151 at the same time. Have you tried just unplugging the 5151 power cord to see if the symptoms are the same ?
 
Last edited:
Tried that with mine and it is an improvement. When I power off cpu and display at the same time, the display collapses to a bright line down the center. When I unplug the display first, it collapses to a one inch band
down the center then extinguishes so not as harsh. I don't see any phosphor burn so it does no damage anyway.

Larry G
 
Interesting, I had training on CRT's and the Sony Trinitron but had not heard of the use of an aluminum film to protect the phosphor.

Larry G
 
Then it looks like this behaviour is by design and most probably won't harm the phosphor coating. My 5151 is 1984 made and should be powered off like that hundreds of times up to today. Anyway, i can make a short cable with a switch on it between computer and monitor to be able to turn off 5151 before shutting down the computer.
 
retrogear said:
Tried that with mine and it is an improvement. When I power off cpu and display at the same time, the display collapses to a bright line down the center. When I unplug the display first, it collapses to a one inch band
down the center then extinguishes so not as harsh. I don't see any phosphor burn so it does no damage anyway.

Larry G
Click to expand...

You're right, mine's the same. I'm very curious about the reason behind this. The beam raster is created in the circuitry of the monitor, not in the PC. Maybe it is easier for the raster to collapse without an input signal for some reason? Nevertheless, I'm going to pull the power on the 5151 first from now on. It seems kinder than the alternative.
 
The MDA display is unique because the horizontal and vertical drive to generate the raster from the pc IS the drive signal, not just for synchronization so when the PC is powered off, the H&V drive to deflect the beam is instantaneously gone but the
high voltage has not had time to bleed off so the beam continues to the center of the tube.

Larry G
 
retrogear said:
The MDA display is unique because the horizontal and vertical drive to generate the raster from the pc IS the drive signal, not just for synchronization so when the PC is powered off, the H&V drive to deflect the beam is instantaneously gone but the
high voltage has not had time to bleed off so the beam continues to the center of the tube.

Larry G
Click to expand...

That explains it. I believe it also explains why on cold boot the monitor doesn't make the static "ch" noise associated with power on until the PC starts sending a video image? Does that make sense? I've always wondered about that behaviour.
 
There is drive and high voltage as soon as the pc is powered on but you don't hear the static "crunch" sound on a monochrome display because HV is only around 10KV instead of approx. 20KV like a color display. More voltage, more static field.

Larry G
 
retrogear said:
There is drive and high voltage as soon as the pc is powered on but you don't hear the static "crunch" sound on a monochrome display because HV is only around 10KV instead of approx. 20KV like a color display. More voltage, more static field.

Larry G
Click to expand...

That makes sense. But mine definitely does a slight noise when the PC starts sending a signal.
 
>That makes sense. But mine definitely does a slight noise when the PC starts sending a signal.

You must have young ears :p
 
pcdata76 said:
Then it looks like this behaviour is by design and most probably won't harm the phosphor coating. My 5151 is 1984 made and should be powered off like that hundreds of times up to today. Anyway, i can make a short cable with a switch on it between computer and monitor to be able to turn off 5151 before shutting down the computer.
Click to expand...

It's not really by design, it's just a side-effect. It will harm the phosphor, eventually.

If you are worried about it (I wouldn't), the way to cure it is to apply a negative voltage to the control grid at power-down. That will eliminate it completely.
 
retrogear said:
>That makes sense. But mine definitely does a slight noise when the PC starts sending a signal.
Click to expand...

That's ultimately due to the sync circuits phase locking to the PC's output. You will hear "microphonics" from the sync and high voltage circuits, and you will hear a rapid change in static charge.
 
pcdata76 said:
Then it looks like this behaviour is by design and most probably won't harm the phosphor coating. My 5151 is 1984 made and should be powered off like that hundreds of times up to today. Anyway, i can make a short cable with a switch on it between computer and monitor to be able to turn off 5151 before shutting down the computer.
Click to expand...

Don't bother. It takes minutes of sustained illumination at normal brightness levels to "burn" the phosphors.

The few seconds your flash appears to exist for will have zero impact (remember these are long-persistence phosphors, they are only actually being illuminated for a fraction of a second, the majority of what you're seeing is the characteristic after-glow), and as we've seen in this thread, is simply a by-product of the monitor's design.

Storm in a teacup! :)
 
bobba84 said:
That explains it. I believe it also explains why on cold boot the monitor doesn't make the static "ch" noise associated with power on until the PC starts sending a video image? Does that make sense? I've always wondered about that behaviour.
Click to expand...

Yeah, there is no a separate free-running oscillator to generate HV and deflections as in modern monitors and TVs when there is no V/H sync signal to lock. That's why you don't see any raster or retrace lines when 5151 is powered on at maximum brightness with MDA cable disconnected. V/H sync signals from MDA adapter are needed to generate HV and deflections as Larry said.
 
SquallStrife said:
Don't bother. It takes minutes of sustained illumination at normal brightness levels to "burn" the phosphors.
Click to expand...

Maybe not. I have just acquired a 5151 (the one I had died) and it has a severe phosphor burn in the middle-bottom of the screen.




Click to enlarge

When I power off the 5160 with the 5151 connected to its power supply, it produces the same glow as the video in the first post. But when I connect the 5151 directly to the wall and power off the 5160, it produces a single line exactly in the same spot where the burn is.


Click to enarge

So I would recommend connect the 5151 to the 5150/5160 power supply to avoid phospor burns.
 
nestor said:
Maybe not. I have just acquired a 5151 (the one I had died) and it has a severe phosphor burn in the middle-bottom of the screen.




Click to enlarge

When I power off the 5160 with the 5151 connected to its power supply, it produces the same glow as the video in the first post. But when I connect the 5151 directly to the wall and power off the 5160, it produces a single line exactly in the same spot where the burn is.


Click to enarge

So I would recommend connect the 5151 to the 5150/5160 power supply to avoid phospor burns.
Click to expand...

If i reboot the computer using RESET key, it produces the same vertical bright glow during reset period so i prefer ctrl+alt+del instead or try to push reset as short as possible.
 
You must log in or register to reply here.
Back
Top