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IBM 5151 Monitor Line Blurring

William_MackNite

New Member
Joined
Mar 16, 2017
Messages
8
Location
Salisbury MD, USA
I recently got this IBM 5151 monitor and it has a problem where it seems like it holds the beam on for too long when drawing a character resulting in blurred text. When first turned on the monitor, the picture was too tall for the screen but I fixed that with the vertical height pot. I tried the other pots that seemed related to the issue but it did not fix it. I then tried the contrast control on the front of the monitor and changing the contrast seems to make the lines not blur as much but did not totally fix it. Does anyone have any suggestions about what I could do to fix it? Or could it be a problem with the graphics card? The last picture shows a dash followed by a equals sign showing off that it seems like the characters start drawing in the right place but then continues too far.
20180117_194533[1].jpg20180117_194521[1].jpg20180117_203158[1].jpg
 
If it was the graphics card that was the problem, how could I tell? I can probe the TTL Video with an oscilloscope, but I wouldn't know what to be looking for. Does anyone know how long the video should be active for when it is drawing one "Pixel" for a character? Also, what should changing the front contrast control do exactly, because when I change the contrast, the character width changes and at some points it just cuts out and I don't know if that is how it is supposed to work or if it is a problem with the monitor. That is the reason that I suspect the monitor over the graphics card. Here is a video of me changing the contrast: https://youtu.be/PYSQxvdu5JE
 
First, you can clear up the noisy contrast control with a tiny spray of electrical contact cleaner inside the pot. Two screws and two push-pins are all that hold it in after you remove the monitor housing back and knobs. I just did mine yesterday.

I'm not convinced either way on the source of the problem. It seems to me that you are going to have to obtain another MDA or Hercules card to test it, which should not be difficult. I can't see an easier option.
 
I don't have another MDA card on hand, but as I did salvage this from a computer listed as dead I guess it it wouldn't surprise me that the video card might be not working properly. (Though that computer's problem was that it was entirely missing its hard disk controller.) I guess I will just get a new one then and test it out. Meanwhile, I will see if the output of the video signal looks correct on the oscilloscope.
 
Meanwhile, I will see if the output of the video signal looks correct on the oscilloscope.
Sync signals pictured at [here].

As for video, the following are some examples of what I see on pin 7 of my IBM MDA card.
* No monitor attached
* Oscilloscope probe set to x10
* Oscilloscope probe ground connected to pin 1

http://minuszerodegrees.net/temp/4/temp_mda_video_1.bmp
http://minuszerodegrees.net/temp/4/temp_mda_video_2.bmp
http://minuszerodegrees.net/temp/4/temp_mda_video_3.bmp
http://minuszerodegrees.net/temp/4/temp_mda_video_4.bmp

The narrowest pulses that I see, are about 100 ns duration.
 
The given pictures look inconclusive to me, but my best guess is abnormally high resistance in the video chain. This could be as simple as a corroded video connector.
 
I measured the resistance directly from the video card's solder point to the monitor's solder point and I got TTL Video: 1 Ohm, Intensity: .5 Ohm. Is this a normal amount of resistance for the Video chain directly from PCB to PCB?
 
I measured the resistance directly from the video card's solder point to the monitor's solder point and I got TTL Video: 1 Ohm, Intensity: .5 Ohm. Is this a normal amount of resistance for the Video chain directly from PCB to PCB?
Sounds good to me.

Does the video signal out of the MDA card look like what was shown in post #7 ?
 
I tried my best to get my 20 Mhz scope to capture the video signal, but it just didn't have enough resolution to get a great capture of it. It looks like the smallest pulse widths were about 100ns, though it could be at most 20% more or less due to the scopes inaccuracy at that range. I guess since the signal is TTL level, I could check it out on my logic analyzer next. If any of the capacitors were failing in the monitor, would checking their capacitance or resistance determine anything or would they all just have to be replaced just in case?
 
I tried my best to get my 20 Mhz scope to capture the video signal, but it just didn't have enough resolution to get a great capture of it. It looks like the smallest pulse widths were about 100ns, though it could be at most 20% more or less due to the scopes inaccuracy at that range. I guess since the signal is TTL level, I could check it out on my logic analyzer next. If any of the capacitors were failing in the monitor, would checking their capacitance or resistance determine anything or would they all just have to be replaced just in case?

Since there is only 15 electrolytic caps inside of a 5151, I'd replace them all instead of pulling, measuring and soldering them back (of course when measured good) one by one. Have a look my thread about recapping IBM monitors here. But your issue may not be related with caps too, try first with another MDA/HGC adapter and be sure that the problem is in the monitor.
 
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