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5151 Montior retrace and raster lines visible

A

atg

Member
Joined
Oct 31, 2018
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Location
East Coast US
Hi all,

I have a 5151 monitor here which just arrived in the mail, and has visible lines on the screen which are too bright. I have tried adjusting the knob on the front as well as the pot for "SUB-BRT" on the side of the logic board. I think maybe the amplifier IC is bad or there is a dead cap in either the brightness or h-blank circuits.

I took a picture of the first time I tested it. There are some vertical blocks from something on the IBM branded MDA card, which ended up going away, but I don't have a pic of it without those.

If anyone knows where I should look (which caps to replace / check), etc., that would be great!

https://imgur.com/a/Q5uWrgL <-- here is the pic. I couldn't get it to upload to the forum

Edit: I should say, I checked the values of the pots which I had adjusted, and they seem to be in the correct range of 100K each
 
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If you could post the circuit on the 5151, then we could discuss the parts of it possibly involved with the problem of visible vertical retrace lines. The image contrast and brightness also does not look(or is not set correctly) normal in the image.
 
modem7 said:
'IBM Monochrome Display' link at [here].
Click to expand...

Looking at the circuit for this monitor (Which is a type I do not have in my collection) there is no internal vertical retrace blanking.

This means that the V retrace blanking is achieved by virtue of the video signal level, during retrace, being at black level(CRT beam just extinguished) or a little "blacker than black"

This beam extinction occurs with the correct setting of the brightness and contrast control. The two should be set so that the graphics or text are as bright as they can be, without defocus with the contrast control, and the then background set to black or just beyond with the brightness control. If this is not possible it could be due to a fault in the brightness control circuit feeding the grid of the CRT, or the video output stage, feeding the CRT's cathode.

On occasions, where a CRT has developed low gun emission, to see anything on the screen the user advances the contrast and brightness controls to see a screen image, and in this case it can be possible that the V retrace lines are not suppressed and the CRT is driven into grid current by the signal.

I'm not saying the CRT is definitely bad, but this would require a check with the scope on DC coupling x 10 probe, looking at the voltages on the grid & cathode of the CRT, to determine if the grid of the CRT is going positive, with respect to the cathode (or cathode more negative than the grid), which it never should be.

Generally the peak to peak video drive at the CRT's cathode from the video signal would be in the order of 30 to 40V and the signal voltage at the cathode should never be lower at any point than the grid voltage, because if the grid is positive with respect to the cathode, it draws grid current and defocusses the beam.

It would be interesting to put the scope on the collector of TR-20 to look at the nature of the video signal there. Some features of the image you posted suggests there might be a problem in the video amplifier.
 
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Chuck(G) said:
Check the brightness potentiometer--you could have an open wiper connection (happens with dirt).
Click to expand...

I checked the pots as they swept from 0K to 100K Ohm (actually the SUB-BRT knob goes to 200K which is not what the schematics say)

So I probably do need to clean them out but they also don't seem to be an open on the wiper side
 
Hugo Holden said:
Looking at the circuit for this monitor (Which is a type I do not have in my collection) there is no internal vertical retrace blanking.

This means that the V retrace blanking is achieved by virtue of the video signal level, during retrace, being at black level(CRT beam just extinguished) or a little "blacker than black"
Click to expand...

I see, thanks!

Hugo Holden said:
... it could be due to a fault in the brightness control circuit feeding the grid of the CRT, or the video output stage, feeding the CRT's cathode.

... I'm not saying the CRT is definitely bad, but this would require a check with the scope on DC coupling x 10 probe, looking at the voltages on the grid & cathode of the CRT, to determine if the grid of the CRT is going positive, with respect to the cathode (or cathode more negative than the grid), which it never should be.

Generally the peak to peak video drive at the CRT's cathode from the video signal would be in the order of 30 to 40V and the signal voltage at the cathode should never be lower at any point than the grid voltage, because if the grid is positive with respect to the cathode, it draws grid current and defocusses the beam.
Click to expand...

Understood. I unfortunately don't have such a probe, but I will order one. (I'm just using the crappy 2 clip lead BNC connector "probe" for my ancient 70s scope) First I'm going to do what I can which is to measure the passives for deviations from the circuit and...

Hugo Holden said:
It would be interesting to put the scope on the collector of TR-20 to look at the nature of the video signal there. Some features of the image you posted suggests there might be a problem in the video amplifier.
Click to expand...

... this I can do, just to detect a signal. I have seen in my google searches that this chip goes bad sometimes: https://www.vogons.org/viewtopic.php?t=68626

Thanks, I will report back.
 
Yes, since with this design of monitor, it relies on the beam blanking from the video signal that is set up by that IC and passed to the video output stage, a plausible cause for the fault is a faulty IC. But also, if it was not that, the two transistors in the output stage ( cascode design) could be checked. The scope would show up right away if the output from the video amplifier was abnormal.
 
So I have the monitor reassembled but with the board exposed so I can probe it. Unfortunately in this setup I am not able to see a picture at all. I may have put something on wrong or a component failed in between dis- and re- assembly. So now I need to diagnose that before I can fix the video.

As for the lack of any video I suspect the flyback. Unfortunately I do not have an HV probe so I might need to get one.

If, as you said, the CRT was marginal it could be that. That would be highly unfortunate!
 
atg said:
So I have the monitor reassembled but with the board exposed so I can probe it. Unfortunately in this setup I am not able to see a picture at all. I may have put something on wrong or a component failed in between dis- and re- assembly. So now I need to diagnose that before I can fix the video.

As for the lack of any video I suspect the flyback. Unfortunately I do not have an HV probe so I might need to get one.

If, as you said, the CRT was marginal it could be that. That would be highly unfortunate!
Click to expand...

Well one thing making it look a little more like a video amplifier issue, than the CRT, was that the image, although defective, it appeared the CRT still had reasonable focus, which is a good sign for the CRT.

The first thing when it is running again, is to scope the video amplifier output and work backwards to the input.

There is no indication that anything is wrong with the EHT value, because the raster size was about right. So you don't really need an EHT probe ..yet.
 
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