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What's wrong with my 5151?

maxtherabbit

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First start it looks perfect, after running for anywhere from 30 seconds to a few minutes the horizontal geometry goes cattywampus and the raster starts quivering. Please take a look at the youtube video.

I've tried:
full recap
deoxit on all pots and the horizontal linearity adjustable inductor
using a different display on the MDA to make sure the incoming sync is still good
 

tonata

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France
Hi,

I think it could be the environment. Do you have something close to the monitor that could produce magnetic interference? Have you tested in another room? Maybe even the computer case can produce some interference.

Also temperature is a factor. Maybe it is not heated correctly or you are testing at very low temperature. My tests at 0 degrees (C) do not go well.

If you find that one of the pots (inside the case) can correct it even temporally then you will know that it could be the pot itself.
 
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maxtherabbit

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Hi,

I think it could be the environment. Do you have something close to the monitor that could produce magnetic interference? Have you tested in another room? Maybe even the computer case can produce some interference.

It's not. Use conditions have been the exact same for several years and the problem developed recently. It has also been getting steadily worse, used to take 20+ minutes of operation to occur
 

maxtherabbit

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Hard to say--I wonder if there's a small arc developing on the HV side of the flyback transformer.

I certainly hope not. The raster never really collapses like it would with a FBT failure. Just shakes a tiny bit at the edges
 

soviet9922

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Have you try to clean the internal adjustment pots for screen size ?.
Also if is not that the powersupply in the monitor may have an issue and the voltage is not estable.
 

maxtherabbit

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Have you try to clean the internal adjustment pots for screen size ?.
Also if is not that the powersupply in the monitor may have an issue and the voltage is not estable.

Yes all pots have been cleaned.

I might have a clue about the power though. The B+ voltage is 14.7 when the monitor first comes on and is working, but it jumps up to 16V when the problem occurs. Replace the 7815?
 
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maxtherabbit

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Well replacing the 7815 made no difference, I guess I'll try the MJ2955 power transistor next :confused:
 

Chuck(G)

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Transistor failures are usually all or nothing--and the 2955/MJ2955/2N2955 is a good sturdy old-school PNP part, but anything is possible.

Does the brightness change when the thing starts misbehaving?
 

maxtherabbit

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If it is an arc in the FBT like you speculated it's not the end of the line. They are available NOS for cheap still somehow
 

maxtherabbit

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Chuck you think the FBT is worth a shot? I can get a 5 pack of them for $25. Starting to get pretty annoyed with this monitor
 

maxtherabbit

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It's as good a shot as anything. Do you own a 'scope? It might be worthwhile probing around a bit.

I do, but my understanding of these types of analog circuits is limited. I'm not sure what to be looking for exactly at this point
 

maxtherabbit

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Well my 5 pack of flybacks came in. They are aftermarket. The one I tried works fine, but it didn't fix the issue
 

Agent Orange

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believe it or not it was the MJ2955 all along

Good for you - glad it's working. Question: How did you check the 2955? Back in my Navy bench repair days, the P/S was the first thing that received attention because they were so prone to fail. I used to look for high AC on the can (collector) using a VOM or the scope if it was handy.
 

maxtherabbit

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I was getting an unusual voltage jump up to 16V at the collector that corresponded to the display becoming distorted after running for a short while. Nominally it should be at 15V since it parallels the 7815. At first I suspected the 7815 wasn't regulating properly and replaced it but that had no effect. Now it is solid

20210310_123646.jpg
 
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