Any other suspects other than the jungle IC?

berniedd · Guest

My 15-inch Mag CRT monitor (model D510) first started its symptoms with an intermittent loss of vertical sweep (single horizontal line across the screen) for only a second or two, then return to normal. Days later, new symptoms appeared: the picture would shrink to 2/3 of its original size, be off-center, and come with a decrease in brightness. Sometimes it would return to normal, sometimes it wouldn't. Although there is still good synchronization and the picture is still intact, it's getting such that I can't do much work with it. I suspect it's the jungle IC - STV7778. Before I replace it, is there anything else I should check? It's got 42 pins and is such a bear to work on.

Guest · Posted: Fri Mar 31, 2006 8:48 pm Post subject:

Have you tested the electrolytic capacitors? These dry up with age and cause noise on the supplies to the chips. Replace them with high temperature (105degC) low ESR ones.

FRick · Joined: 23 Feb 2006 Posts: 7

Base from previous observation of similar troubles (momentarily troubles resulting to permanent if continued used) is a often cause of a cold soldering or the leads on the pins of some parts was cracking and when heated during used, they will separate and causing the problem.

But its hard to figure out where this section is: You stated symptom - "first started its symptoms with an intermittent loss of vertical sweep (single horizontal line across the screen)" which is a good guide leading to vertical section.
Your second symptom - "Days later, new symptoms appeared: the picture would shrink to 2/3 of its original size, be off-center, and come with a decrease in brightness" which i assume it could be in the power supply section.
Try to find for some of crack soldering of pins on this two section or on some other section too, for you said "Sometimes it would return to normal, sometimes it wouldn't." which is a good sign of a loose connection cause by a cold soldering.

Good Luck...

frick

Guest · Posted: Sat May 13, 2006 11:40 pm Post subject:

Well, I soldered up all those dry joints but still no dice. I finally replaced the jungle IC. The brightness problem disappeared but the change in picture size remained. I can describe it as "the resolution changed by itself." I can make it fill the whole screen by adjusting the horizontal and vertical width controls. But the condition is intermittent, and sooner or later the picture would be too wide for the screen, and I would have to readjust the size controls again.

Guest · Posted: Sat May 13, 2006 11:50 pm Post subject:

I'm now suspecting the microcomputer chip. What do you guys think?

Guest · Posted: Tue May 16, 2006 6:02 pm Post subject:

I would still suggest capacitors, it does sound like a 'dry joint' but these can happen inside the capacitor itself. If you have the manual, scope all of the powersupplies, if you have an ESR meter, test all of the electrolytics. If you only have a meter try and test all of the electrolytics, or just try to 'wiggle' all of the electrolytics. They should move slightly in one direction only, if they move in both directions, replace. Maybe changing the faulty capacitor would have saved you the trouble of replacing the jungle or maybe it caused the problem with the jungle.
Is the B+ healthy?

Please let us know how you go. Smile

dolescle · Joined: 08 Nov 2006 Posts: 7

I'll have to agree with the post above you could try replacing the capacitors, some maybe leaky, I had this problem once, it was a capacitor on the vertical section, you could also check potentiometers near the horizontal and vertical sections, some of these components may corrode in time..

hotwaterwizard · Joined: 26 Nov 2004 Posts: 72

Here is the Datasheet for STV7778

http://www.eserviceinfo.com/download.php?fileid=26091
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