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Old 19th Jul 2019, 10:07 pm   #10
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Commodore 1084-D2 Monitor Repair

What is happening here is that a narrow pulse is cutting off the CRT beam current of all the three guns at the same time.

So I will make some suggestions how this could happen.

Wherever this pulse is coming from you also know know it was affecting the pal decoder circuit too(because it caused a color phase error when you were feeding the monitor with composite video, of course that is now gone on an RGB signal).

Since that circuit is in a completely different section of the monitor, it now appears likely this pulse is on one or more of the the power supply rails.

To get the three guns of the CRT cut off together requires the pulse at the three cathodes all going positive at the same time. So that means this disturbance is affecting the RG& B channels together, so again likely the power supply to the video output stages, so as suggested by Refugee check the filter capacitor there.

Or the could be an abnormality in the V blanking signals that also are normally common to the three stages.

The other way the three beams could cut off together is if there was a negative pulse on the CRT grids (which are normally common as one control grid in the CRT's gun) . Sometimes, depending on the design, blanking pulses can be introduced there rather than into the video amplifiers.

It would be easy to find where this problem was, with the scope, working backwards from the CRT's cathodes (once the grid circuit was excluded) and finding where it was coming from, either injected into them as a faulty V blanking Signal, or a a large glitch in the power supply rail. To check for noises/pulses on the power supply rail with the scope simply set it on AC coupling so it ignores the large DC offset.
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