Weird CRT thing I'm not sure what it's caused by

[cstine]

I have an otherwise pristine Apple Studio Display that's got a weird thing going on, and I'm trying to figure out if this is a thing I could deal with, or a find-someone-comfortable-with-high-voltage thing because I'm entirely not.

It's making a click, then the image is expanding, then slowly shrinks back to normal, repeat. Took a video of what was going on, and it's here: https://tube.uncomfortable.business/w/kmPymiao9VRqQMVtU49FZD

Since everything else is fine (it's clear, focused, bright and such) it gives me the maybe-you-could-fix-it vibe, but I'm way over my head, so here I am.

 

[GiGaBiTe]

Clicking or popping with image expansion like that probably means the insulation inside the flyback transformer is breaking down and allowing internal arcing. If you can see the flyback transformer, look for cracks in the casing or signs that its been hot, like discoloration.

 

[retrogear]

The neck board at the focus pin will have a spark gap so if the high voltage gets too high it will arc across the gap just like a spark plug. Yes it could be the flyback BUT
dust can collect in the spark gap and cause an arc with normal HV. You can pull the neck board off without worrying about discharging the HV on the tube then blow
some forced air at the socket and might get lucky. I serviced CRT's for a living for 25 years.

Larry G

 

[daver2]

I agree. Go with the simple things first and make all of the dust and fluff homeless first.

Use a hoover (on suck) first and a can of 'air' to dislodge any stubborn stuff. Just be gentle not to damage anything.

Removing the case of the monitor, and observing it in the dark - taking every precaution not to get too close and touch anything, may reveal where the arcing is occurring.

Dave

 

[Hugo Holden]

It's making a click, then the image is expanding, then slowly shrinks back to normal, repeat. Took a video of what was going on, and it's here: https://tube.uncomfortable.business/w/kmPymiao9VRqQMVtU49FZD

That is not what is happening on the Video. The image is not expanding, it is shrinking with a reset that puts it back to the larger size which is corresponding to a lower EHT voltage (final anode vlotage) on the CRT.

The entire raster scan (both on its horizontal and vertical axis) starts out over scanned (larger, suggesting the EHT is too low) then with time it shrinks proportionally (EHT increasing) and then there is a reset (likely an electrical discharge if that moment seems synchronous with the click) and the EHT goes back to a lower value and the larger raster scan.

The reason the raster gets smaller with increasing EHT is that the size of the scan ((both on the H and V axis) is inversely proportional to the square root of the EHT voltage. This is the relation for magnetic deflection. .However with electrostatically deflected tubes, such as those on scopes, thhe size is directly inversely proportional to the EHT voltage.

In any case, the value of the EHT is oscillating between two states in a manner reminiscent of a relation oscillator. Firstly though, after powering on the EHT should be stable and not increasing slowly. That needs an explanation in itself. It might be that as it increases beyond a certain voltage, one of the spark gaps is arcing over, or even an arc from the the external anode connection to the external aquadag, discharging the EHT to a lower value again. It is odd that later it stabilizes out and the oscillation appears to stop.

In most VDU's though, the EHT value is sensed and there is a shutdown circuit if it gets too high, so as to generate x-rays, but normally with that circuit, it doesn't oscillate because a latch deploys cutting off the drive to the EHT generator.

I agree with the remarks though, the first thing is to clean all of the dust out and clean around the anode cap area and see if that helps.

One other potential explanation, the DC power rail to BOTH the horizontal and vertical scan output stages may not be stable, and increasing with time after power up, but that is less likely as a scenario, because, normally that does not have a significant effect on the vertical scan output stage, so the picture would tend to shrink in size more on the horizontal than vertical axis, but it appears about equal in this case, suggesting an EHT issue.

 

[andy]

I have seen that type of behavior when there is arcing from under the anode cap. You need to remove the anode cap, and thoroughly clean the rubber cap, and the CRT glass in that area. Before reconnecting it, it's not a bad idea to apply a thin coating of silicone dielectric grease to the underside of the anode cap. It's available from auto parts stores for use on spark plug wires, and helps prevent arcing.