Bizarre CRT bloom / wobble / shadowing on... all of my compacts?

[s_pupp]

Bloom isn’t very noticeable at average brightness with retrace lines invisible. But every time I open a window or menu the horizontal geometry wiggles and jumps, and at above average brightness the shadowing presents itself.

I’ve read up more on bloom, could just be a worn out tube or flyback or a flaw in HV regulation. But I’ve now tested on five compacts, six CRTs. All have this identical issue. Only compacts. I’m baffled!

I think you have a good point regarding the possibility of a worn CRT. An aged CRT is definitely one thing all the compact macs have in common.

 

[chiptripper]

I think this is normal. Increasing the cutoff makes the screen brighter, but also larger.

At the point where retrace lines are visible, the picture content affects geometry.

Yeah, on my compacts, if cut off is too high, the screen is too bright and also bulges/increases in size. But it doesn't wobble.

I think you have a good point regarding the possibility of a worn CRT. An aged CRT is definitely one thing all the compact macs have in common.

Update: I tested a (new to me) clean, low-use Classic II. I didn't recap the analog board, just swapped in a recapped LB. It is much more stable than my other compacts. However, if I crank up the cutoff, it exhibits the same horizontal instability, just to a lesser degree. (Rock steady voltages during this testing.)

Conclusion: these computers just... do this. What feels like a comfortable brightness for my 38-year-old eyes is higher than the horizontal circuitry can handle. If cutoff / brightness is too high (but not high enough for refresh lines to be visible), high-contrast screen activity causes instability in the form of buckling and bowing at the left and right sides of the screen.

Maybe someone else here can perform this test on a compact and confirm?

The amount of instability really does seem to depend on how worn out the flyback and / or CRT is. Looks like I have 5 CRTs with a moderate degree of aging and wear. Wish I had a HV probe to test the voltage drop difference under the anode cap, but for now I'll just make peace with running them at lower brightness.

One thought: for analog boards I use low-ESR caps with identical capacitance, but often higher voltage. Seems practical to, say, buy ten 100uf 25v caps rather than a mix of 100uf caps at 16v, 20v, 25v. I know in most cases this is completely fine, but I do wonder if over-spec caps in the horizontal circuit could exaggerate this issue. That, I suppose, I could actually test.

 

[ymk]

Conclusion: these computers just... do this. What feels like a comfortable brightness for my 38-year-old eyes is higher than the horizontal circuitry can handle. If cutoff / brightness is too high (but not high enough for refresh lines to be visible), high-contrast screen activity causes instability in the form of buckling and bowing at the left and right sides of the screen.

I tend to run my tubes on the dim side, so this isn't something I've noticed before.

I'd check out the horizontal deflection with a scope. If the signal envelope mirrors the distortion, the source is likely the analog board.