Color Classic CRT geometry adjustment

[Action Retro]

Hey all!

I cant' for the life of me figure out how to fix this geometry issue I'm having. The bottom of the CC screen is slanted up on the left-hand side, even though the top appears to be straight to my eye.

Also, the image needs to be rotated a hair counterclockwise.

Attached is a picture of the screen. Anyone know how to adjust this?

 

[This Does Not Compute]

Fixing a rotated screen image involves physically turning the yoke assembly at the rear of the CRT...I don't think the circuitry from that era supported rotating the image in software. The procedure is documented on page 129 of the Color Classic service manual. Doing the adjustment requires more courage than I think I could muster.

 

[Action Retro]

Haha, maybe it's safer to just tilt my head slightly when I use it

 

[wottle]

I can't remember if there was much play in the mounting screws that hold the CRT to the front frame, but if possible, you could loosen the 4 corner screws and try to shift it in that direction a bit. Quite a bit safer than messing with the yoke (both to you and the glass nipple).

 

[This Does Not Compute]

I mean, it's not an impossible task, but the service manual assumes you're performing the adjustment with the system powered on (so you can watch the image change). One could instead do the adjustment iteratively, where you power down the machine, discharge the CRT, adjust it a little, then power back on and see how close you got. Much safer (assuming you're comfortable working with CRTs to begin with) but probably a lot more time-consuming if you have to take multiple attempts to get the image adjusted just right.

 

[Action Retro]

Haha, I'm definitely going to give it a shot.

 

[pfuentes69]

For me it's terrifying every time I see Adrian (Digital Basement) doing that with the bare hands, but apparently it's just 12V there.

 

[rikerjoe]

I’ve done the iteration method of yoke manipulation that Colin mentions with satisfactory results. It just takes time and patience. Likewise, I’m not as brave as Adrian grabbing the yoke on a live machine.

 

[GiGaBiTe]

For me it's terrifying every time I see Adrian (Digital Basement) doing that with the bare hands, but apparently it's just 12V there.

It can be a bit higher than 12v, but not more than 50v. The real problem is that it's high frequency AC in the dozens of kHz. The higher the switching frequency of AC, the easier it is to get bit by it at lower voltages, at least in my experience.

The safety factor is working on the equipment plugged into an isolation transformer. That way any potential shock is just going to be localized to your hand, instead of your hand to some grounded surface where it can go through your entire body.

Still, a CRT yoke isn't what I'd consider excessively spicy electrically wise. Whenever I have to adjust a CRT yoke, I just put on a thick PVC glove and make sure I don't get sharp pointy bits stabbing through it.

 

[JDW]

Not sure why I came across this thread today, but if you closely examine the photo in the opening post, you will see the displayed content is in fact NOT ROTATED as evidenced by the fact the left and right edges are perfectly straight up and down. If the entire screen was tilted, the left and right edges would not be straight up-and-down. It is only when the entire screen is rotated that you would ever twist the plastic Yoke Collar.

In other words, something else is causing the bottom left corner area to be raised too high (higher than the right side). My guess is the flat magnets on the back of the CRT, but that is only a guess. Would be great if @Action Retro could chime in about whether the problem was ever resolved, especially because Apple documentation makes no mention of that problem.

 

[chuck.dubuque]

I had that problem with my the monochrome green CRT from a dead Apple IIc monitor that I placed into my SE/30 MacEffects case. Had to twist the yoke for the overall tilt, I had to twist the magnets to fix issues with the deflection like "that corner that is too high/low". And you have to do this all with the screen powered and the computer presenting an image. The magnets seem to be designed to stay in place, so they are hard to twist, and when you let go they drift back towards where they were before. So you have to turn to slightly overshoot what looks good, and then hope when you let go it reverts to where you want it to stay. Rinse and repeat until you get the picture you want. No way to effectively do that with power off.

Usual CRT cautions apply. Don't touch anything else besides what you are adjusting. If you have to (my hands tend to wander/get fidgety), put your other hand in your back pocket to avoid ground across your heart. Make sure someone else is nearby if you can. Be deliberate and mindful and it should be non-eventful.

 

[JDW]

Unfortunately, the magnets used inside the Color Classic are not the same. They are flat and glued in place on the CRT. No twisting tweaks possible.