So I recently acquired a PET 4016, which I cleaned up and got working again. However, I'm running into an issue with getting the picture centered right on the CRT. It's not terribly obvious in the photo as it is in person I think, but it's way off center to the right and the edges are not at all straight. The text looks slightly crooked on bootup, and as I type across the screen it dips a little and then comes back up as it gets to the end of the line. Now what I don't know is if this is normal behavior for a PET or if I'm just blind and missing some obvious adjustment I should be making (I already lowered the vertical height and slightly rotated the deflection yoke which has improved but not fixed the issues). I'm by no means a CRT expert beyond the basics and safety precautions, so some suggestions would be very helpful.
It's a slight issue, not a huge factor in the functionality of the computer of course, but annoying enough to me that I'd like to fix it if possible.
Yeah, that's an elderly CRT. There's no one magic fix for this, but a few things you will do in combination. The good news is that it's monochrome so purity and convergence, the two most frustrating factors in adjusting a CRT, are nonexistent.
1. Yoke position, rotation and angle. Loosen the clamps, remove any rubber wedges, and screw around with it. This will affect rotation, trapezoid, and centering on the screen. Use the wedges to lock in the yoke angle once you find one you like better.
2. Meandering lines, wiggles, corners and other distortions are tuned out by taping or gluing little magnets or small pieces of a type of magnetic metal called mu-metal at various places on the tube, mostly near the neck but ahead of the yoke, to affect the deflection locally. Little refrigerator magnets, the kind made out of the classic black ceramic-like material work well for that. You might even find spots on the tube where these were these were present, but have fallen off after the adhesive crumbled to dust.
3. Components have changed value, causing the now 45-year-old factory tune to no longer be valid. Consider checking and/or replacing electrolytic capacitors in the power supply and deflection electronics, including on the neck board. Large 'pillow' film capacitors would be next most likely to degrade, and then resistors. It doesn't look that bad, though. If you do find yourself having to tilt the yoke so much to get the image to center that it's getting trapezoidal, capacitors are probably bad. I think I would check for obviously bad electrolytics and probably look deeper only if I can't get a satisfactory result out of yoke and magnet tuning. Capacitor ESR can increase too high even if it looks intact, so don't write them off as 'good' unless you've tested that, you may need to revisit if replacing visibly bulging caps didn't fix it. Also if the CRT changes geometry as it warms up, or if it pulses shakes or 'breathes', there's definitely bad capacitors.
This is fantastic information, thank you. I think the yoke is pretty centered now, and it's not doing any of the things you suggested would be caused by bad capacitors. It's really just the right side and bottom of the picture that are just not straight lines at this point.
I was able to generally see a change by holding some magnets up to the neck, but unfortunately I don't think I have any that will work as a permanent fix; they're either too weak or too strong. So I might have to order some, I'm just worried that I'll buy them and they'll have the same problem of not being the right strength.
