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Mac Plus CRT/Analog board distortion.

mac

New member
I recently liberated my old Mac Plus from my parent's garage where it had been sitting for probably almost 20 years. The story of how I got it is fun. When I was a young teenager, maybe 13, I went to a little flea market event that a local community college near me used to hold every year. A young woman there was selling her old Mac Plus, in the carrying case, with everything included. I bought it for something like $15 or $20, and she told me something like "this got me through college, and I hope maybe it gets you through college too." I used it all the time after that, but when I was an adult and moved out, somehow it ended up in my parent's garage, where it stayed for the next 20 or so years. I could never find it, and had been looking for it for years. Anyway, one day my parents finally brought it over after finding it in their garage.
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So I started it up, and initially it had the problem of the screen cutting out occasionally. So I opened it up and resoldered a bunch of cracked solder joints mainly in the connector to the motherboard and the other major connectors. There were also some other random joints that were cracked which I also resoldered. I bought a Hakko FX-888D just for this (I didn't want to use a crappy iron on something so important), and it worked great. That solved the initial problems, but there's some distortion in the image.

The top of the image is bowed down in the middle, and the right side of the image at the top is compressed/shifted to the left. Interestingly, the image itself doesn't seem to be very distorted, it's just kindof the wrong shape/not square. I measured the image, and it's about 6.75 inches tall and 4.5 inches wide, which is a little compressed compared to the 7.125 x 4.75 that the Dead Mac Scrolls lists.

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I don't remember it having this distortion when I was a teenager, and it did sit in a garage for 20 years. But then again you never know, maybe I just didn't notice or ignored it at the time.

The analog board visually looked to be in pretty good condition, but there were a couple of interesting things. C15 or C16 has some kind of white ring at the top, maybe some kind of burn? C36 or C35 was bulging ever so slightly, but not too bad. The other caps looked fine visually. Interestingly there are no rifa caps. Instead there's some big old plasticy thing, like C32. I think there might have been some work done on the board at some point because all the solder joints for the flyback connector appear to have been resoldered: there's a TON of old flux around those joints. Likewise, there are one or two solder joints here or there that look like they'd been manually resoldered at some point, mainly down near the power supply stuff, but I think it was only like 3 or 4 solder joints total, maybe. However, the static shield was still attached to the analog board via two sided tape, and there was no evidence of old two sided take that had been removed (it seemed like the original two sided tape?). So possibly this work was done at Apple? I don't really feel like taking it apart again right now to get some pictures, but I will do so in the coming days.

According to the Dead Mac Scrolls, this kind of distortion is a result of "loose yoke magnets" and the solution is to "adjust the loose magnets." How do you tighten the yoke magnets though? Or do I just need to adjust them? I've never heard of anyone "tightening" a loose magnets before, nor have I heard of "loose yoke magnets." What does "loose" yoke magnets mean exactly? I'm happy to do this, I just don't understand exactly how to do this. If it's just regular yoke magnet adjustment, then it seems odd to list it as "loose yoke magnets." That would seem to indicate I should tighten them somehow or something. But maybe it just means to adjust them. Interestingly, the CRT component inside looked as thought it had never been touched before. It was a real pain to get a screwdriver under the yoke cup thing, you could tell that part of the CRT and yoke had never been touched by anything, it looked brand new.

The dead mac scrolls also mention C1 if the display is distorted horizontally, but actually the image isn't really that distorted (oddly enough). The shape is wrong, but the image itself seems weirdly accurate. And this doesn't explain the bowing at the middle of the top. Likewise, the zener diode CR15 is mentioned if the display is severely distorted, but I don't think the distortion is that bad. I can always replace these anyway just in case, but I get the feeling the problem is the "yoke magnets" being "loose" (at least according to the Dead Mac Scrolls).

I have the caps to replace all the caps in the system if necessary, and I'd be happy to replace any other components, it's just a matter of what to do. Any ideas or suggestions? How do you "tighten the yoke magnets?"

Here are a few more closeups of the distortion:

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bdurbrow

Well-known member
DISCLAIMER: This is just based on my general knowledge of electronics; I have no personal experience with fixing this on a Mac Plus. Hopefully somebody else with actual experience will make a post.

I think that what they mean is that the yoke magnets have loosened over time, and that during handling (or just due to gravity) they have shifted slightly, resulting in distortion.

After discharging the CRT, check the magnets for looseness. If loose, use something non-conductive to adjust them while the machine is on, so you can see the results. As you are aware, this is a high voltage environment when running, so take precautions (nonconductive shoes, plastic chair, work with one hand, etc). Based on online photos; the magnets for this particular CRT seem to be attached to the yoke such that they can spin, and are held in place with some sort of glue; it's possible that that glue has degraded, allowing one or more magnet to shift. After adjustment, a dab of hot glue should work to hold them in place again; yet be easily removed should the need arise.

Centering requires adjustment of the other set of yoke magnets.

Your description of the capacitors on the boards indicates to me that the machine is due for a total capacitor replacement; and considering it's age that's not surprising. Bad capacitors on the analog board can also cause CRT distortion.

There are some adjustment tunable inductors on the analog board that affect image size; if you care, you could adjust those to bring the size into spec.

This PDF has some instructions on how to handle the issues:


Note again, that I've not actually done any of this myself; it's just based on general gathered electronics knowledge.

Hope that helps!
 

bdurbrow

Well-known member
That’s odd… they were there when I read the post sometime around midnight (it’s 7am-ish now). I didn’t see anything bad in them either (exposed credit card numbers, stuff in reflections, etc… just a Mac). Huh.
 

mac

New member
Hey sorry about that, I accidentally left some personal information in the metadata in them. Here they are:

Mac1 - Copy.jpg

Mac2 - Copy.jpg

Mac3 - Copy.jpg

Mac4 - Copy.jpg
 

dkjones96

Member
Is it parallel on the left but not the right or is the whole image turned? If the whole image is turned you can correct that by loosening the yoke clamp on the tube neck and turning it a bit. It might be sticky after all these years but it'll come loose. Not sure why but pretty much all of my compact macs have needed a bit of yoke rotation.

The magnets it mentions adjusting are for geometry issues and unless some of them are loose you shouldn't need to touch them. If the white paint they put on them is still there it shouldn't need to have them adjusted. They will be the small tabs at the yoke next to the clamp I talked about earlier.
 

mac

New member
As far as I can tell, it seems to be correct on the left. On the bottom right it seems fine, but as the right side goes up higher, it increasingly goes to the left. If it is tilted a bit, I think that might be separate from the compression on the right side. The left side seems very even to me. I admit the picture maybe looks like it's tilted a bit, but I think that's mainly an illusion from the top being bowed and the right being compressed as it goes up higher. If you look at the left side of the image in the picture, it seems to maintain a fairly consistent sized gap between the image and the edge of the screen. Whereas the right side goes in further and further as the image continues up from the bottom. And likewise the top of the image is sagging down or bowing down a bit.
 

mac

New member
Thanks for the feedback so far everybody!

So far, I'm thinking I'll test the magnets for loose-ness, and if any are loose I'll try adjusting them. It'll be delicate work if any are loose given the high voltage nature of the whole thing. However, before that I might try replacing the capacitors just in case. I have all the capacitors to replace everything, so that should be no problem.

When I open everything up again, I'll post back here with some pictures of everything. Maybe you guys will see something I missed!
 

dkjones96

Member
Only the red wire to the anode has high voltage on it. The deflection coils in the yoke all run on about 30V so as long as you don't mess with the big red wire the yoke is relatively safe.
 
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