Disconnecting CRT from powerboard

[Quadraman]

I accidentally snapped the end of the yoke off a SE CRT recently but I can't get it disconnected from the power board. The red line with the suction cup at the CRT end is what I can't get disconnected. Do I disconnect it at the CRT or at the other end?

 

[tomlee59]

Could you post a pic? Your description is a bit confusing. The yoke is not connected to the suction-cup thingumabob. Rather, it's the assembly that surrounds the neck of the crt (hence the term "yoke"). The HV feeds the crt through a recessed button in the bell of the crt, and the thick wire (usually red, but sometimes other colors, like black) from the cup connects to the high-voltage ("flyback") transformer on the analog board.

Take a look at the pics in http://68kmla.org/files/classicmac2.pdf to see how the HV lead terminates in a spring-loaded clip. That will give you an idea of the mechanical operation you need to perform to extract the wire (assuming that this is your problem).

If the Mac was powered up in the recent past, you will likely want to discharge the crt prior to poking about, otherwise you will become the discharge path. The shock will not harm you, but it can be most unpleasant. Instructions for discharging it are also in the linked-to doc.

 

[Quadraman]

No pic. I snapped the end off with the prongs that connects to the video board. The CRT is useless now. I just need to disconnect that red wire so I can throw it out.

 

[Osgeld]

you can disconnect it from the crt, not sure about the board

there's a pair of pincers in that suction cup that keep it in place, feel around the rubber, you should be able to figure out its orientation, now squeeze that area and it should pop out

you could use tools, but beware, its easy to rip that rubber suction cup and if you wanted to reuse it (which i assume you don't by your post) it would be dangerous, but more importantly if there is still a charge in there it might make you shout a couple hell worthy words

 

[Scott Baret]

I usually use an insulated pair of pliers to remove my anode caps (the correct term for the "suction cup") from CRTs. I carefully use the pliers to pinch one of the two pincers inwards and then carefully pull up to remove it from the hole on the CRT.

As far as discharging--from my personal experience, you should have nothing to worry about if you are working on a Classic (I or II, either analog board), even if you just switched it off and pulled the power cord. They discharge as soon as the machine is switched off. I've never felt anything through my pliers or fingers when operating on a Classic. The same can be said about later SEs, although I've heard the earlier SEs didn't self-discharge. I've done CRT transplants on SEs from 1988-1991 and have never had a problem with discharge. My guess is that Apple added the self-discharge function when the analog board was redesigned to accommodate the new fans.

If the SE has a "rat cage" fan, it's probably a wise idea to discharge the CRT. This can also be said about any Plus or earlier model. Of course, if you wait a long time, the CRT will lose its charge as long as the Mac is unplugged. (I replaced a CRT on a Plus after it had sat idly for ten months; this machine had lost its charge in the CRT by then).

Granted, some folks do prefer to discharge every CRT regardless of computer model or length of time since the Mac was last plugged in. Most manuals will tell you to do this and so will those who always err on the side of caution. However, these discharge steps really aren't necessary with the newer compacts--I've worked on dozens of them without using the discharge procedure and have yet to be shocked.

(I should point out that I have yet to perform a Color Classic CRT replacement. If anyone has experience with these let me know since I want to replace a CRT in a Color Classic sometime soon and would like to know if I need to dust off the discharger).

 

[Quadraman]

Ok, one other thing. Can I remove the yoke from around the long end of the tube and save it for parts or is it permanently attached? There are places where it looks there are dollops of hot melt glue or some other sort of adhesive.

 

[Osgeld]

its glued on but removable, its glued on cause depending on how its twisted it will effect the orientation of the screen, once in the right place, plop

 

[Quadraman]

I got the yoke off now (it just needed a little twist once the screws were loosened to break the grip of the glue). The only thing I need to get off now is the metal band around the front of the tube that the screws go through to bolt the tube to the front of the case. It looks like there a clamp that slides back but what's a good tool to use for this? So far I've managed to not have this thing explode in my face or electrocute me and I want to keep being safe about this.

Another question, I have a Plus that stopped working a while ago. Is the tube the same in that model as the one in the SE? Does it matter if it's not?

 

[Osgeld]

dont know about the band, and as far as the vacuum you can safely release it through the little spout it was formed with

maybe someone can chime in on how to actually do it, because I never have

and its probably discharged by now

 

[ppuskari]

All I can say after watching this thread is that I hope I never have to remove a crt from any of my compact macs.

Learned a lot though!

Thanks guys.

 

[MacJunky]

Does your replacement CRT lack a band or something?

 

[Quadraman]

Does your replacement CRT lack a band or something?

I don't have a replacement CRT but I want to save the frame and attaching band in case I find one that doesn't have it. I don't want to throw away anything that could potentially be reused. I could smash the CRT to get it off because it's going in the trash anyway but that would probably be too dangerous.

 

[tomlee59]

Do not remove the band -- there's no non-destructive way to do it, and no real non-destructive way to reuse it, unfortunately.

 

[trag]

Not sure if you're already past this part...

The suction cup and red (usually red) cable to the side of the CRT is an integral part of the flyback transformer. If you wish to reuse your analog board and flyback transformer, do not damage that cable/connector/suction-cup.

(BTW, I can no longer find a source for new flyback transformers for the compact Macs. If anyone knows of a source, I'd love to hear about it. I don't need any now, but we don't want any new old stock of those old parts to end up at the recycler.)

As another poster wrote, there's a sort of spring clip inside a hole in the CRT under the suction cup. Typically, one very carefully inserts a flat head screw driver under the suction cup, slides it to the hole, uses it to press one side of the spring clip inward and then gently pull the suction cup up a little to get that side of the clip out of the hole. Repeat for other side of the clip and remove cable/suction-cup from CRT.

 

[wally]

...I want to save the frame and attaching band in case I find one that doesn't have it...

The metal band around the CRT is more than a means to secure the four corner mounting lugs. It is an implosion resisting band applied and tensioned by the CRT manufacturer to many hundreds of pounds of tension. It applies an inwards force especially at the four outer corners of the glass where the front plate meets with the funnel section. This force puts some weak spots in compression, an advantage since glass is stronger in compression rather than extension. It makes an implosion less likely, and in the event of one reduces the velocity of many of the fragments.

On an intact CRT with good vacuum, it would be unwise to remove it. On a broken CRT it still could be a bit hazardous to release the spring tension suddenly by cutting the band. On a larger 23 inch size CRT the recommended tension is 800 to 1500 pounds! 8-o Although there have been subsequent refinements, best I can tell the origins of the band and its technical functions are best described in US patent 3,350,154. Those with the interest and bandwidth can download it (943 KB PDF) from http://www.pat2pdf.org/

 

[stevep]

Does your replacement CRT lack a band or something?

I don't have a replacement CRT but I want to save the frame and attaching band in case I find one that doesn't have it. I don't want to throw away anything that could potentially be reused. I could smash the CRT to get it off because it's going in the trash anyway but that would probably be too dangerous.

That attaching band is built in as part of the CRT isn't it? I would think any replacement you get will have one included.

 

[Osgeld]

The metal band around the CRT is more than a means to secure the four corner mounting lugs. It is an implosion resisting band applied and tensioned by the CRT manufacturer to many hundreds of pounds of tension. It applies an inwards force especially at the four outer corners of the glass where the front plate meets with the funnel section. This force puts some weak spots in compression, an advantage since glass is stronger in compression rather than extension. It makes an implosion less likely, and in the event of one reduces the velocity of many of the fragments.

that's counter intuitive but makes since |)

 

[wally]

That attaching band is built in as part of the CRT isn't it? I would think any replacement you get will have one included.

Yep. Proper replacements come with the permanently attached implosion control band with mounting lugs.

 

[wally]

that's counter intuitive but makes since |)

Me too, you would think if something might break, the last thing you would want to do is put and leave any kind of additional unnecessary forces on it. But prestressed concrete is a somewhat similar kind of invention where internal cables compress a beam lengthwise. This is so that the beam sagging between supports lessens the net compressive force but does not cause the concrete to go into a stretch mode and pull any cement particles apart. Less compression is different strengthwise than outright stretching at least in concrete and in glass. I think the mechanisms for this are completely different. In concrete the adhesive bonds are weak but the sand and cement particles touch in compression. In glass I think the tiny surface scratches open up into big propagating cracks under tension, this is how glass is commonly cut.