Flyback Swap

[karmaman]

I know this isn't really about TV's, but I was hoping I could get everyone's advice on doing an FBT swap for an old iMac I have. It's the first model ever released, quality-built in the US, and they would be near-indestructible if it wasn't for a glaring design flaw: the placement of the FBT. Instead of being placed next to the tube, the chassis is mounted vertically next to the tube and the flyback hangs over the tube, making it run extremely hot. About 3-4 years after the iMac had been released, reports of iMacs with dead FBT's started circulating the internet, and now that these machines are considered vintage, It's a well-known fact that they are notorius for having bad Flybacks (they overheat, the insulation on the wires inside melts, and they short). I've got a Revision A iMac, manufactured September 1998. It's in great shape except for the FBT, after it's been on for 10-30 minutes, the screens starts flickering every few minutes and you hear a quick zap from inside. It bugs me when I use it to know that it's only a matter of time before the FBT completely shorts and the iMac refuses to start up (the classic symptom of a dead FBT). I have found a replacement fly (LG #: 6174Z-1003G) online for $30 and a page with detailed instructions on doing the swap (http://www.macopz.com/columns/imacrepair/) and feel fairly confident about doing the swap, the only part that gets me is discharging the tube. Basically stick an insulated screwdriver under the cap, and make sure the screwdriver is connected to ground, right? When I google it, all I get is idiots saying "zomg teh flyback is very dangerous why risk ur life over sum old computer go buy a new one flybacks can kill u". I know it's not true, but I just wanted to check with the pros. Can you guys tell me how to do it? Also, a lot of websites recommend connecting the screwdriver to the ground pin of the power connector on the iMac itself, so when the CRT discharges all that current goes to the ground on the chassis, and to do it without having the iMac plugged in.... wouldn't that screw everything up? Well anyway if this doesn't belong here, please feel free to delete. Thanks!

[radiotvnut]

I always discharge CRT's by placing about a 1 megohm resistor in series with the HV anode and the CRT ground strap. Just connect one end of the resistor to a good insulated screwdriver and the other end of the resistor to ground. Slide the screwdriver under the anode cap. You may have to do this several times to completely discharge the CRT.

[MRX37]

http://www.repairfaq.org/sam/crtfaq.htm#crtsdc

This should help. If not that article, then something on that site should help you.

[ChrisW6ATV]

I just use a "clip lead" (a wire with alligator clips on each end) with one end on the chassis (make sure you pick a part that is connected to the CRT ground, or just clip to the CRT's ground strap/connection) and the other end on the screwdriver shaft. Yes, obviously an insulated-handle screwdriver. I slide the screwdriver tip under the HV cap until it touches the metal clip. I also use the same grounded-screwdriver assembly to go ahead then and unclip the HV cap from the CRT. Once you did the discharge, it should be OK to lift the edge of the rubber/plastic cap to see how to unclip the wire. Once you have unclipped it, do not touch the anode terminal of the CRT, as it will "self-recharge" to some degree. Discharge/ground it again just before you reinstall the HV wire again after your repair.

I should also point out that a charged CRT, especially on a small tube as in an Imac, probably does not have nearly enough capacity to "kill anyone". I got zapped from a plugged-in and live HV cable on a 23" B&W monitor chassis once. It felt like I got hit by a truck, but it was not fatal. HOWEVER... Do not take this comment to mean in any way that such an activity is safe at all, and do not try this (touching a live flyback output) at home, or at work either for that matter!

[zenith2134]

The HV is probably as high as 25kv, but probably less (15" screen, right?) This of course is extremely low current, in mA's so likely just a big surprise but not death. Can NEVER be too careful, however.

iMac's were extremely nice when they were first introduced, and like most pc monitors, the fly is the first thing to go. Sucks that many get trashed as a whole because the monitor breaks down. Good luck.

[andy]

You don't have to worry about the HV as long as you discharge it like people said. I've repaired a lot of imacs and getting to the flyback is by far the hardest part.

So, imacs are becoming collectable? I still see these being recycled regularly. I lost interest when their value dropped below what it cost to ship them.

[site123a]

If you don't have wired alligator clips you can use two long screwdrivers instead, just put the flat one under the anode and the other on the CRT ground, then touch the two together. Hold the two screwdrivers together for 15 seconds then put them out...

[karmaman]

Thanks everyone, I feel a lot more confident about doing the swap now. I fired up the iMac today and the tube is very bright and colorful, it's a little soft but when running at 640x480 it looks pretty good (I use this machine for old games, and most them run best at 640x480). The tube is surprisingly an LG, I was under the impression that they made junk but this tube has been through a lot (the iMac belonged to a graphics studio for 6 years) and it's held up pretty well. I'll order the FBT on Sunday and let you guys know how it goes. Just two things: Where is the ground strap on the CRT, and does LG make junk? I bought a DVD drive from them that was DOA but how good are they in terms of Tubes? Also, has the quality changed since their GoldStar days?

[MRX37]

I dunno if LG makes junk per se. Not enough experience with the brand.

You might want to seriously check out that link I posted. repairfaq.com goes into great detail about CRT and other electronics repairs.

[karmaman]

@MRX37: I forgot to mention in my last post, I've read the repairfaq at least twice, it taught me a ton about CRT's, great resource, it's been a while since I read it though I'll probably read through it again tonight.

A really weird habit I have (that started after I read that repairfaq, which in turn sparked my curiousity about CRTs) is that I can't seem to stop drawing picture tubes on my Geometry notes. The teacher will start droning on about theorems and it just happens. First I draw the shape of the tube, then the yoke, then the neck board, then the flyback suction cup, then the chassis under the tube... happens in English class and French too...

[MRX37]

Ah. Seems like you got CRT's on the brain, kinda like the guy in Eric H's avatar.

[radiotvnut]

The ground strap will either be some sort of spring or a band of braided wire that makes contact with the aquadag coating on the tube. There will be a wire soldered to this spring or braided wire that connects to the CRT neck board, the chassis, or both.

[ChrisW6ATV]

I used to draw San Francisco cable cars on my notes in school, and now I live near there...

The "ground strap" would refer to any bare wire or metal bracket(s) that make contact with the coated back surface of the CRT, and other metal that those items are connected to.

The company that is now LG has "made junk" at one time or another, as have many other big electronics companies. Their old stuff from the Goldstar days was mostly mediocre at best, as was much of the electronics from Japan at one time, and Japanese cars as well... We all know what happened later. LG is now a leader in many fields (such as ATSC digital TV, which they bought along with the rest of Zenith), and so is Samsung, for example. Samsung was a low-end Korean company at one time (though they made a cool part-tube CB radio called the Dak in the 1980's), and now their LCD panels are the best in the industry.

[karmaman]

So when I discharge the tube to the ground strap, should the iMac be plugged in?

[radiotvnut]

No, the device should be unplugged from the AC outlet.