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I just got a Mac Plus off ebay which is in excellent condition with original boxes and manuals, but the screen will be on and then go blank. I tapped the top of the box and it came back on. But after awhile it goes off again. I opened it up and check the connections that I could find, but it didn't help. If I move the Power/Sweep board when the screen goes out, it will come back on. But after awhile, even moving the Power/Sweep board doesn't help. Any ideas?

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Hmmm. That's a tough one. I don't have any experience repairing those old CRTs, but please be careful in there!
I have been trying to be very careful. I downloaded the apple safety bulletin and repair manual before I started. At this point, I haven't had to do anything where I am touching it without a plastic handle screwdriver. The thing that bothers me is that when I discharge the crt (using a screwdriver and jumper wire with aligator clips) I don't hear a spark or snap. How do I know when it's safe? It would be nice to hear from someone who has experience working on these.
Although some people get quite wound up about the subject of the dangers of CRTs, the actual risk involved is pretty small. So long as you have the Mac switched off and unplugged whilst working inside it and you exercise a sensible level of caution around the HV circuitry, you should be fine.

There is a good article about this on LowEndMac, written by Prof. Tom Lee who is a very reliable source when it comes to these things.

I myself have had a CRT discharge shock off a 14" 1980's monitor I was working on and I would really only describe it as being uncomfortable. It was down to carelessness, I inadvertently touched a bit of the flyback circuitry and got the discharge down my arm, but it was very brief and I'm here to tell the tale! Needless to say I am now more careful when working around CRTs but I wouldn't say that the shock was by any means life-threatening and it caused no injury.

As for the issue regarding the dark screen on the Plus, I have had a similar problem with mine before and I actually found that replacing the PRAM battery sorted it. The Plus batteries are AA-sized 4.5V cells and I believe it is still possible to get them from some places. Alternatively you could build a clock battery out of three AA or AAA cells in series, as I did with mine. It's a good place to start but there may be something else going on, especially if tapping the case can affect the picture.
Thanks for the link. That's a good article. I'm going to check the pram battery first. Maybe the connection is bad or dirty on the battery holder. It's part of the power/sweep board. That may explain why moving it brought the screen back on. Thanks for the tip.
One thing to remember when working with electricity is to only use one hand. This prevents any accidental current from taking a path from one arm to the other and exposing your heart to the current. Put your left (assuming your right handed) hand in the back of your pants (trousers for the UK) while you investigate with the right.
I couldn't find a 4.5v battery, so I bought a 3 cell "AAA" battery holder from Radio Shack and connected it to the battery terminals, but it didn't help. I got another mac plus Sat. from ebay for parts, so I swapped the power/sweep board and then the monitor, but it didn't help. I'm pretty sure the mac plus I got Sat. is junk too. It looked like it had sat out in the rain for a couple of years. I guess I'll have to run across another one on Ebay. (that works!)
I had to order the 4.5v battery online. If they are not fixable, how about Macquariums?
From the looks of the last one I got, I think it already had fish in it.
It sounds like you're having quite a bit of bad luck with these Pluses. The trouble with machines of that era is that it is getting harder and harder to find working ones. I have one Plus at the moment but I might get a spare one if the chance arises, just in case.

I did however recently find an SE/30 that had been sitting out in the rain for at least two weeks and after opening it up and drying it out it powered up and worked fine. The original 40MB hard drive has since died (not too surprising) so I have replaced it. In damp conditions, the metal framework inside these compacts rusts very quickly as does the floppy drive mechanism. I think I caught my SE/30 in the nick of time, a few more weeks exposed to the harsh, Scottish weather and it would have been ruined.
Mine did this after 1 year in 1990!

It was so sensitive you could simply shout at the machine and it would work. It got progressively worse.
I took the cover off, and found that a very slight movement of the video board would bring the display back.
I eventually found a cold solder joint on a wire (not a chip) I resoldered it and biggity bam I was back in business.
Some guys on a vintage forum told me to check the solder joints on the video board. I ended up re-soldering a bunch of joints and eventually got the right one because it works perfect now. I'm glad I got it to work because this computer is in mint condition. It looks like it was in the box most of the time. Thanks for the input.

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