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Mac Plus w/ intermittent video (tapping the case "helps")

fluxsmoke

Member
I recently acquired a very nice Macintosh Plus, but when I booted it this morning, the screen didn't display anything. When I tapped the side of the case, picture appeared, and after a little more tapping it stayed on. It booted up as normal otherwise.

I had just opened it up and removed the internal floppy, which was malfunctioning (I think it needs to be cleaned & lubricated). To do that, I removed the motherboard, but I believe I reinstalled it properly. I can't see anything inside that looks sketchy or loose; any suggestions?
 

desertrout

Well-known member
Hi! Welcome and congrats on the new Plus!

That symptom is a classic case of dry / cracked solder joints on the J1 connector on the analog board. Those and many other joints have a tendency to crack over time, causing intermittent connectivity.

If you're remotely comfortable with a soldering iron, it's an easy fix - merely remove the old solder on those joints and replace it with new solder. It's recommended that you do a close visual inspection of the backside of the analog board for cracked joints, but it's usually isolated to J1, J4, and the flyback.

Larry Pina's book (http://www.maccaps.com/MacCaps/Repair_books_files/Macintosh Repair and Upgrade Secrets.pdf) is a great resource for typical troubleshooting and repair procedures.

And make sure to ask more questions and keep us updated!
 

fluxsmoke

Member
Thanks for the blazing-fast reply! It's funny, I took another crack at searching for the problem a few minutes after I posted and came up with a PDF that suggested the same thing: http://www.maccaps.com/MacCaps/DIY_Information_files/Classic Mac Repair Notes .pdf

I'm quite comfortable with a soldering iron (see username, I guess), so I'll take a shot at that today if I get time.

I've previously seen credible articles online that say the anode is well-insulated, so if you're not going to fool around disconnecting it, you shouldn't bother discharging the tube--and that PDF I found backs this up. If I re-flow the solder with the analog board still in place, are there any points on the back of the board that I need to be especially cautious of? My intent is to touch only the connectors in question, but no plan survives contact with the enemy...
 

desertrout

Well-known member
I figured with that name you'd probably be fine. ;)

Honestly for the minimal effort required to discharge and remove the anode and the safety benefits, just do it. As you rightly say, mistakes happen all too easily. Besides, two of the pins on J1 are part of the horizontal circuit, as are most of the components on that top part of the board, and that's where all the high voltage action is.

Reflowing / working on the board in place is easy, just put the Mac on its side.
 

cheesestraws

Well-known member
Yes, do discharge the CRT anyway. The risk/effort tradeoff is intensely favourable...

Are you looking at the US or International/240V LB? At least on my 240V ABs, there's a cluster of capacitors around C38 that seem to maintain mains voltage for longer than one might expect after unplugging: I always explicitly discharge those with a screwdriver when I work on the board.
 

fluxsmoke

Member
Here's a fun one: if I turn it on, sitting upright, the screen is off. If I pick it up and rotate it about 45 degrees to the left or to the right, the screen turns on. This is extremely reliable (and kind of funny)
 
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