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Dead Mac 512k

MacMischief(iskra)

MacMischief(iskra)

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Joined
Jul 20, 2023
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32
Location
Molboro NJ
Hello vintage Mac friends! Yesterday I purchased myself this Mac 512k at a yard sale for $65. As we all know these machines are little toasters, so most of them dont work. I took I took it home and opened it up (please note that three of the five torques T15 screws where missing exept the screws under the handle). I checked it out and besides a little dust, all the caps looked very clean. When I powered it up, the speaker made a pumping noise and the floppy made a small and short vrrrr noise along with the speaker. There was no display at all. Today I realized that when hitting the side of the machine that had the power/analog board the machine began to chime and a couple of dots appear on the screen for a second. After awhile of putzing around inside with my multimeter (I can barley use a multimeter) while the computer was on And touching the voltage regulator, with the multimeter, and it sparking (I did this twice) the machine would chime on powerup, and the heater would glow. This, of course, was short lived, and it went back to its same shannanigans. Then after banging around on the board some more I realized that when moving the flyback transformer cable around the thing would chime and spin up the floppy every time. I left it to do this for about ten minutes to go write this message, and came back to hear the floppy drive making a weird grinding noise ( as shown in the video below) like it is trying to eject a disk. Going back to it again, it now does nothing(back to the same things as stated in the begging)even with me moving the transformer cable about. I really dont know whats wrong so I hope you great folks who know more then me do. Anyway I have included a video below. Thank you so much for reading and have a great week!
 
Sounds like typical analog board issues of bad solder joints from the extreme thermal stress. If you look closely at the solder joints, you may see a ring inside a ring, cracks in the joints or floating disconnected pins, which are obvious signs of overheated baked joints. Though some cracks can be microscopic.

Get a good soldering iron and reflow every single joint on the analog board with new leaded solder. You also may want to do the CRT neck board as well.

Once you do that, power the machine up again and see what changes. If the machine comes up, I'd recommend recapping the analog board and replacing the RIFA caps, since they're known to short and go bang, or sometimes go on fire. The only cap you shouldn't touch (unless there's a problem with it) is the 3.9/4.5uF horizontal deflection capacitor. These aren't made by anyone anymore, and require using a microwave film capacitor as a substitute.

https://tinkerdifferent.com/threads/128k-analog-board-logic-board-capacitor-list.824/ - The 128k and 512k are similar, the analog boards may differ a bit so use this as a guide and note any differences on your analog board.

If your machine doesn't come up after reflowing the solder joints, you get to delve into the wonderful world of analog electronics troubleshooting, and high voltage power supplies.
 
GiGaBiTe said:
Get a good soldering iron and reflow every single joint on the analog board with new leaded solder. You also may want to do the CRT neck board as well.
Click to expand...

Leaded solder? I didn't think that was appropriate for electronics.
 
oldpcguy said:
Leaded solder? I didn't think that was appropriate for electronics.
Click to expand...
63/37 Sn-Pb solder is approved for mission-critical stuff. Pb-free stuff is for the ordinary market and is far less forgiving. I think you may have that confused with acid-core solder, or even the 50/50 solder that was used in plumbing.
 
oldpcguy said:
Leaded solder? I didn't think that was appropriate for electronics.
Click to expand...

What makes you think that? All electronics from the dawn of the vacuum tube up until 2006 when the ROHS ban took place used almost exclusively some mixture of leaded solder. 60/40 or 63/37 were the most common formulas.

You do not want to mix ROHS solder with leaded solder, it creates a junk alloy that makes terrible solder joints. It's readily apparent in most cases when you mix the two, you'll end up with a crystalized mess that looks like a dull gray, instead of the normal sheen. It tends to be brittle and crumble apart.

MacMischief(iskra) said:
So you all think using lead solder is necessary or could I use the regular stuff?
Click to expand...

Leaded solder is a requirement, its what was originally used. Either 60/40 or 63/37 is fine. Don't mix leaded and ROHS solder, you'll have a bad time.
 
If you clean the RoHS solder off the joint and re-solder with Pb-Sn, joints should be fine. I don't know if it works the other way.

I confess that I laid in a big stock of 63-37 Kester a very long time ago (local electronics store going out of business), and have never had to resort to RoHS stuff, thank heavens.
 
Chuck(G) said:
63/37 Sn-Pb solder is approved for mission-critical stuff. Pb-free stuff is for the ordinary market and is far less forgiving. I think you may have that confused with acid-core solder, or even the 50/50 solder that was used in plumbing.
Click to expand...
You're right, I am confusing it with acid-core.
 
So I looked at the back or the analog board, and I saw no broken solder joints are there any other visual clues that could help me find the broken one?
 
MacMischief(iskra) said:
So I looked at the back or the analog board, and I saw no broken solder joints are there any other visual clues that could help me find the broken one?
Click to expand...

The joints generally get rings in them like these:
SebDLMCrack.jpg


Despite the arrow, all bottom three joints are cracked, the top ones aren't far behind.

Cracks can also be microscopic, and you won't see them at all, unless you have a scanning electron microscope.

Here's a SEM picture of a cracked joint, you'd never see this by eye.

iu


It's just best to do what I said earlier, reflow every single joint on the analog board.
 
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