Bringing a Mac Plus back from the dead!

[JNizzle]

Hello people!

I recently acquired a Mac Plus while doing a clean out of my grandfather's old factory. The keyboard and mouse were FUBAR, as they had been left under an old can of paint which leaked all over them. He said that there was also a hard drive somewhere, but we couldn't find it.

I scrubbed as much old paint of this Mac as I could, and besides from a rather even coat of yellowing, it seems to be in pretty good nick.



I took it home and plugged it in, and it made the start up bong and the floppy drive made noises, but the screen never fired up.

I cracked open the case and besides some dust, everything seemed to be in relatively good shape. No caps were leaking, and nothing looked corroded.

I also discovered that this Mac had 4Mb of RAM. Nice!

I came to the realisation that this Mac had deeper problems. I noticed when I found the Mac, that it had LOTS of screen burning, which indicated to me that the CRT as a whole could have just bit the dust, or that the flyback transformer was completely screwed.

Further poking around inside the machine also revealed that the CRT power connector looked like it was burned or melted.



I thought while I had it open, I'd perform the cap change just in case, for longevity if I ever got this thing working. I couldn't find all the correct caps, so I used some higher volt ones, and there was one blue non-polar one that I didn't change at all.

Upon firing this up after the change, it worked! I have no idea why the cap change fixed it, non of the caps seemed damaged in the slightest, I verified this when I removed them. No residue or anything!



So now I have a semi working Mac, but I have 2 issues which I'm unsure of how to fix. The computer makes a REALLY REALLY high pitched squealing noise. I think this may be the flyback transformer, but I'm not sure. UPDATE: When I fired up the machine to take the photos in this post, the noise was gone! I'm unsure if it will come back, has anyone else had this problem?

It's also VERY hard to insert floppies into the drive. The seem to be stuck on something, I'm not sure what though (I've made sure it's aligned correctly). Also, sometimes when I put them in, the jitter around in my hand like crazy until whatever mechanism in there stops, and I can finally insert the disk! And on top of that, the auto eject doesn't seem to be working, but I'm not that familiar with macs that boot directly from disk, as my other mac (a mac classic) is currently not working AND it has a hard drive.

I also have no boot disk to test this machine further. Let me know what you guys think!

 

[JNizzle]

As I've been turning this Mac off and on throughout the day to test how my repairs have been holding up, I've noticed another weird thing.

There is no startup bong and the screen does this stripy half-on kinda thing, where the top of the screen just isn't working and the other half is showing horizontal stripes.

Tapping on the side of the Mac restores the picture and I hear the healthy startup bong. I've opened the Mac up again to see if anything is screwing up.

This is an odd case!

 

[tjjq44]

Bad solder joints on the analog board! Re-melt them (especially the flyback and connectors). For floppy, you'll have to disassemble the drive, there are several tutorials about that

 

[JNizzle]

Alright, so I resoldered all the contacts around the flyback transformer, and I resoldered the area around that burned-out CRT power plug that I mentioned, because it was covered in a dusty black residue.

I'm not sure if this fixed my problem though. I seem to be getting a more solid distribution of horizontal white lines, but my computer hasn't booted since I resoldered. Any ideas?

I also haven't heard that whistling in a while, so that may have fixed itself.

 

[JNizzle]

Okay, now some deep dark stuff is going on. I plugged it in, and it's fuse blew in half.

It tripped the whole house, everything went dark.

I can't do any more testing until I find a new fuse, but this has me thinking, is there just some potentiometer I just need to adjust?

 

[LOOM]

That reminds me of this thread: https://68kmla.org/forums/index.php?/topic/22511-mac-plus-tripping-home-electric-when-switched-on/

Recapping C33 and C36 fixed that issue for me (it tripped the fuse at home). When I opened it those two were cracked open. You probably want to order new ones, and order the correct capacitors with the original voltage for those you didn't have the correct value of at the same time  :beige:

 

[JNizzle]

That reminds me of this thread: https://68kmla.org/forums/index.php?/topic/22511-mac-plus-tripping-home-electric-when-switched-on/

Recapping C33 and C36 fixed that issue for me (it tripped the fuse at home). When I opened it those two were cracked open. You probably want to order new ones, and order the correct capacitors with the original voltage for those you didn't have the correct value of at the same time  :beige:

Okay, noted. I will replace those caps as I just checked, and they do look a little old and cracked.

Also, I was under the impression that any capacitor could be replaced by one with the same capacitance, but any HIGHER voltage, as the voltage on a capacitor is only it's minimum voltage.

Correct me if I'm wrong.

 

[IlikeTech]

This is true, however, don't go too high, because the caps can have trouble staying formed.

 

[JNizzle]

This is true, however, don't go too high, because the caps can have trouble staying formed.

Yeah I think I've chosen caps that are a little too high for what was originally on the AB, but I'll swap them out later. I swapped the purple 100uF 200v capacitors out for 100uF 400v ones.

Also, you mentioned that the caps might have trouble staying "formed". What exactly does this mean? I tried googling it but I'm not getting clear stuff or I'm just getting walls of text.

 

[bibilit]

I don't think this screen is the result of any capacitor.

probably something going on the Logic Board side (memory maybe)

remove the Logic board and check anything obvious, if a simm is faulty (if present) can be your problem here.

 

[apm]

The stripes are definitely a logic board problem. If tapping it fixes it, then it's probably bad connection. I'd look at the +5V and ground lines from the analog board to the logic board. It's hard to think what else would restore both the chime and the video simultaneously.

Really dumb question, but did you make sure to replace the insulating sheets on the back of the analog board and underneath the logic board when you recapped it? Shorting the analog board to the conductive inside of the case would be a good way to blow something up spectacularly!

 

[IlikeTech]

I don't know.  400 volt might not be too high. I heard that forming is a problem, but take it with a grain of salt because I only have a vague idea of what it means.

 

[JNizzle]

Alright, thanks for the help everyone.

I bought some more caps to replace the two film ones on the AB, they're not identical, but they're electrically the same. I bought some fuses as well.

When I get home I'll fix it all up, and I'll try re-seating the cables to the logic board. I'll also try re-seating the RAM.

 

[JNizzle]

Alright people, I'm still having issues with the mac.

I replaced the two film capacitors with two caps of the same rating and voltage, the dude at Radio Parts (aussie electro store) said they should do the trick.

I soldered them in and replaced the fuse with an identical one.

However, I didn't even get to turn the mac on before it tripped my house. I plugged the cable into the wall, plugged the other end in my mac, switched it on at the power point, and everything went dark!

I think I may have a short somewhere, but I'm unsure.

 

[IlikeTech]

Yea, you seem to have a short before the switch

 

[sigtau]

Unplug it, discharge all of the caps, then check for multimeter continuity between ground on the analog board and the live/neutral prongs of the AC input.  It sounds like somehow those two got shorted together, because the safety features of your breaker are kicking in when electricity returns to ground instead of the neutral line like it should.

I don't have the Plus board in front of me so I couldn't tell you where to check, but if you do this, you can at least confirm that's the issue.

 

[tjjq44]

maybe you shorted something when you remelted solder... It worked a little bit the first time before it trip your house but maybe the short made a component fail and then this failure makes your mains crack up? 

Give a last try with logicboard out of the computer just to see, if it doesn't trip your house and you hear kind of "flop flop flop" repeating from the mac, then your logicboard should be the matter, but I don't think so... there is a good chance it'll act the same way whith or without the logicboard attached

 

[68krazy]

Maybe you've got a pinched ground wire somewhere, or something similar.

 

[JNizzle]

I was poking around in the power supply with my multimeter, and those two caps that I replaced have contact with ground.

I'm at least 90% sure that's where my problem lies, as I was at least getting a PICTURE before I replaced them.

What actual purpose do they serve?

Also I've yet to find any continuity between any component and the live and neutral prongs of the AC input.