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Heavily Upgraded 512K Logic Board - Thoughts?

mitchW

Well-known member
Hi, I had a 512K in the attic for the last 10 years. It has a bad analog board and the mainboard was upgraded to some point. The machine also had some Hyperdrive parts there (like the PSU, fan and interface board that plugs over the 68000). Also the mainboard displayed a sad mac and didn't work.

Well, I pulled it out of the storage today and found that the logic board was upgraded a lot. There is a MacSCSI card that is inserted into the ROM slots and the original ROMs are inserted into it. Also there was a lot of work around the RAM. I think the RAM is actually upgraded to 1MB.

Does anyone know what did they try to accomplish with all this?

Thanks!
 

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cheesestraws

Well-known member
Too bad the machine itself is in extremely bad shape - painted exterior, non-original CRT (Philips unit), missing floppy drive, etc

Well, it may not be a pristine collectors' piece, but that too tells the story. It sounds like it was a machine that was kept working because it was needed to work, and upgrading wasn't an option. The marks of a long service life are historically interesting in their own right.
 

mitchW

Well-known member
I will be putting my Mac Plus on my bench next week (for exchange of the RIFAs), and I would try to also get this logic board going. As I said, last time I think it only showed sad mac.

So diagnosing such an upgraded board would probably not be easy. I am leaning towards removing the upgrades and getting it going. If those upgrades would have been intact, then I would of course leave it like that. But I think it is better to part the machine out and sell the remaining bits of upgrades to someone that needs them. Like I said, the Hyperdrive card + PSU is there, but the drive itself is missing and the power wires are cut. MacSCSI upgrade is missing the SCSI cable.

But still, has anyone seen such a memory upgrade before?
 

3lectr1cPPC

Well-known member
I'd love to see photos of the externals of the computer! Sounds like you've found your very own "cursed mac".
 

mitchW

Well-known member
I forgot how much I paid for it, but it was too much back then. I think around $60, and that was in 2014. I stripped the paint, but the paint stripper somehow weakened the plastics so it broke on few spots on the top grille, which I glued back, but I think it would now need another coat of paint to hide the defects. Before it had a bad snow white paint job with paint already flaking off, so I would go for a light beige one, and I think it would turn out fine.

After that paint stripping "accident" I tried to at least get it going, but analog board didn't want to cooperate. I got it going by replacing some resistors and HOT, but it would burn the HOT (Horizontal Output Transistor) in matter of few seconds. With flyback removed, it kinda worked, but of course with no picture. I then tried the logic board on my working Mac Plus, and also didn't work (sad Mac).

So I then put it all together and put it in the attic for the last 8 years (I made a mistake before, it was 8 years, not 10)
I will get some photos of the case when I bring down

But like I said, there is no more original (non-modified) components left in this Mac. Even the metal frame is bent slightly to accommodate different CRT. So I think the best thing would be to part it out...
 

joshc

Well-known member
So I think the best thing would be to part it out...
Depends if you can make a working machine out of it or not.

I would personally keep the logic board upgrades and repair it as is. Bad RAM chips are a common problem with Macs of this age, so it’s a case of finding which ones are causing the problem.

The analog board should also be repairable, again just a case of more diagnosis to see what’s going on.

So it just boils down to how much time you want to spend on it.
 

mitchW

Well-known member
I would be better (and cheaper) off if I just get a working 512K. For the repair of that one I would need to get:
- floppy drive and its bracket + cable
- new flyback transformer or a working analog board
- repair the logic board (possible, but difficult)
- new casing if I can't make this one looking presentable
- keyboard and mouse

So essentially almost everything except the CRT.
The one who had this Mac really used it to death, with upgrades and new CRT added at some point. Most likely when the flyback failed, the machine was retired.

Anyways, I will see what I will do :) Really doubt it will work ever again, though
 

mitchW

Well-known member
I put the logic board into my Plus and it showed: 028000 error code. If I am not mistaken, this means the chip in location G12 is bad. But that board is DIY upgraded which error code might mean something different

Any suggestions? Or should I just remove the memory upgrades and go from then?
 

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mitchW

Well-known member
I tried it, but it G12 is actually 2 chips on this board, the original one and the other one "piggybacked" - soldered on as a part of a upgrade.
I tried another one on the top of those 2 and it didn't help.

So now I removed the memory upgrades. I am missing one 74F253N (location G13), so I guess I'll have to wait till I get another one :)
 

mitchW

Well-known member
I replaced the missing 74F253N and removed all the upgrades. I am getting now error 20100, so looks like G5 is the culprit now
 
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