68kMLA Classic Interface

This is a version of the 68kMLA forums for viewing on your favorite old mac. Visitors on modern platforms may prefer the main site.

Click here to select a new forum.
Cheap 512k or 128k motherboard replacement
Posted by: Macintoshguy1984 on 2016-12-07 17:55:58
Im looking for a cheap ($30-$40) 128k or 512k replacement motherboard. The old one doesn't work anymore and cannot be repaired. could i use a plus board?? and keep the 400k floppy drive?
Posted by: Carboy7 on 2016-12-07 18:47:50
Welp, there's your problem there. You can't get 128k logic boards for cheap, because rare ermagawd. 😛

Posted by: jsarchibald on 2016-12-07 18:55:12
Plus board won't work, ports are different at the rear because of the introduction of SCSI.  So you can only use a 128K or 512K board in it, unless you want to replace the rear bucket.

As these boards don't have caps, I would think they are generally repairable (unless the battery leaked on the board).  RAM can be replaced, and traces can be added to get these boards running again.

Last thing - you won't score any of this stuff cheaply, as they are in demand.  Best to keep an eye out on BIN items, or buy a system for parts, as generally it is the Analog board that goes faulty in these earlier Macs.  You could end up with a working logic board for a 512K, but the chances of a 128K are slim.
Posted by: unity on 2016-12-07 19:15:09
128k boards are indeed hard to find cheap, but two recently sold on eBay in separate auctions for $6 each. So, it can happen. I have many extra 512k boards, but I wont be able to test any for a while.
Posted by: CC_333 on 2016-12-10 10:07:39
Can a 512k board theoretically be downgraded to a 128k?

c
Posted by: BadGoldEagle on 2016-12-10 10:17:20
Yes it can be done. You need to remove one chip (it's supposed to be just above the 68000 in that photo):

Screen Shot 2016-12-10 at 07.14.30 pm.png

You'll notice 128k boards don't have this chip while 512k ones do. 

Then it's just a matter of replacing the RAM chips.
Posted by: unity on 2016-12-10 14:55:46
Or just install the 128k resistor, as marked on the board. I never did it, but my understanding is that installing the resistor knocks it down to 128k. Also I have yet to see in person a 512k Hybrid board from factory as a 128k, although they exist. I think I collect machines that are just too early to have one. The later "Macintosh 128k" badged ones (on back) should have this board.
1