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Help fixing up some Macintosh Plusses


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I should preface this by saying that I'm a beginner when it comes to trying to diagnose and fix compact Macs, but I'm learning. I did manage to fix an intermittent screen problem on my Mac SE by reflowing some joints on the analog board, for example.

 

I have three Mac Plus units. One of them works fine. One of them has a Total Systems accelerator in it and has had occasional issues booting. And one of them has never worked since I've had it (it showed nothing on the screen and just made some screeching noises periodically). But yesterday I tried reflowing the solder joints on the analog board of the non-working unit to see if that would do anything, and in fact it did look like there might be some cracked joints there. I was able to get the screen to come to life, but it's just horizontal lines that will eventually turn into garbage, still accompanied by the occasional screech.

 

Then I tried using the unit with the accelerator, and while it booted fine the first time, when I tried rebooting it, it showed basically the same symptoms as the non-working unit, which I thought was interesting. Looking through The Dead Mac Scrolls led me to believe these symptoms were indicative of problem on the analog board, but it occurred to me that I could test this by swapping the logic board from the non-working unit into my Plus that was definitely good. And as it turned out, I got the same exact symptoms on the good unit when I used the logic board from the bad one. So I'm assuming the issue is with the logic board, but I am not sure what it could be. There is no obvious damage. I did try reflowing the solder joints for the connector that goes between the analog and logic boards, but that didn't seem to have any effect. I'm not really sure where to begin with the board that has the accelerator on it, so I haven't tried anything on that one. Does anyone have any advice?

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I don’t think I actually tried it before, but yes the logic board from the good machine does work in the bad machine, so it does look to be a logic board issue. I haven’t tested voltages because I don’t really know how, although I do have a multimeter. 
 

I guess what I want to know is if there are any common failure points on a Mac Plus logic board. Since I have two machines showing the same issue more or less, I’m wondering if there is some common fault between them. 

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I did try swapping the RAM earlier but neglected to mention it. But I've never taken out a Mac's ROM chip (and wasn't even sure if they were socketed until I checked which ones they were). After doing some swapping around of chips, I can say that it is definitely the ROMs that are at fault. When I put the ROM chips from the working Plus into the non-working one, the non-working one starting working, and vice versa. So I decided that I would put the good set of ROMs into my accelerated Plus, since it is my favorite and WAY faster than a normal one. That entailed removing the upgrade board, which I had never done before, but it turned out fine and now that machine is working. But the interesting thing is that I took the ROMs I took out of the accelerated machine and put them into one of my other ones, and that one started working as well! As I mentioned, the accelerated one had been a little flakey, so it's possible it will stop working again. Or it's possible that the chips just weren't seated properly or something and now they will be fine. The third machine that never worked for me still doesn't work though, so I will be interested in getting a set of replacement ROMs for that one.

 

Does anyone know what causes ROM chips to go bad?

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