Mystery Mac Plus Problem

[Reasons.]

I've been working on some Mac Plus repair today, and have reached an impasse. The original problem was that the computer would not turn on (with no humming). According to The Dead Mac Scrolls, this was caused by a burned out transistor and rectifier at Q1 and CR5, respectively. I replaced those parts, but then a new problem emerged: the computer still wouldn't boot, but there was a new hum. That apparently indicated a busted flyback, so I replaced it with one from an analog board I had hanging around. After that, the computer would give me a startup bong but wouldn't display anything. The floppy drive would occasionally spontaneously eject, too. Now I'm stuck. This specific issue doesn't seem to show up in DMS, and I really don't know what to do. Any thoughts? I'd hate to have gotten this far only to find that the thing is just screwed.

 

[Johnnya101]

Well, I think its much more narrowed down now. Could there be any loose connectors by the CRT or wherever for video? I dont have a Plus, just guessing.

 

[Reasons.]

I desoldered and desoldered pins 1 and 3 on J4, to no avail. I don't think that's what it is, though I'm open to suggestions.

 

[Johnnya101]

Looked it up quickly, and it MIGHT be a problem with the video plug to the analog, or as I said before, could be other cables.

Do you hear anything CRT related? At least it powering on?

EDIT

Heres the link: https://discussions.apple.com/thread/2110431?tstart=0

 

[Reasons.]

So, another interesting wrinkle. I tried swapping the logic board with another Mac Plus I have. When I put a known good logic board in the machine I was working on, the CRT lit up. It wasn't an actual image, though. It just lit up, edge to edge, with a light glow. Also, the floppy drive wasn't getting enough current to run properly, it seemed. When I put the logic board from the machine I've been working on in a known good body, though, there was a different problem. It displayed the blinking floppy, but just the blinking floppy. the whole rest of the screen was dark. It wouldn't boot, either. So now I am even more stuck, if such a thing was possible.

 

[Johnnya101]

So both had a problem?

Well the good thing is that you know that the CRT works. The rest sounds like a bad logic board or something. I have no idea, since I haven't had a Plus yet, so Im going to leave it to the pros. It might be something at the video part of the main board or something. Might be something preventing video output from working.

 

[GregorHouse]

So, another interesting wrinkle. I tried swapping the logic board with another Mac Plus I have. When I put a known good logic board in the machine I was working on, the CRT lit up. It wasn't an actual image, though. It just lit up, edge to edge, with a light glow. Also, the floppy drive wasn't getting enough current to run properly, it seemed. When I put the logic board from the machine I've been working on in a known good body, though, there was a different problem. It displayed the blinking floppy, but just the blinking floppy. the whole rest of the screen was dark. It wouldn't boot, either. So now I am even more stuck, if such a thing was possible.

Hello, Reasons,

Have you checked the voltage on the motherboard? You could have a problem there.

The easiest and safest way to do it is on the external drive port. You should have 5V on pin 6, 12V on pins 7 and 8 and -12V on pin 5. For more information look at pages 8 and 9 on classicmac2: https://68kmla.org/forums/index.php?/topic/19751-compact-mac-repair-guide/

Good luck!