Macintosh Plus | CRT works only with the back case off.

[ktg5]

Hey,

it's been a while since I was on here - wanted to share my problem with people here to see if anyone could help me with an issue I've been having with one of vintage macs.

In the midde of 2019 I got a Macintosh Plus from my dad's friend - the thing boots with one small problem - the CRT only works when the back case is off. I checked the Analog board for about 30 minutes - no cracks on the board or capacitors leaking from what I could tell. I checked the motherboard as well - seemed fine as well. Then after all of that, I thought that maybe something was touching the case thus the CRT not showing anything? But I wasn't too sure.

Anyone know how I could get this built-in CRT working again?
I can provide photos of the Macintosh if needed.
Thanks.

 

[MacTech68]

Does the Analog Board have a white plastic insulator card fitted ?

If you have a EMI shield on the motherboard that fits over the I/O ports, try removing it as a test.

Make sure the four torx screws holding the CRT to the front panel are not loose.

Confirm that the black with white stripe lead from the CRT socket has been fitted under one of the CRT mounting screws.

Resolder the video pin of the video/power connector, roughly in the middle of the power/sweep board. The video pin sits by itself at the end of the connector (note the pic below shows the power/sweep board with the screen's front facing down).

Some general pictures may assist if the above does not help.

 

[ktg5]

Hey MacTech68,
Thanks for the reply, here’s what I’ve got:

Does the Analog Board have a white plastic insulator card fitted ?

Yes it does.

If you have a EMI shield on the motherboard that fits over the I/O ports, try removing it as a test.

That made the screen show up.

Make sure the four torx screws holding the CRT to the front panel are not loose.

Checked it, seems to be not loose.

Confirm that the black with white stripe lead from the CRT socket has been fitted under one of the CRT mounting screws.

Checked it, seemed fine.

The last parts I didn’t get to test, is it ok for the Mac to be running without the shield?
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Also I found out that the floppy drive doesn’t eject disks which is something we can get to later if that’s something worth talking about.

 

[MacTech68]

The EMI shield on some early machines didn't have an electrically insulated side against the bottom of the motherboard. Later ones had a sheet of plastic glued to shield. You could run the machine without the shield or get a sheet of plastic (like over-head transparecy thickness) glued to the EMI shield's upper side.

As for the floppy drive, the old grease will need to be removed from the mechanism or you have a shattered cog in the eject motor's gearbox.

Here's a good starting point:

Mac Plus floppy drive - help!

Hi all, The floppy in my Mac Plus was obviously seized up, so I attempted the relubrication procedure described here: http://68kmla.org/wiki/Floppy_Drive_Lubrication It did not fix my drive, but it DID fix the eject mechanism. I could put disks in, they would fail to format, and then I could...

forums.macrumors.com

 

[ktg5]

So I just came down to my garage to see if the Macintosh was going to work again, so I turned it on and it wasn’t showing on screen again.
Should I try re-soldering the video pin or something else?
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Also would temperature affect anything?

 

[MacTech68]

Temperature will certainly affect the cracked solder joint on the video pin.

Go ahead and resolder.

 

[ktg5]

Temperature will certainly affect the cracked solder joint on the video pin.

Go ahead and resolder.

Just resoldered the pin, seems to work.
Thanks a lot.
If I have any other issues, I’ll report back.