Macintosh 128k chimes but no video - logic board issue

cadenya

New Tinkerer
Oct 30, 2021
2
0
1
Hey guys, it's been awhile since I've worked on any Macs. I have a Macintosh 128k (later revision board) that will chime but not show any video. I've tested the board in a known working case. When I first opened it up the board was slightly corroded, and after an alcohol wash it still doesn't work. Any ideas what the issue could be? I'm not sure what part of the board controls the video signal so I can't go around testing components just yet. Thanks!
 

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cadenya

New Tinkerer
Oct 30, 2021
2
0
1
Screenshot 2024-01-17 at 6.22.45 PM.png
Found this thanks to someone in the BlueSCSI discord, should I just start at J4 (Pin1) and check everything down the line with my multimeter?
 

Burpethead

New Tinkerer
Jun 5, 2022
3
4
3
Hello! I have a Mac Plus with the same issue - their analogue boards are very similar. For me, I did a recap, and had video for about 5 minutes after recap, then the screen went black. I know it's the analogue board because swapping it out with another returns video. I have also swapped CRT's and their power cables. So it must be a resistor that popped or something on the board. I double checked all the capacitor connections, but I think testing them would mean taking them all off again right? Perhaps one of the new capacitors is bad? I would love to know what it is. I have a multimeter but don't know what to do or how to proceed. Have a recapped analogue board waiting for an answer....
 

kozak

New Tinkerer
Jan 30, 2022
3
3
3
NYC
kozak.digital
It’s pretty common to end up with cracked solder joints on those analog boards, especially on the connectors. I would try reflowing the solder for all three connectors on the analog board (deflection yoke, CRT neck board, logic board) and see if that fixes the issue.
 
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