Macintosh classic brightness issue

[TimTheEnchanter]

Recently my friend has bought himself a Macintosh classic. It’s in amazing condition the only problem is that he decided to replace the capacitors on the digital board himself. Whilst doing this he ruined a few traces and did a terrible job of soldering. After he had done this and run into some issues he gave it to me as he no longer wanted to keep it. I would say this computer looks “out of the box” new but for the issue that the screen wasn’t working. I took it apart and fixed most of the issues that I found apparent and tried again and as luck would have it, it now chimes and displays a screen! But there’s one issue... the screen flickers horribly! Not as in loosing connection but the brightness is all over the place! I’ve never messed with one of these and have no idea where to start my search for issues. I’ve already looked at all the solder connections on both boards and reflowed that of witch I found needed to be taken care of.  All of me research pointed me to this site so I come here hoping I get some help! I’ve attached a video of the screen to show you exactly what the screen is doing. Feel free to ask questions as I am really looking to get some help. Thanks!



View attachment 489EA673-D066-40FC-A9E9-8456F6F69BC8.MOV

 

[TimTheEnchanter]

While I was waiting for a response I did some probing on some display lines and I saw voltage  fluctuating but then it just stopped? It must me a power supply issue and now that it’s not acting up I don’t know where to follow. If anyone has any ideas let me know. 

 

[TimTheEnchanter]

I took some measurements off of the connector to the HDD and all voltages are spot on. 

 

[joshc]

@TimTheEnchanter There's more than just the flickering issue. I see a grid behind the picture as well. Also, that display really needs adjustment, it is not as wide or tall as it should be.

I would start with inspecting the capacitors on the analog board, they most likely need replacement, or at the very least you should resolder all the joints.

 

[TimTheEnchanter]

@joshc the screen size issue is due to the variable inductor’s ferrite rod being shattered. I need to order a part for that. The caps on the look to be in amazing condition but that’s not always a good way to tell if it’s good or not. I really would like a good schematic that I can follow to diagnose the board completely otherwise it’s going to be very hard for me to follow. 

 

[techknight]

Classics almost always have to have the caps changed. 

But variable brightness is usually noise on the control signal, Could be caused by leaking electrolyte, could be caused by bad solder connections on the connector itself, on either end. 

The brightness signal for the analog board comes from the Logic board, so it could even be that! especially if there is cap goo everywhere. 

you have to inspect everything.