Mac Classic screen shake

[This Does Not Compute]

I'm working on a Mac Classic. I recapped the motherboard and it boots and works fine. However, I noticed that the screen image has a weird subtle shake/wobble to it. I haven't seen this sort of thing before, I attached a video clip that illustrates it (watching the menubar is the easiest way to notice it). Anyone know what's causing this? The caps on the analog board visually look good (and they're all 105C-rated Nichicons), so I'm hesitant to replace them if the problem likely lies elsewhere.

 

[Byte Knight]

I'm working on a Mac Classic. I recapped the motherboard and it boots and works fine. However, I noticed that the screen image has a weird subtle shake/wobble to it. I haven't seen this sort of thing before, I attached a video clip that illustrates it (watching the menubar is the easiest way to notice it). Anyone know what's causing this? The caps on the analog board visually look good (and they're all 105C-rated Nichicons), so I'm hesitant to replace them if the problem likely lies elsewhere.

I had the same issue with my Classic - recapped the logic / analog board and reflowed a bunch of solder joints without any improvement. I finally turned up the voltage adjuster on the analog board a tiny bit and all was good after that.

 

[This Does Not Compute]

I had the same issue with my Classic - recapped the logic / analog board and reflowed a bunch of solder joints without any improvement. I finally turned up the voltage adjuster on the analog board a tiny bit and all was good after that.

OK, some digging suggests that there are feedback resistors that can drift over time, so voltage sag makes sense. My question now is...where's the voltage adjustment on this analog board? Not seeing anything obvious other than the video adjustment pots on the back.

EDIT: Never mind, found that this revision still has the PP1 adjustment pot, it's just slightly hidden by the big 5V PSU.

 

[macintosh2002]

Hi, please check cracked or high resistance solder joint on PP1 potentiometer (can be found in the middle of the AB). Voltage should be +4.95V to +5.05V. You can check the +5V at the floppy-port on pin 6.

 

[Sideburn]

OK, some digging suggests that there are feedback resistors that can drift over time, so voltage sag makes sense. My question now is...where's the voltage adjustment on this analog board? Not seeing anything obvious other than the video adjustment pots on the back.

EDIT: Never mind, found that this revision still has the PP1 adjustment pot, it's just slightly hidden by the big 5V PSU.

View attachment 13398

I can hear your voice and speaking style…
IN your posts.

(I have a Sony monitor doing something very similar but it’s waving and rising vertically)

I have yet to figure it out. Replaced caps and bridge rectifier and the driver chip.

 

[lfletche]

Hey Colin, I had a similar issue with a Classic AB, recapping it fixed the voltage problems it had but I still had the shake especially when the power supply was first turned on. I ended up replacing the DP3/DP4 diodes as well as the OP1 optocoupler and that seemed to do the trick - it was rock solid after that.

Regarding the capacitors - I’d suggest at least maybe lift a few of them off in that big cluster especially and check underneath them. I don’t think I have seen a Classic AB to date that doesn’t have some sort of cap leakage at least starting to happen. I’d double check under them and if they are still all good you can easily just put them back on.