Macintosh SE scanlines on display - another analog board fault!

[jake18125]

My macintosh SE (that has been throwing me issues after issues) has yet another fault on the analog board!

First it was the TDA1170 squashing the display, and once I had replaced that, the display has very prominent scan lines. Adjusting the cut off had no effect, and the two resistors (R20 and R21) that the Dead Mac Scrolls suggest could be faulty are absolutely fine.

Anyone got any ideas? 

 

[rplacd]

Another forum member knows the AB's components better than I do, but the scanlines might be fixed by simply adjusting the brightness – I've fixed visible scanlines like these before.

 

[jake18125]

Another forum member knows the AB's components better than I do, but the scanlines might be fixed by simply adjusting the brightness – I've fixed visible scanlines like these before.

Unfortunately adjusting the brightness and the brightness cut off had no effect 

 

[techknight]

you probably have a bad transistor on the CRT neck board. might need to test it with a DMM on diode check. It could also be the logic IC thats on the analog board, but, you wouldnt get a video signal at all if that were the case. 

 

[techknight]

One more thing, check the solder joints on the analog board where the CRT Neck board plugs in. This breaks often and needs resoldered. if your missing your 30v+ scan derived voltage supply this will happen as well. 

 

[jake18125]

you probably have a bad transistor on the CRT neck board. might need to test it with a DMM on diode check. It could also be the logic IC thats on the analog board, but, you wouldnt get a video signal at all if that were the case. 

Swapped the neck board with a spare I have, no change unfortunately 

 

[jake18125]

One more thing, check the solder joints on the analog board where the CRT Neck board plugs in. This breaks often and needs resoldered. if your missing your 30v+ scan derived voltage supply this will happen as well. 

Hmmm I’ll reflow the joints. Hopefully that’ll help

 

[jake18125]

Swapped the CRT board, reflowed all the joints, compared all the values to my SE/30s analog board and I can’t find the fault 

 

[jake18125]

GOOD GOD thats one for the books. I fixed it! 

The 470k resistor that goes between the cut off potentiometer and ground had burnt out. Swapped for a replacement 330k (only one I had on hand, but the difference can be offset by the potentiometer) and we have a happy mac without scanlines!



Also including the bomarc schematics with the resistor that failed highlighted if anyone else has this issue at some point 

 

[DW1992]

..:: Resurrecting this old thread since I see this question asked all over other forums, and I just experienced the issue myself ::..


Cheers Jake for the detailed repair notes!
I had the exact same problem with an SE I recently picked up.

Recapped the power supply and analog board, but the screen was over driven (see pics).
Unsoldered a lead for R20 and R21, which both measured correct values.

When I checked R25, tucked way up under the fly back cage, it read open.
Replaced it with a new 470K resistor, powered up, adjusted the brightness control and voila! Sweep lines are gone!

Thanks again!
I hope this helps others like it helped me.