68kMLA Classic Interface

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Display Issue on Macintosh Classic
Posted by: AnwarHegazy on 2025-10-02 11:13:12 Hello everyone, I need help with display issue on a Macintosh Classic. But first let me give you small report about what I have done. The computer had a black screen, i discovered that the switching IC (TDA4605) was damaged, I replaced it and I recapped the analog board. I took some measurements and i turn PP1 potentiometer to the max to get the following values: [1] Measurements of LM317T (IP2) as follows: - Input pin ( I ) 20.7v - Output pin ( O ) 18.9v - Adjust Pin ( A ) 17.7v [2] External Floppy Connector Measurements as follows: - Pin 5 was -11.7v - Pin 6 was +4.8v - Pin 7 was +11.6v - Pin 8 was +11.7v [3] BP2 Hard drive connector as follows: - 5v rail was +4.89v - 12v rail was +11.58v Computer Boots up but the problem is the screen keeps scrolling. as you'll see in the attachments. I appreciate your help in advance.
Posted by: bibilit on 2025-10-02 11:26:00 If you need to turn PP1 all the way up, you still have an issue with voltages. PP1 is a fine adjustement only. Voltages are ok, but probably an issue with cap goo around. Two diodes should been replaced as a matter of fact (dp3 and dp4 IIRC) But you also have an issue with vertical hold, and your screen is far too bright. I will clean potentiometers and try to adjust them (those are located on the upper part of the analog board)
Posted by: AnwarHegazy on 2025-10-02 11:30:00 If you need to turn PP1 all the way up, you still have an issue with voltages. PP1 is a fine adjustement only. Voltages are ok, but probably an issue with cap goo around. Two diodes should been replaced as a matter of fact (dp3 and dp4 IIRC) But you also have an issue with vertical hold, and your screen is far too bright. I will clean potentiometers and try to adjust them (those are located on the upper part of the analog board) I've already replaced DP3 and DP4 as well as the IFRBC40 MOSFET, although it was working.
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