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Macintosh Plus Display vertically compressed
Posted by: keelus on 2023-02-02 13:15:48 Hello! I've been working on my Mac Plus for a month and I got this issue out of nowhere. (The diagonal lines are from the camera) As you can see, the display is very compressed vertically, strange display resolution. I tried adjusting the height potentiometer, but it's maxed out, if i turn it in the other way, it goes even more compressed. This is what I have done to this Mac Plus so far: For other issues I had (no display, clicking, and sad mac): I replaced these electrolytic capacitors for brand new ones: C2,C3,C6,C7,C10,C12,C18,C24,C27,C28,C30,C31,C32,C34,C35,C38,C39,C42,C45. I replaced the U3 optocoupler for a 4N35. Brand new RAM (x4 256k) Trying to fix this issue: LM324N - Brand new Q3 (2N4401) - Brand new Q6 (2N4401) - Brand new Q4 (MPSU51) - Brand new CR15 - Brand new I need help to fix this. Does anyone went throught this problem/know what could be causing it? Also (might be a dumb question), if I buy a different potentiometer to control the height, could I make it stretch a lot so it matches the correct vertical height? Any kind of help or suggestions will be appreciated. Thanks for your time, keelus
Posted by: WarriorRocker on 2023-02-02 13:42:18 Looks like the horizontal drive is working properly and the max potential of the vertical drive is way below spec. Have replacing any of the transistors affected the display in any way? I would focus on the vertical drive portion, could be crazy but resistors and diodes can fail. Worth probing them in-circuit and seeing if you get sane values, ie not short or open. Also (might be a dumb question), if I buy a different potentiometer to control the height, could I make it stretch a lot so it matches the correct vertical height? Most likely not unless the actual potentiometer is bad, if you are able to adjust it then its probably fine. The range of output is locked by the circuit itself, the value of the potentiometer is the overall resistance range available through the ring.
Posted by: keelus on 2023-02-09 06:17:39 Looks like the horizontal drive is working properly and the max potential of the vertical drive is way below spec. Have replacing any of the transistors affected the display in any way? I would focus on the vertical drive portion, could be crazy but resistors and diodes can fail. Worth probing them in-circuit and seeing if you get sane values, ie not short or open. Most likely not unless the actual potentiometer is bad, if you are able to adjust it then its probably fine. The range of output is locked by the circuit itself, the value of the potentiometer is the overall resistance range available through the ring. Replacing the transistors made NO effect. Still with the same problem. The R2 and R3 resistors work as they should, and the zener diode CR15 has been replaced also althought it made no change. Do you have the vertical portion schematic, or, more or less what should I do? The range of the potentioneter goes between 0 and 450ish. I think is a 500k resistor so i think it's good
Posted by: WarriorRocker on 2023-02-09 16:06:42 https://windswepthomeblog.files.wordpress.com/2021/09/plus_analog.pdf This document has some of the sections like Vertical Sweep covered. Looks like Q1 and Q2 according to the schematic are the main drive transistors for the vertical sweep.
Posted by: Iesca on 2023-02-09 18:24:58 Here's a later version of that document (2007, v2)
Posted by: keelus on 2023-02-10 07:45:17 https://windswepthomeblog.files.wordpress.com/2021/09/plus_analog.pdf This document has some of the sections like Vertical Sweep covered. Looks like Q1 and Q2 according to the schematic are the main drive transistors for the vertical sweep. I see. I forgot to add that I replaced the Q2, and made no change. I still need to replace Q1, but can't find a shop nearby (spain) that would ship a MPS-U01
Posted by: MacintoshProblems:P on 2023-03-31 11:29:37 Have you tried opening it up and adjusting the potentiometers on the side? They might've corroded and that would be my guess for this issue. I'll attach a picture of what it should look like. idk if you already fixed it or the potentiometer is broken
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