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Unchanging voltage with voltage resistor change

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First post - thank you for the awesome information I've been able to track down so far. I haven't tried everything yet, but I think I've got enough to start a thread. Apologies if this is covered elsewhere and I missed it.


I've had my grandmother's OG Macintosh (I think it's legit, Model M0001) sitting in a corner of basement for 10 years, and I finally decided to try it out. Symptoms: flip the power switch, sometimes it gives the classic "bong" and sometimes it gives a muffled, slow-mo version of the sound. The screen doesn't show anything, or sometimes shows garbled checkerboard, or sometimes just vertical lines with varying levels of brightness - very inconsistent. I bought some torx heads and started following the Classic Mac Repair Notes. I resoldered J1, removed & tested C1 (it's good) and resoldered it, and tried powering on again. Same symptoms. 


I resoldered the flyback transformer and tried again - same symptoms.


I noticed that jiggling the J4 and J2 connectors would result in screen flicker, and if a pushed one or the other just right it would show a stable (but garbled checkerboard) image on the screen. I checked & cleaned connectors, and resoldered J4 and J2. I tried again - now no "bong" at all, no image, nothing I could do to make it change.


I finally tried testing voltage on the pins of the external floppy reader port. 12V reads 12.5 volts, 5V reads 4.8V. I tried turning the voltage resistor (while the unit is on... is that right?) and the voltage doesn't change at all. If I turn it off, turn the voltage resistor, and turn back on - still no voltage change (12.5 and 4.8). I turn the voltage resistor ALL the way to the high side, turn the machine on, now I get nice clean "bong" sound, sad mac icon (still mostly garbled screen) and (so strange) but readings of 9V on the 12V pin and 3V on the 5V pin. What?


If I turn the voltage resistor all the way down (clockwise from the back) I can go back to 12.5V and 4.8V, no bong, no image. I can repeat the symptoms based on the absolute high or absolute low settings of the voltage resistor.


Next steps will be to continue to go through the Classic Mac Repair Notes. I just figured I would start this here to see if anyone had a similar situation or other ideas. Thank you!

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I removed and tested a bunch of capacitors that the Classic Mac Repair Notes indicates are prone to failure, all were good (C24-C26, C29-C31). I did find out that CR29 was bad, so I replaced it with an equivalent diode (hopefully). I plan to wire it back up and test this afternoon. 


Main issue I've identified so far: Measuring the 12V and 5V bus while the computer is running reads inconsistent results. Sometimes it's 12.5V and >5V, sometimes it's more like 9V and 3V. It's stable while the computer is on, but each time I turn it on it goes to one or the other voltage level. The voltage adjustment resistor (R56) does nothing noticeable, which seems very odd.


I am going to go search for how to test the two rectifers attached to heat sinks (CR20 and CR21). They don't seem to be working, but I just tested them using the diode function on my multimeter.

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Thanks Bibilit, I finally found another post that was having a similar issue. I think it might've been you that talked about the U3 component. I think you are right, I just don't know how to test it. This computer is actually my dad's, so I told him what I had done so far, recommended U3 and the CR20 and CR21 for replacement, and suggested he find someone who is less likely than me to destroy it with poor soldering skills! I will let you all know if he finds someone and if those replacements work. Thank you!

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