68kMLA Classic Interface
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| Click here to select a new forum. | | Mac IIfx startup circuit | Posted by: LeSpameur on 2025-02-18 02:20:17
I was given a broken Macintosh IIfx motherboard from Maxwell battery leakage.
Only one of the battery leaked, and not any IC were damage (I think). The most of the damage on the interrupt and reset switches. I cleaned the motherboard in IPA several times, I reflowed the legs of the IC’s, replaced most of the resistances, and I repaired a few traces and one via.
At this point it boots and everything works fine : the pram, the interrupt and reset switches, keyboard and mouse… But I can’t get the power button at the back to work, the only way to turn it on is via the keyboard, and when I turn it off in Mac OS, it says "it is now safe to switch off your Macintosh", so I have to unplug the cable, which is bad.
I tested every single resistance, trace, of transistor, and it looks fine.
I’m not an expert in schematics at all, but I started to surround the working parts of the startup circuit.
Any thought ?
Thanks,
Hugo | Posted by: LeSpameur on 2025-02-18 02:24:22
I also replaced UJ18 and UI18 with brand new HC132 chips, but it made no difference. | Posted by: LeSpameur on 2025-02-19 02:17:06
When I manually inject 5V to Q2, it starts up, and when I manually inject 5V to pin 4 of the power button, it starts up as well. | Posted by: Jockelill on 2025-02-24 00:40:36
Have you replaced UB1? Startup is one circuit and shut down is another. I had same issues as you (also one battery bombed) and had to replace UB1 and also repair some vias in that area. | Posted by: pezter22 on 2025-03-05 18:42:58
When I was trying to figure out my Startup/Shutdown issues with my Mac IIFX I took measurements to see if they were getting proper voltages to the NAND chip (UJ18). You definitely have a broken trace leading up to the power on button. You will just have to follow the schematic.
With the batteries in place check the voltages on UJ18 I made this diagram to check if I had voltages on the IC a few years ago. Voltages with the two batteries in place should be the combined battery voltage usually around 6+volts. The Function Table should tell you how the voltages corresponds with the associated pins and their gate (High Voltage +High Voltage=Low Voltage, Low Voltage+Low Voltage=High Voltage and so on).
If a voltage is incorrect with a pin follow that trace to the corresponding component near by and repair the trace. You should also have battery voltages on the power on button.
From the looks of the board you did have a lot of damage. I had similar damage that I just couldn't repair. There was just too much from two Maxell batteries vomiting on my board. | Posted by: LeSpameur on 2025-03-12 01:58:41
I found the issues : R5 wasn’t correctly soldered and damaged, which explained that it couldn’t turn on via the power button, and replacing Q2 fixed the shutdown issue. | | 1 |
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