Mac Portable 5V Line and Troubleshooting

[campbellashe]

Just wondering if this is what I should expect on the 5V line. I'm new to occiloscope probing and need some guidance as to interpretation... I have a new to me Mac Portable that I'm trying to troubleshoot. I've replaced the caps but have the original hybrid module. There is some evidence of cap corrosion on the board. I get random errors (sad Mac) on boot after the initial good chime. Using a bench supply at 6.5 volts via the J17, I am probing at the VCC pin (28) on the RAM and + side of C16 with the following tracing included below.... I think it tells me there is a lot of noise but want some guidance.

The error codes I get are:
0100 0282 / 0000 1FF2
0000 0003 / 0000 FF00

I did find a broken trace at C15 -- I have 0300 172C / 0000 1FFA before I fixed that trace.

I have gone through Androda's pre- hybrid check -- testing resistance and that check out okay.

 

[David Cook]

If this is a 5120, it is a common issue that powering up sound (for the chime) introduces electrical noise into the 5V line and scrambles the CPU, resulting in sad mac.

Connect a bodge wire to the sound power signal as shown below. If your Mac boots reliably, you can just leave this workaround in place. A number of us have tried isolating the component that has aged poorly (or become damaged by corrosion) that is causing this issue on so many 5120s. Nothing I've tried replacing has actually fixed it.