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Macintosh Portable inconsistent 5V+

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I would appreciate some guidance...


I recapped "recently" (like year ago or so) acquired M5120 and plugged it in for the first time. I got a chime and a sad mac. While trying to make it easy to access the board while powering it, it did a smoke on pin 10 on J22 (yes, it is a ground pin but the Portable case is conductive, so it touched something else where). Anyways, searched through the forum found out that pin 3 on Q4 should give you ~5.2V. I get 5.15V sometimes (usually after a small break without a charger and with a charger from PB 165c, 3A) and I can get it to chime again. Most of the times I get 0 (on battery) or 1.3 (if charger is plugged in).


The charger gives 7.6V whenever I've measured it and possibly even charges the battery (or at least I've seen it increase from 6.3V to 6.36V but it might also be the battery recovering). I have used IPA and a brush to clean board where necessary. I've never cleaned my boards by washing so if that would be the next course of action, how do you wash them? With plain water and a brush and do you use regular household oven to dry it?


Also suggested on the forum is to use flux and heat station on Hybrid, I don't currently have access to a heat station so can't really try it out. Pin 3 on Q16 and Q20 gives either 7.6V if on a charger or 6.3xV if on a battery. I guess that is correct. Pin 1 on Q16 is around 3V. Pressing RESET + NMI does not change anything on the 5V (so it stays the same it was prior pressing them, be it 0, 1.3 or 5.15).









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Hmm, did some trials and it seems that the computer starts up (almost) every time if I plug in the battery first. It doesn’t boot up with charger only (I guess this is expected) and I need to RESET + NMI it to get it to boot if I plug in the battery after the charger. So... most probably it works from power mgmt perspective and the sad Mac is because of a bad trace etc. 

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(a) The charger can't produce enough power.

(b) Without the battery the power subsystem is unstable and generate high voltages on the +5V line, frying chips. The power sub-system is not failsafe.


Only ever start the machine using a properly charged battery or using a regulated (important) 6V power supply via the battery connectors.


There really should be an FAQ at the top of this area with this information in bold at the top. Preferably with flashing lights and klaxons.

Edited by Stephen_Usher

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If I only had one :) Anyways I found the problem (I had accidental dropped solder on a memory chip, it is visible on the logic board picture above) and the computer now boots to a flashing disk screen. The HD is marked as bad, and it didn't do much than whining when attempted to start it up.


Regarding the PSU there is a long thread from 2011 to 2013 discussing about it. I know the thread is old and more knowledge has been gathered, should one use 2/3A PSU or no?


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The Portable was designed to run on battery, not without one and not only using the external psu. 


So the main goal of the psu is to keep the battery charged, the original psu (1,5) is more than enough to keep that goal. 


Using the 2 A is not an issue if everything else is in good condition (Logic board)


the 3 A is to be avoided, the risk is heavy to blow something. 



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