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Portable M5126 garbled screen, no chime... :(

MacGyver

Member
Hi there :)

Years ago, I finally got hold of a M5126 backlit Portable. It was untouched, never before been worked on. I was not surprised that it didn't boot and only produced a garbled screen, similar to a Simasimac.
I removed the caps, cleaned the board thoroughly, soldered new caps. Same thing. The board is as clean as can be, no dull vias visible, no corrosion from cap goo. I gave up back then, couldn't find a good reason why it shoudn't work. Thanks to Corona ;), I rediscovered the Portable and decided to tackle the problem again. I removed all the caps, double-checked every pad, via, track in the surroundings of the caps, ultrasonic-cleaned the board again, dried it, resoldered new caps. Same thing.

Maybe somebody can point me in the right direction to check things. After applying 6.5V to the battery terminals, the board pulls around 60mA on my lab supply. After PMU reset, it goes down to around 20mA. I can then power the unit up with the space bar, it pulls ~780mA with the backlight running. Yes, most of the time it starts up with the backlight running. I hear a faint whine/hiss from the speaker, which changes pitch after about half a second and then stays that way. Most of the time it powers up to horizontal bars with pixels, slightly changing every time I power it up, with sometimes varying display contrast. Sometimes it powers up to a completely black screen.

I do have SMT soldering /rework equipment, a scope, etc., however, I am kind of lost here what to check first and hope that somebody has experience fixing these boards and can point me in the right direction of a probable fault. Hopefully the screenshots give some clues? I'd appreciate any help and shared experiences which might help get my 5126 back running. I do have a 5120 parts donor board, however that seems to only be of limited use for a 5126 repair. I'd go as far as saying that the Portable probably died before it died because of bad caps. I had 5120 running which were far worse looking in terms of cap leakage...

Thanks!

MacGyver

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Stephen_Usher

Well-known member
Try resetting using the debug and reset buttons at the same time.

My non-backlit machine shows similar patterns when first powered up as it starts in an inconsistent state and crashes.

 

techknight

Well-known member
you probably have a bad SWIM. 

You can try removing it if your comfortable with PLCC chips, and try again. 

 

GregorHouse

Well-known member
I had the same exact symptoms. In my case it turned to be a dead via under a chip, because of the corrosion caused by a leaking cap. The electrolyte filtered under the chip, so the corrosion wasn't visible until I lifted it.

It was a via between chip U14A and U14J (I don't remember the exact pins), that got damaged by C11. My tip would be to triple check all the vias surrounding capacitors and lift the chips around capacitors that leaked most.

 

MacGyver

Member
you probably have a bad SWIM. 
Techknight, you were spot on! I removed the SWIM, then I got a good chime and the "grey" RAM test screen. On floppy access it crashed, of course, without the SWIM. So I replaced the SWIM with the one from my donor board and it booted System 6 just fine!
Now I got just one Portable M5120 left to fix... This is also a tough one because it has vertical bars on the screen, every 8th vertical line is black, but not always. I remember an old thread where this was a problem on an SE/30 I believe, and it was a tough one... ;)

 

techknight

Well-known member
I figured that was your issue. Ive worked on enough of these to know. :)

These fail because the voltage regulator starts to run wild, way past 5V. sometimes up to the main 7.5V DC input from a power adapter if its connected. 

The regulator runs wild for 2 reasons. 1, bad caps, of course. and sometimes 2, people trying to power strictly from AC adapter without the battery. Bad idea. 

 
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Stephen_Usher

Well-known member
I never understand why Apple didn't fit a zener diode and fuse to stop this causing damage... Or design a proper failsafe power sub-system.

 

techknight

Well-known member
I dont recall those jumpers? but I very rarely saw backlit machines. 90% of what I ever saw were non-backlit models. 

if your talking about the switches, they switch external ROMs in and out of circuit. 

 
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