Mac IIcx, Bad Motherboard?

[F. Hopper]

well, i've been tinkering with the Mac IIcx i got a while back. The first time i turned it on it seemed to run ok, for having been gutted of its hard disk anyway. Now it seems to be dead in the water with no display at all, you plug it in and it immediately turns on, and the power switch won't turn it off. I did notice that a small crystal Y4 had become broken off the motherboard somehow, so I soldered it back, not sure if its on correctly, though.

Anyone have any ideas? bad motherboard?

 

[F. Hopper]

I'm guessing by the view count vs reply count that i've answered my own question. I guess I will patiently wait for a motherboard to appear on ebay.

 

[chrismacguy]

I'm guessing by the view count vs reply count that i've answered my own question. I guess I will patiently wait for a motherboard to appear on ebay.

Id check the PSU is producing more than the power to light the light on the top (My PowerBook 1400 once displayed the on-light constantly and it was a dead DC board - it didnt make the Bong) - If its not that its likely the logic board. I must also congratulate you on your continuing efforts to keep these elderly machines going.

 

[MacTech68]

Several ideas:

Check that the power switch at the rear isn't locked to the "auto on" position. Turning it 90 deg locks it pushed in (a cheap "power on after power-fail" setting).

Check that there isn't something leaning on the keyboard power button. Yep, I actually had a customer ring for this 'fault'. Try disconnecting the keyboard too.

Remove the PRAM battery and mains power overnight. Check that the battery measures at least 3.4 Volts or replace it.

Most likely the surface mount capacitors have leaked and corroded tracks. The caps look like this. I repaired quite a few IIci and IIcx machines cleaning the board, replacing the caps and using prototyping wire to jumper etched/damaged tracks. Not easy or short work but if you find all the broken tracks and replace the caps with SMD Tantalums, it can be most rewarding.

Leaked caps will often appear to have a "wet" area around them, and you may even see blackened or copper oxide (blue) colored corrosion.

 

[F. Hopper]

i can rule out the power switch, keyboard power button, and corroded tracks, as I have been testing the motherboard out of the case plugged into nothing but the monitor.

 

[MacTech68]

i can rule out the power switch, keyboard power button, and corroded tracks, as I have been testing the motherboard out of the case plugged into nothing but the monitor.

Hmmm...

Try removing the PRAM battery and leaving it with no power (mains included) overnight. Check both ADB (keyboard) ports for shorts both sides of the filter module. Pulling pin 2 to ground momentarily powers on the machine.

I would even suggest gently warming the surface mount capacitors with a hair-dryer and re-testing. This was the old Sony method of diagnosing failing SMD electros.

Otherwise, the soft-power-on signal to the PSU module could be faulty within the PSU itself. The following diagram (albeit for a IIci) should help.

http://home.earthlink.net/~gamba2/images/macIIcisch.GIF

 

[F. Hopper]

pulled the PRAM battery, will let it sit over night. I don't think its the PSU as i have already tested the board with another known good one.