• Updated 2023-07-12: Hello, Guest! Welcome back, and be sure to check out this follow-up post about our outage a week or so ago.

IIci logic board issue

Byte Knight

Well-known member
Hey guys,

I recently acquired a IIci that just gives me a gray screen upon boot. I've got 12V and 5V at the HD power connector, so I'm assuming the PS is good. The board has some corrosion in the expected places near the surface caps, and C10, C13, and the IC at UD13 basically fell off with a little probing. I gave the board an isopropyl bath, recapped it, and put the IC at UD13 back in its spot. I checked all the traces (I think) in the affected areas and get good continuity. Unfortunately I continue to just get the gray screen upon boot. I can turn the computer on with the keyboard and button on the back but can't turn it off without unplugging it.

IMG-5886.jpg IMG-5887.jpg

I'm also getting continuity with all the 5V rails to all the GND rails, even at the PS receptor (pins 2,3,4 with 5,6,7). I'm guessing that's not a good thing for normal computer function. Any idea what would cause this?

Thanks!
 

imactheknife

Well-known member
Your start up circuit still not working properly if you have to unplug it. Would check little diodes and your hc132, hc74 chips. Do you get s chime?
 

volvo242gt

Well-known member
Has there been any battery damage? Almost seems like there might be some ROM issue, in addition to a rotted trace somewhere.
 

Byte Knight

Well-known member
Your start up circuit still not working properly if you have to unplug it. Would check little diodes and your hc132, hc74 chips. Do you get s chime?
I get no chime when I turn it on. And thanks for the help!

What do you think about the 5V and ground rails having continuity?
 

Byte Knight

Well-known member
Has there been any battery damage? Almost seems like there might be some ROM issue, in addition to a rotted trace somewhere.
No battery damage. I thought about the ROM too, so I tried a Mac ROM-inator II in the rom slot and removed the jumper but got the same gray screen...
 

imactheknife

Well-known member
No battery damage. I thought about the ROM too, so I tried a Mac ROM-inator II in the rom slot and removed the jumper but got the same gray screen...
So, probably not rom related in my opinion if rominator gave same result. . All the chips around all the caps can either have bad traces, corrosion still. Have you checked continuity on all those?
 

Byte Knight

Well-known member
So, probably not rom related in my opinion if rominator gave same result. . All the chips around all the caps can either have bad traces, corrosion still. Have you checked continuity on all those?
All of the traces are surprisingly good. Looking at the schematic, I'm beginning to wonder if the chip at UB13 may be bad and maybe shorting out the 5V to ground? I'm probably at the point where I need to start swapping out components...

Screen Shot 2022-12-04 at 11.06.05 PM.png
 

mdeverhart

Well-known member
What resistance are you seeing between 5V and GND on the logic board? Do you have the power supply connected when you measure it, or just the board by itself? If you power up the logic board, what voltage do you measure across the yellow capacitors? When you measured 12V and 5V on the HD connector, was the logic board plugged in as well?

I wouldn’t expect it to do much of anything if 5V was hard-shorted to GND, so I’m a little confused.
 

mdeverhart

Well-known member
Thinking about this some more, I don’t see anything in the soft power circuit that would cause the issues you’re seeing, other than something that would drag the power rails out of spec. The schematic you posted show what appear to be the primary voltage rails:

+12V, +11V, +5V, +4V, +5V Standby, -12V, and -5V

Can you take measurements of all of those rails when it’s powered on and showing the gray screen?
 

AwkwardPotato

Well-known member
The +5V rail you measure at any one point on the board is the same +5V you'll read at any other point, same with ground, each is just one big net. If you put your meter in ohms mode and measure across +5V and ground, you can expect it to read something in the few tens of ohms. If there was truly a low-ohms short on the +5V, the protection in the power supply would've kicked in and the computer would've been totally lifeless.

If the cap damage was bad enough for a chip to almost detach from the board, there's likely some trace rot as well, especially up in the audio section. The startup circuit may have problems of its own but they shouldn't result in a gray screen. Make sure both Sony sound chips produce an appropriate reset (A/BRST#) signal (should be low when the computer turns on and then go high a fraction of a second later). Then check that those reset signals make it to where they need to go everywhere else on the board. After that, start checking traces between Sony chips<->ASC<->MDU+CPU<->ROMs.
 

mdeverhart

Well-known member
The +5V rail you measure at any one point on the board is the same +5V you'll read at any other point, same with ground, each is just one big net.
Sorry, yes, I wasn’t clear - it wasn’t that I was expecting the 5V rail to vary across the board - I just wasn’t sure which voltage rail any of the capacitors were on, and figured it was worth checking a few of them to see if we could get data on some of the other rails. Checking based on the schematic makes more sense than random checks like I originally suggested 😀

Good call on checking the reset lines from the audio chip and the RAM/ROM traces.
 

Byte Knight

Well-known member
The +5V rail you measure at any one point on the board is the same +5V you'll read at any other point, same with ground, each is just one big net. If you put your meter in ohms mode and measure across +5V and ground, you can expect it to read something in the few tens of ohms. If there was truly a low-ohms short on the +5V, the protection in the power supply would've kicked in and the computer would've been totally lifeless.
Ok, thanks for the explanation - that was driving me crazy. I'm measuring 16 ohms from the 5V to GND. My IIsi doesn't give continuity from 5V to GND so I thought something was drastically wrong with the IIci.
If the cap damage was bad enough for a chip to almost detach from the board, there's likely some trace rot as well, especially up in the audio section. The startup circuit may have problems of its own but they shouldn't result in a gray screen. Make sure both Sony sound chips produce an appropriate reset (A/BRST#) signal (should be low when the computer turns on and then go high a fraction of a second later). Then check that those reset signals make it to where they need to go everywhere else on the board. After that, start checking traces between Sony chips<->ASC<->MDU+CPU<->ROMs.
Good to know. So looking at the audio circuit below, I see that A/BRST go into pin 6 of the L & R sound chips. Do you know where the signal goes from there?

Screen Shot 2022-12-05 at 10.34.04 PM.png
 

mdeverhart

Well-known member
It looks like /ARST is the main reset for the logic board. As far as I can see (looking at the schematics on my phone…) /BRST only goes to the MDU (U11). The audio sound chips drive the reset to the other components.

If you have an o’scope or logic analyzer, it would definitely be worth checking if /ARST and /BRST are asserted low at power on and then released.
 
Top