IIci turns on by itself...

Colleton

New Tinkerer
Apr 20, 2022
14
6
3
Okay, I have another IIci logic board that turns on by itself and will not turn off via software.

I have built a PSU extension cable so I am finally able to make measurements on the power on circuitry.

As far as turn-on goes, on a known good logic board with the PSU plugged in UE13 pins 1 and 2 are both a TTL logic high (5vdc), producing a TTL logic low (0vdc) on UE13 pin 6. With this state, the machine will not turn on until the power switch is depressed (normal operation).

Just FYI, all of the caps, chips, transistors and diodes in the power-on circuitry have been replaced.

On this machine, UE13 pin 1 is a TTL high (5vdc), but pin 2 is only 1.4vdc, so the NAND gate on UE13 sees this as a TTL low, producing a TTL high on pin 6. This turns the machine on as though the power switch was pushed. If I jumper 5vdc to pin 2 the machine will not turn on until I remove the jumper. I've measured continuity between UE13 pin 2 to C12, and from C12 to R43 and R44. I measured continuity from the PSU connector pin 10 (5vdc continuous) to R44. I have good continuity everywhere I've measured. I have replaced C12 and C13 just because but there was no change.

The one thing I've noticed is that when I measure continuity from PSU connector pin 10 to the positive side of C12 on a known good board it reads 100K in about 1.5 seconds. When I make the same measurement on this board it takes 30 seconds and never quite reaches 100K. The 100K ohms of resistance that it's reading is caused by C13 as I am not measuring through C12. I've removed C13 and get an instantaneous 0.3 ohms of resistance from PSU pin 10 and the positive side of C12.

It looks to me as though there's a short somewhere on the 5vdc continuous line but I can't find it. Any thoughts or suggestions on how to isolate this problem would be greatly appreciated.
 

jajan547

Active Tinkerer
Mar 25, 2022
732
295
63
North Carolina, USA
Okay, I have another IIci logic board that turns on by itself and will not turn off via software.

I have built a PSU extension cable so I am finally able to make measurements on the power on circuitry.

As far as turn-on goes, on a known good logic board with the PSU plugged in UE13 pins 1 and 2 are both a TTL logic high (5vdc), producing a TTL logic low (0vdc) on UE13 pin 6. With this state, the machine will not turn on until the power switch is depressed (normal operation).

Just FYI, all of the caps, chips, transistors and diodes in the power-on circuitry have been replaced.

On this machine, UE13 pin 1 is a TTL high (5vdc), but pin 2 is only 1.4vdc, so the NAND gate on UE13 sees this as a TTL low, producing a TTL high on pin 6. This turns the machine on as though the power switch was pushed. If I jumper 5vdc to pin 2 the machine will not turn on until I remove the jumper. I've measured continuity between UE13 pin 2 to C12, and from C12 to R43 and R44. I measured continuity from the PSU connector pin 10 (5vdc continuous) to R44. I have good continuity everywhere I've measured. I have replaced C12 and C13 just because but there was no change.

The one thing I've noticed is that when I measure continuity from PSU connector pin 10 to the positive side of C12 on a known good board it reads 100K in about 1.5 seconds. When I make the same measurement on this board it takes 30 seconds and never quite reaches 100K. The 100K ohms of resistance that it's reading is caused by C13 as I am not measuring through C12. I've removed C13 and get an instantaneous 0.3 ohms of resistance from PSU pin 10 and the positive side of C12.

It looks to me as though there's a short somewhere on the 5vdc continuous line but I can't find it. Any thoughts or suggestions on how to isolate this problem would be greatly appreciated.
I had a very similar issue on an LC475. The vias near the power connector were corroded, machine powered on and ran but any instance of a power down in the menu the machine would lock up.
 

retr01

Senior Tinkerer
Jun 6, 2022
2,473
1
796
113
Utah, USA
retr01.com
Hi @Colleton! :)👋

Have you checked the power switch button on the back of the IIci? Often, it is why the IIci powers itself on as soon as it is powered off. According to Apple:

The Macintosh IIci uses a switch-and-shut-down circuit to control the power supply. The circuit controls the power supply through the power failure warning signal on the NuBus interface. The circuit attempts to switch on the power supply when the power switch is pressed and for two seconds afterward. ADB keyboards have a secondary power switch. Pressing either power switch discharges a capacitor (through a resistor) that activates the power-on circuitry. The capacitor receives its charge through a soft-power circuit that is active even when the computer is off. As long as the computer is plugged in, the power supply will switch on the computer within two to four seconds. This circuit works in conjunction with the locking power switch on the rear of the computer. This switch can be locked in an on position, which allows the computer to restart as soon as it detects AC power. In effect, if there is a power failure and the computer shuts off, it will restart as soon as power is restored. If this switch is in the off position, the computer will not turn on automatically. This feature is most valuable when using the computer as a file server. The power-off function is under software control. In the Finder, the Shut Down command from the Special menu activates the power-off function. This software control allows the computer to finish pending activity before switching off. On the other hand, the power-down switch generates a signal that turns off the computer after two milliseconds without going through software.

1.26/Basics October 1991 Macintosh Ilcx/IIci/Quadra 700 - Apple Service Technical Procedures Macintosh Family Volume Two (1990-1992)

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Cheers!
 
Last edited:

speakers

Tinkerer
Nov 5, 2021
98
76
18
San Jose, CA
peak-weber.net
Don't get me started about this logic :cool:... Here's a couple of my experiences that:
  1. On a Iicx with similar logic, the machine would start using the power switch but not do a soft-startup. This was the result of a re-capping boo-boo with a couple (C15/C16) being reversed.
  2. On a IIci which I brought back from the dead, there was the opposite problem: after being left plugged-in but powered-off for a number of hours, it would fail to soft-start. I replaced UE13 thinking it suspect but to no avail. It turned out to be gunk/short under capacitor C13. Re-installation after cleaning and application of more solder mask fixed it.
 
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Colleton

New Tinkerer
Apr 20, 2022
14
6
3
@jajan547 : I've looked at the vias connecting C12 to R43/R44 (and on back to pin 10 of the PSU connector). Everything looks good and ohms correctly. I have cleaned out and re-soldered the via for C12 using a bit of fine wire to ensure connectivity on each side.

@retr01 : Yes, I've checked the switch, it is not set to always on.

@speakers : Yeah, the solution is never as straight-forward as you would like. I have inspected to make sure that all of the components are installed correctly. As any technician knows, the first thing you should do when you encounter a problem is to check your work.
 
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