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 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.
