Macintosh Portable M5126 SCSI 5V

[jcs]

I recapped an M5126 and swapped its PMGR chip and everything seems to be working now, except internal SCSI termination power. With it booted, I show 12V on SCSI pins 31 and 32 but only about 2.3V on pins 33 and 34. The middle pin on the floppy interface shows 5.2V, as do other 5V things like the I/O interfaces.

Q211 shows 6.6V. I found this diagram for the M5120 showing 5V on SCSI comes from C20 but there's no C20 on the M5126. What feeds the 5V into the SCSI connector?

 

[finkmac]

The Macintosh Portable does not supply termination power.

 

[SuperSVGA]

It comes from Q20 to the left of the battery connector. The gate of Q20 is connected to R173 on the bottom side, which then goes to the Power Manager. I believe the Power Manager should be pulling the gate low when the SCSI +5V is on, so you should see near 0V on the gate.

 

[jcs]

It comes from Q20 to the left of the battery connector. The gate of Q20 is connected to R173 on the bottom side, which then goes to the Power Manager. I believe the Power Manager should be pulling the gate low when the SCSI +5V is on, so you should see near 0V on the gate.

Thanks. I read 0V, 5.2V, 5.2V on Q20. As soon as I test R173 with my multimeter, it activates the 5V going to the SCSI port and powers up the drive...

 

[SuperSVGA]

Thanks. I read 0V, 5.2V, 5.2V on Q20.

Well the middle leg of Q20 should connect directly to the SCSI +5V, so if you're seeing 5.2V there then you should see it on the SCSI pins as well.