A3000D SCSI Diode 800 Clarification

[danbeaver]

To Clarify: The black stripe of Diode 800 points TOWARD the motherboard.

A failure of 5 volts on pin 25 of the SCSI port (failure = lack) when powered up, or pin 25 "goes to ground" when powered down means diode D800 is going to ruin your day.  I have two A3000D's and the silk screening is opposite on both (one points in and one points out) and neither diode was correct, with both pointing out.

     __________________
  x x x x x x x x x x x x x
\  o x x x x x x x x x x x /
  __________________

o = Pin 25 looking end-on of the female SCSI socket

[danbeaver]

Yes, I found everything BUT which way the diode goes.

Yep, it's certainly been documented over the years.
http://amiga.serveftp.net/A3000_HardwareGuide/scsi-termpower.htm

Another common problem is that people plug a Centronics printer cable into the external SCSI port, which shorts the +5V termination supply to ground, which causes the diode to go short circuit and the excess current causes the track on the bottom side of the board to fuse open circuit.

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[danbeaver]

Mech,

 I knew you would know, but after a very bad guess(or two) I just pulled the schematics, checked it with a multimeter and found the the black stripe "points toward the port" but that pin is going toward the motherboard.

Funny that it worked the way it had blown (+5 powered, but ground when power off).  Also had to resolve whether the new GALS I burned were flakey.

Oh, I found blacked traces underneath, but they tested good.

Would one place the fuse in line (between the diode and port) as makes the most sense to a chemistry major, or some other place?

[danbeaver]

Again as a chem type person, I would want the fuse to blow before the diode; granted with my soldering skills, I would end up replacing the fuse and diode at the same time. But anything that works to protect the MB traces is better than redoing them.

@Mech (Mike), Do you work on (others) stuff like this?  I try to refurb my stuff, but there are sticky areas (SMD for example) that didn't exist when I was a kid (soldering my O27 train stuff).  I learned my DC current from that hobby.  I remember you reprogramming NEC USB (EP)ROMs so I know you have a "skill set."