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There are two common reasons why it might be doing this: 1. A short. The power supply protects the board if too much current is being drawn. For example, if a tantalum capacitor has gone bad and is shorting the +5V and GND. With power off and a multimeter, check the SCSI power connector pins...

To close the loop on this thread, the pencil marking around R151 saying "delete" means R151 needs to be removed from the board to switch a Centris 650 to a Quadra 650/800. With R151 removed, then the LED jumper toggles between Q650 and Q800. See...

What size would you like it to open to instead?

Another oddity on this particular Macintosh II are three ground contact fingers atop the power supply. I haven't seen this on any other Macintosh II power supply. The contacts do not appear on other power supplies with the same part number: CR-45 (68-1073-5) 699-0389 REV E6, M23. Yet, they seem...

I found another variant. This is 820-0163 A (circa late 1987 early 1988). It has the part renumbering like it's successor, 820-0163 B. It also has the holes for battery holders, like its successor. However, the polarity peg holes are non-plated on A, but plated on B. Most significantly...

Have you run MacTest Pro on the IIfx? It might be worth just double checking that there isn't a hardware issue. https://macintoshgarden.org/apps/apple-mactest-pro

I have not encountered that problem, but I appreciate your write up. I will remember to check this in the future. Thank you.

Very cool (pun intended). Have you found that power usage drops significantly when the screen is blank? And thus, the fan is needed less? I wonder whether it would be better to monitor temperature instead.

Could you tell me a little bit more about your technique. Do you have a particular brand of reef lighting that you recommend?

This is fascinating. Thanks for bringing it to our attention, @terminalcancerman

I can't begin to tell you the number of times I've taken pictures of some repair that I performed, only to see it up close and think "There's no way I can post that!" So, I appreciate your acknowledgement that sometimes it doesn't have to be pretty to illustrate a point.

Good advice!

@MacOSMonkey definitely welcome back! You set me up for success by suggesting using a 27C256 and providing the .bin file. It worked perfectly, the first time. The SuperVideo software recognized and displayed the version as 1.3. That was the good news. The bad news is the performance is...

Fascinating.

Wow! Amazing. By the way, there was a revision to the board. They moved a capacitor. Maybe to help pick and place of the IC? Very questionable to run a trace under a tiny SMT, though.

I confirmed that the Mac IIcx does not natively support the HD20. I confirmed that the Mac IIcx does mount the HD20 when booted from 6.0.3 with the Mac II HD20 init on a 800k floppy. The Mac IIcx crashes when booted from 6.0.8 with the Mac II HD20 init on a 800k floppy. I'm beginning to think...

I can confirm that the IIx does not include native support for the HD-20. The IIx does mount an HD20 when booted from 6.0.3. However, I'm getting a crash with 6.0.8 with the Mac II HD20 init. The IIx with HD20 also worked in 7.1! I booted this from a hard drive with Mac II HD20 in the...

Yes, but the IIx also doesn't have an external floppy port, yet is listed as supporting the Floppy EMU on the internal port. So, I was just wondering why he didn't put the same note on the II. That's an awesome factoid that I was unaware of. My first Mac was a dual-floppy SE!

@bigmessowires Nevermind. This is not compatible with the IIfx. So, no HD-20 support.

Bus error on the IIfx with System 6.0.5 and 6.0.8.