Mac Plus doesn't boot when peripherals are attached.

[Maku]

Grabbed a "non-working" Mac Plus off a friend recently and first boot worked just fine, got the beep and "no disk" icon. Then I connected the mouse, keyboard and external floppy drive (as internal was supposedly dead) and the Mac stopped booting. Garbled screen, no beep, you know the drill.

Tested the voltage rails and they seem to be off:
5V reading at 4.2-4.4V
12V reading at 10.9~11.2V

Voltage potentiometer doesn't seem to be doing anything, BUT reducing the screen brightness lets me boot with mouse attached (with brightness at max even just mouse being connected crashes the CPU).

Anyone ever seen anything like this? Never services a Mac before, not sure what the next steps are.

I ordered replacement cap kit, hoping this helps...

 

[cheesestraws]

5V reading at 4.2-4.4V
12V reading at 10.9~11.2V

yeah, these are definitely too low

BUT reducing the screen brightness lets me boot with mouse attached

This all sounds to me like the PSU is on its way out. Or, perhaps more accurately, has already left. The PSU on these things was somewhat anæmic when it was new and time has not improved things. Recapping the analogue board is worth a try: one of the major issues with the PSUs in these is that they cook themselves, and electrolytic caps aren't good at heat.

There's a good PDF out there that gives a tour of how the AB works: do you have a copy?

Finally, please be careful when working with the AB, as with all mains stuff. I don't know how comfortable you are working with the mains, but two especial points about the Plus AB:

• One of the heatsinks is live. Be careful sticking your hands in towarrds the AB with the power on
• I'm pretty sure some of the big caps don't have bleeder resistors - make sure you short them out with a suitable screwdriver before poking, or you may get a belt.

 

[Maku]

I found this PDF regarding the circuitry - is this the one you're talking about?

Mac Plus, analog board schematic and Tech Notes : Free Download, Borrow, and Streaming : Internet Archive

archive.org

I've read about this possible being caused by faulty U3 chip, not sure where to get an replacement though...

 

[bibilit]

Hi, yes probably U3, easy to remove and replace (socketed)

Can be found easily online

 

[cheesestraws]

I found this PDF regarding the circuitry - is this the one you're talking about?

Yup, that's the one. That will tell you what's going on in the PSU.

 

[Maku]

Yup, that was it, replaced U3 and it's working now! Unfortunately seems the keyboard I got is dead, so will need to continue working on it.

I guess some people have project cars, I seem to have a project computer.

 

[bibilit]

Probably the cable and not the keyboard.

Keyboards are not troublesome, the cable in the other hand can.

 

[Maku]

Probably the cable and not the keyboard.

Keyboards are not troublesome, the cable in the other hand can.

I've checked the continuity and it sees fine.

 

[Maku]

Happy to hear other diagnostic ideas though!

 

[bibilit]

Can also be a broken trace, in the connector side of the keyboard, or in the Logic Board side

 

[Lord Refa]

I recently was getting a pair of SE's running and found that when plugged in, the SE wouldn't come on, turned out to be a bad cable. May want to check that with a different one to see if that helps.

 

[Maku]

Oh, it was the U3 chip, replaced now and it's working great.