Failing DISK II controller

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Last seen: 4 days 21 hours ago
Joined: Sep 15 2023 - 11:16
Posts: 2
Failing DISK II controller



I made a mistake, I inserted a disk II controller le wrong way...

several chips burnt... including some on the analogic board of the Disk II drive....


the analogic board of the disk II is repaired and works

all the burnt chips on the Disk II controller replaced including the P5 prom  (all are working)


the Apple try to boot but the engine never stop and it never read

I tested all chip

(everything works fine with a second controller)

(even if I swap the chips between the good and the failing board)


there are no traces of burnt tracks on the board


any idea of the failing element ?  perhaps a transistor ?








Last seen: 1 hour 32 min ago
Joined: Dec 20 2003 - 10:38
Posts: 539
What about the capacitors?

What about the capacitors?

Last seen: 18 min 42 sec ago
Joined: Jul 5 2018 - 09:44
Posts: 1890
Aren't there a couple of caps

Wayne has a point there...  aren't there a couple of caps on the Disk II Controller card that might pop if shorted?  Also possibly a resistor that might burn through.  The resistors are easy to check, the caps you'd have to remove from circuit unless they are obviously blown.  It should be possible to look at Q1 with a logic analyser also.


CVT's picture
Last seen: 3 hours 45 min ago
Joined: Aug 9 2022 - 00:48
Posts: 677
I would align 2 Apple II slot

I would align 2 Apple II slot pinout diagrams in a 69 position and see what went where. Of particular interest will be the power rail pins. Also I would check the +12V on the motherboard, as it powers the motor of the floppy drive.

Last seen: 4 days 21 hours ago
Joined: Sep 15 2023 - 11:16
Posts: 2
Blown Disk II controller
Wayne wrote:

What about the capacitors?

no traces of burn the capacitor and resitors looks ok


I checked what happened when I plugged the board the wrong way

apparently +12V was on +5V

+5 on +12V

and -12 / -5 on A5/A6 (I guess it explain why the prom was blown)


Apparently the 2 MPSU51 were connected to +12V instead of  5V

that's why I think they are blown too...


the  P5 prom remplaced by a SN74S471 works, the card is recognized


I ordered some MPSU51 to replace them


no problem with the machine, the machine works fine with another Disk II controler


thanks to all











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