68kMLA Classic Interface

This is a version of the 68kMLA forums for viewing on your favorite old mac. Visitors on modern platforms may prefer the main site.

Click here to select a new forum.
Macintosh SE Dead ADB Ports/IC
Posted by: jasonfortheworld on 2026-04-24 10:12:32
Does anyone have any experience with dead ADB ports on an SE? I stupidly plugged an S-Video cable that bridges pins 1 and 2 together and 3 and 4 together which I'm pretty sure killed the ADB ports on 2 SEs that I have.

I tested one of boards and it looks like the Tokin filter is still good which makes me afraid that I either fried the ADB chip or the VIA. I don't see any physical damage on the board.

I'm not sure how best to test ths ADB and VIA chip for damage, or if there are even replacements out there.

I came across an old thread about making modern replacements for the ADB chip, but it's a bit beyond me. I also saw there's an adapter board for ADB chips from later Macs.

Also, the computer boots up just fine, but no combinations of keyboards and mice will work.
Posted by: robin-fo on 2026-04-24 10:36:15
Did you test the ADB transistor?
Posted by: jasonfortheworld on 2026-04-24 10:38:16
Did you test the ADB transistor?
I have not! I've never worked the ADB system before. Do you know which transistor it is on the board?
Posted by: finkmac on 2026-04-24 10:40:23
aren't the ADB ports usually fused? Maybe not on the SE though.
Posted by: jasonfortheworld on 2026-04-24 20:47:08
aren't the ADB ports usually fused? Maybe not on the SE though.
I'm not to sure. I don't see anything that would be an ADB fuse, but I'm not the most familiar with the ADB circuitry.
Posted by: robin-fo on 2026-04-25 08:41:43
I have not! I've never worked the ADB system before. Do you know which transistor it is on the board?
Posted by: jasonfortheworld on 2026-04-27 09:24:00
I checked out the transistor and was getting what I think are some bad readigns from it after compairing it to a good SE logic board.

  • Red probe to Base and Black probe to Emitter
    .6v - matches a known good board

  • Red probe to Base and Black probe to Collector
    .57v - matches a known good board

  • .7v and .688v for swapping each probe - matches a known good Board

  • Black on collector and red on emmiter is.163v while red on collector and black on emitter is 135 ohms (there was no voltage reading for this one). This does not matche my good board. On the known good transistor black on collector and red on emmiter is .32v while red on collector and black on emmiter is .27v

I'm going to order some transistors and see if that's the issue. Fingers crossed it's this easy.
Posted by: jasonfortheworld on 2026-04-29 15:34:28
Back to the drawing board. Looks like the discrepancies in the two transistors were thanks to me testing it while still on the board in both instances.

I swapped out the transistor anyway just in case but unfortunately it didn't fix the dead ADB ports.
Posted by: jasonfortheworld on 2026-05-04 17:02:06
Well I think I've reached the end of what I'm willing to do. I programmed a new ADB chip using this HEX:

github.com

GitHub - lampmerchant/macseadb88: Firmware for PIC16F88/87 as a drop-in replacement for the Macintosh SE's stock ADB controller.

Firmware for PIC16F88/87 as a drop-in replacement for the Macintosh SE's stock ADB controller. - lampmerchant/macseadb88
github.com github.com

But no luck unfortunatly. These board are beyond my desire for fixing.
1