PowerBook 3400c unexplained behaviour

[Tazintosh]

Good evening folks,
Long time no see, sorry about that.

I own a PowerBook 3400c (200 MHz + 16 MB expansion module for a total of 32 MB).
Recently, I bough a 180 MHz one, but equipped with the 128 MB expansion module!!! I think I was searching this module since 7 years.

Long story short, I took the 128 MB module, put it on my 200 MHz, restarted and… well, it's complicated.
Need the long story long:

• The Mac chimes.
• Memory module is recognized (black screen right after pressing the startup button is —much— longer than with 32 MB total), no memory related "beep", etc.
• Once the back screen (checking memory?) is done, the HD starts spinning.
• The computer then switch to the "white/grey" screen. It's the one shown right before the extensions loading process.
• Problem is that the computer will never go beyond this screen… unless… (more on that soon)
• Sometimes on this grey screen, I'm getting the cursor (top left), sometimes not. Sometimes it cannot be moved, sometimes it's awfully laggy.
• IF AND ONLY IF, I plug an ADB mouse, the Mac will try to boot, BUT I'm getting the "? floppy" icon. This is extremely weird since the disk was working and booting before disassembling.
• If I then unplug the ADB mouse, the "? floppy" icon will stop blinking and if I plug it right back, it will blink (!!!) (blink = by design)
• If I then plug a PCMCIA card on it with a system, the mac will boot instantly from it.
• Once on Finder, still no mouse moving (neither with ADB or Trackpad), all must be done by keybaord.
• Inserting a CD on the drive will never mount it. And if I open the trackpad panel, the system will complain the Trackpad is not available on this computer (!). About those two (cd and trackpad, I'm not sure though if it's system related since I've not digg into it, the PCMCIA was provided and already installed with the 180 MHz model.
• I've physically swapped the two trackpads without success.
• I've rolled back to the original configuration (32 MB) without success.
• I've done multiple PRAM resets. Multiple Power Management resets.
• No fall, no hit, no PRAM battery installed, no leaks.
• I'm normally not a complete newbie at this kind of manipulations.
• I've swapped the two HDs and both are non recognised, for which my guts clearly screams "Impossible!".
• I've also remounted the 180MHz and… it does exactly the same thing!!! (to the only difference the trackpad is fluid on the grey screen)

Since then, I've been able to get the 180 MHz running again.
Not sure precisely what —made— it though.
Tried again resets, had to remove the CD drive (this for sure), got it to boot on the PCMCIA and eventually, the internal drive did showed up.
Since I got a cursor working I could also select it as default boot drive.
Restarted, set all extensions to System Complete and put back the CD drive.

Now it's booting fully normally.

"Mine" however, is still blocked at this "grey" screen and deeply dependent on an ADB mouse being plugged or not. Even if the cursor is unusable.

But read this one, personally, I never encountered this in my life:

• Plugged-in ADB mouse, with some change chance, system boots and extensions loads.
• While extension were loading, I unplugged the mouse and the extensions stopped loading (!). Plugin it back and the extensions were continuing (!!!), and so on (!!!!!!)
• Once booted on Finder, I opened a window in List view: unplugged the mouse, pressed 5x Keyboard down arrow: nothing. Replugged the mouse and the selection on screen moved by 5 (!!!!!!!!!!!!!!!!!!!!!!)

Sadly without a working cursor, I cannot do everything and I remains blocked.

I never add issues with the keyboard, it's always working (ADB plugged or not).
But only plugin an ADB mouse allows any process to continue (even thought the cursor movement makes it literally unusable).
Unplugging it stops any process.


Situation update:

• I've removed internal HD and CD drives, keyboard and trackpad from the equation: Computer boots. Plugin in an ADB mouse/keyboard allows to control it.
• Attaching back the internal HD (actually trying with multiple HDs): it's not mounting, not discovered in Disk Utility, rarely only, system will ask to reformat it (and fail). Drives have been formatted clean using my Pismo and an attached USB IDE drive.
• I've swapped the Power Supply Board which had a noisy capacitor: no better.
• Removed the internal drive, attached back the trackpad only (tried with the trackpads from both 3400's: Computer won't continue booting until an ADB is attached.

Note that when attaching the trackpad, I've noticed (and suspected for a long time already) a lot of "interferences". If I never touch the trackpad (with ADB mouse plugged-in), the mouse will behave almost normally, BUT, if I ever touch the trackpad, the cursor will auto move, giggle all around etc. Again, this with —both— trackpads.

AI opinion:

The PowerBook 3400c uses the ADB (Apple Desktop Bus) for the internal trackpad, keyboard, and external mouse port. The symptoms you described—laggy cursor, boot hanging at grey screen, extensions pausing when mouse is unplugged—are classic signs of ADB Bus noise or a partial short.

Why the mouse "fixes" it: The ADB bus is likely flooded with garbage signals from the damaged trackpad connection. Plugging in the mouse adds electrical resistance (termination) to the bus, which stabilizes the signal just enough for the CPU to process the next interrupt. When you unplug it, the noise returns, and the system hangs waiting for a clean signal.

Why the drive icons blink/don't mount: On Old World ROM Macs (like the 3400c), if the ADB controller is stuck or flooding the CPU with interrupts, the rest of the boot process (SCSI/ATA probing) times out or gets suppressed.

• I've searched for anything that could be responsible for the "interference noise" and have yet not found anything wrong (to my eyes). Unplugged, the IR, the Brightness & Speaker Grill, etc.
• I've removed 90% of all computer's parts, inspected, mounted them back, no better.

What worries me the most is the "Replace the logic board" which is written down way too often on the troubleshooting pages of the repair manual of the 3400c. 😵

If you have —any— clue, I would greatly appreciate it!

Best,