Macintosh LCII failure

[indibil]

Hello.

I haven't received any replies since my last post. Perhaps I didn't use the oscilloscope correctly; it was my first time, but I think it illustrates the problem.

Today I ran a test. I had the motherboard from an LC3 and swapped the two sound chips. The LC2 with the LC3 chip has the same problem, and the LC3 with the LC2 chip works perfectly.

So, if the fault is located further back from the measured point, and it's not the sound chip, what could be failing? A transistor? A ceramic capacitor?

I'm lost. Can anyone guide me?

Thanks.

 

[Garrett]

That's interesting that when swapping DFACs, the issue stays with the board, not the sound chip.

I had an LCII with bad audio once, and replacing the DFAC did solve my issue.

 

[indibil]

Hi!

I have some news. This afternoon I started testing. I focused on the MC34119 (op-amp), and coincidentally, when I soldered a wire to pin 4 to check for a signal with the oscilloscope, the Macintosh LC powered on without sound.

I checked that pin and it wasn't connected to R74 and R75, so the trace must have been damaged, and when I soldered the wire, I must have broken the connection. I repaired the trace and now I have 100% working sound. I have the Lemmings song stuck in my head, haha, I had it playing for over 10 minutes verifying that everything was working.

Thanks to everyone for your help.

 

[David Cook]

Hi!

I have some news. This afternoon I started testing. I focused on the MC34119 (op-amp), and coincidentally, when I soldered a wire to pin 4 to check for a signal with the oscilloscope, the Macintosh LC powered on without sound.

I checked that pin and it wasn't connected to R74 and R75, so the trace must have been damaged, and when I soldered the wire, I must have broken the connection. I repaired the trace and now I have 100% working sound. I have the Lemmings song stuck in my head, haha, I had it playing for over 10 minutes verifying that everything was working.

Thanks to everyone for your help.

That's wonderful news! Congratulations. Your persistence paid off.

 

[iigs123]

I'm glad you got it working! It can be frustrating to dig deep into this. I'm feeling inspired to get back to my Q610's goofy vram issue!

 

[Arrowhead]

Not to bring up an old thread, but I have an LC II that bongs nice and loud; the screen goes from gray to white... but no mouse pointer, no keyboard light... the fan spins. Swapped PSU with known working unit, same results, removed the 2 RAM simms, same issue, removed and reset the ROMs, same issue.

Any pointers on what to check or test? The board was recently recapped, and all the caps are installed correctly, with no pads lifting, etc.

Brian @ Arrowhead

 

[Mark_W]

the screen goes from gray to white

Is VRAM installed? LC II has no onboard VRAM and if no VRAM is installed, it will boot to a blank screen. Also try a different VRAM stick.

Also plug in a hard drive or bluescsi and see if the motherboard tries to boot from it.

 

[Arrowhead]

Doesn't the LC II have 256k of VRAM on the motherboard? I have tried both it's original hard drive and my external BlueSCSI, all the same result.

Brian @ Arrowhead

 

[Mark_W]

Doesn't the LC II have 256k of VRAM on the motherboard? I have tried both it's original hard drive and my external BlueSCSI, all the same result.

Brian @ Arrowhead

No, only LC III and LC 520/550 has onboard vram.

There's no activity on scsi bus at all? You can also try to insert a floppy disk, see if it tries to boot from floppy drive.