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Macintosh Classic stalls booting when anything is plugged into the SCSI bus

Verault

Well-known member
I figured the same on the voltages. Not low enough to be a problem.
I opened another thread asking about the SCSI chip: https://68kmla.org/bb/index.php?thr...-onboard-scsi-chip-and-have-a-question.39143/

I ordered the -40 variant and it came last night. I put it in today and when I did all I got was a b/w checkerboard. I check continuity on every pin it was in there fine. I took it out and got a sad mac (no scsi chip in place). I put the old chip back in and I started getting a

0000000e / 0000ffff sad mac error​

I looked it up and signs pointed to the UH6 74ls174. I took it off and noticed pin 1 and pin 16 both were tarnished and were not making real contact from leaky caps in the past Im sure). I cleaned the pads but a pin broke off the IC. I swapped it for another and installed it and checked continuity. As soon as I did I got the machine back to the state it was at before. I can boot off floppies but SCSI still stalls. I dont know if that -40 SCSI IC is incompatible or bad at this time because of the rabbit hole with the LS174 chip. I will check the pinouts in your last post and see what I find and post back.
 

Verault

Well-known member
So Going by your image I have traced continuity from pins 8 to11 and 17 to 19. Only one doesnt have continuity Pin 18 of the scsi chip should be going to pin 8 of the 680000 cpu. I am getting NO coninuity. Could you please check that your get it on your board? I am getting continuity from pin 18 of the scsi chip following the trace and it goes under the SCC chio UD5. I can get it that far but I am guessing it goes to a VIA under that chip and then I dont know where.

Edit: I have a battery damaged parts board and I cannot get continuity between pins 18 of the scsi chip to pin 8 of the CPU on that board either. Please confirm if you can or not.


Ok I have removed the ZILOG SCC chip from my bad board to find and follow the trace of pin 18 from the SCSI chip. IT goes around a bunch of vias and I follow it to pin 54 of UI4 BBU. I can get that on my bad board and the one I am trying to repair. So I get continuity on all of those pins.
 
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Verault

Well-known member
I tried installing the 53C80-40 into the machine and again and the same thing. The machine just shows checkerboard. so the 53C80-40 is not compatible with the 53C80 that came in the classic. Im at a loss unless I can get a working 53C80.
 

ymk

Well-known member
I checked my board and SCSI controller pin 18 doesn't run to 68000 pin 8. It does run to pin 54 of the BBU. I mixed up the /LDS and /LDS' labels:

1633918683889.jpeg
 

Verault

Well-known member
So I installed the New zilog Scsi chip. Same exact problem. But now my machine is only reporitng 512KB with no expansion RAM card installed. And 2048KB with the expansion ram card! What gives? I am starting to think the problem was something other than the scsi chip all along and now its manifesting in the RAM? What do I do now? This machine is frustrating me so much and I already sunk all this time and money into it.
 

ymk

Well-known member
Wild guess, one of the higher address lines? Eliminating the SCSI chip as a potential cause is still progress.
 

Verault

Well-known member
YMK I appreciate all your help I honestly do. But seems noone else is chiming in at all and this machine has turned into a money pit. Im just going to shove it in the closet and go back to repairing some MAcintosh II boards as those are more straightforward than these classics. I cant say I am a fan of the classics design I just wanted one in my collection. And of the 3 I worked on they have all been problematic and terrible. So thanks again. But seems this machine is dead for the time being.
 

ymk

Well-known member
I think a Classic PCB is in development. I don't blame you for setting it aside.

I've got several logic board puzzles in storage, but too many better things to work on.
 
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