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Thread: Mac LC Woes - ADB Bus Issue?

  1. #1
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    Default Mac LC Woes - ADB Bus Issue?

    So, Got a LC unit, DOA , full recap, ( PSU and Logic Board ) , system boots up, but no mouse or keyboard, I am getting 5V to the Keyboard.. I am suspecting a bad EGRET chip... can't seem to locate the Schematic, i did compare to LC II and LC III and did seem to have simular design, but not exact part location or part numbers... Hate to toss a perfecting good unit becuase of no ADB Bus, does anybody know if the EGRET chip had a second ADB Port?

    Any suggestions on what to do ?

    I was hoping to replace my Performa 575 with this LC as it has a solid case.. ( My 575 case was shattered in a move, and now is in a box labeled "basket case" never to be at a retro convention again )

  2. #2
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    So you have 5V to pin 3? ADB signal is line 1 on the plug and if it is like the mac II line goes through a RFI filter and has a 470 ohm resister tying it to the 5v line.
    What I collect: 68K/Early PPC Mac, DOS/Win 3.1 era machines, Amiga/ST, C64/128
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  3. #3
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    Quote Originally Posted by Unknown_K View Post
    So you have 5V to pin 3? ADB signal is line 1 on the plug and if it is like the mac II line goes through a RFI filter and has a 470 ohm resister tying it to the 5v line.
    Yes, 5v at Pin 3 , Ground on Pin 4, I am going to see if i can find the actual transister that ties Pin 19 and Pin 20 to the ADB Data bus, i bet that sucker is bad, there is one transister located near the ADB Inductor/filter and the Egret IC.. but none of the pins were tied to ground, so i am thinking that maybe the issue.

  4. #4
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    The LC III doesn't list the part number, but the LC II shows it as a 1AM which is a MMBT3904 seems pretty standard design, Pin 20 ( PA6 ) to ADB Data Pin 1, and connected to the Collector of the 3904 via 22 Ohm resister and pulled up to +5 via 470 Ohm Resister, The Base of the Transister is driven from Pin 19 ( PA7 ) via a 1K ohm Resister

    So looks like Pin 19 is Transmit, and Pin 20 is Receive

  5. #5
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    ok, i think i may have found it, it , i appear to have an "open" between Pin 20 ( PA6 ) and the Resister junction of the 22 Ohm Resister and Pin 1 of the ADB Port.

    pin 1 is reading 470ohm to 5v rail, and 22ohm to Collector of the Transister tied to GND/Pin19(pa7).

    However, Pin 20 is floating between the EGRET and ADB Bus.

  6. #6
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    ok, issue fixed, there was a broken trace at a via , i removed the egret chip and cleaned up under it, but that was not needed, the broken trace was located before the egret.


  7. #7
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    Quote Originally Posted by ki4swy View Post
    i did compare to LC II and LC III and did seem to have simular design, but not exact part location or part numbers...
    The LC and LC II are basically the same machine, but the LC II has a 68030 instead of a 68020. The LC III is a completely new and superior design that has nothing in common with the former two machines other than form factor.

    The LC and LC II are compromised designs where the 32 bit CPU is placed on a 16 bit bus and has an artificial 10 MB memory limit in the memory controller. The LC III is a completely different design with a faster 25 or 33 MHz 68030 on a proper 32 bit bus and can have up to 36 MB of RAM and 768k of VRAM for 15 bit color at 640x480.

    I have an LC III, and have also had the former two machines in the past, they were quite dreadful. I even have a PDS upgrade card for them which brings it up to LC III processor specs, but the 16 bit bus and limited memory really kills performance.

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