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Thread: International (PAL) Apple IIe circuit diagram

  1. #1

    Default International (PAL) Apple IIe circuit diagram

    Hi,

    I'm playing with Apple IIes again having just picked a couple up for coffee money.

    I've fixed one (two shorted caps). The second is proving more problematic. There is no clock signal on the CPU preventing it from firing. However, I can't trace this back as I don't have a circuit diagram for these PAL "International" Apple IIes. I have plenty of diagrams and literature for the U.S. Apple IIe (SAMS computer facts, Understanding the Apple IIe) but they all have circuit and layout diagrams which relate just to the Apple IIe on that side of the world. The PAL international version is very different. It's not just the layout of the chips. Different ICs are used.

    Click here for a large picture.

    If anyone can point me to a PAL circuit diagram (or just knows what to check) I'll be grateful.

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  2. #2
    Join Date
    Jun 2003
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    Northwest (UK)
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    Default

    Hi Tez,

    Just uploaded this PDF...

    'Understanding the Apple IIe' (136mb) It might be the same one as you have but this copy certainly has video schematics for the PAL IIe. Let me know if you need any more help fault finding as I have a couple if IIe's with this board.

    Cheers,
    Dave

  3. #3

    Default

    Hi Dave,

    Thanks for that. Yes, I already had that document and noticed the PAL video circuits but they don't seem to help much. It just deals with the video circuit and doesn't seem to show the pathways associated with the clock.

    I must point out I don't have a scope at the moment...just a logic probe. Also I have a working board set up right next to the faulty one so I can compare readings.

    Here's the problem. I've been using the SAM's troubleshooting guide on page 0014 of the March 1985 Apple 2e 6502 Computer Repair Information document. It says..

    If the reset clircuit is working (it is) check the 1MZ clock waveform at pin 37 of IC UC4. If the waveform is missing (it is!) check the waveform at pin 14 of the Program Logic Array IC (UD1). (I did, and there seems to be a waveform there).

    Now here's where it get's tricky. The guide then says "If the waveform is present at pin 5 of IC UB8 and pin 6 of IC UB8 logic reading is Low, check IC UB8 by substitution."

    The Sams documents says UB8 is a SN74S02N. There is only one of these chips I can see on the PAL Apple IIe board, to the right of the IOU. However, pin 5 and 6 of 74S02N both show a pulse on the faulty board(?). On the working board though, there is no pulse(?) Pin 5 is high and pin 6 is low with no pulse on either, which SAMs would indicate is faulty. Yet this board is working fine?? This is the opposite to what I expected.

    My conclusion is that this 74S02 on this PAL Apple IIe perhaps plays a different role to that on a US one? Maybe the clock circuit uses gates in different logic chips. If I had a schematic of the clock circuit in this machine I could trace it right back to the crystal.

    I appreciate that a pulse might not necessarily mean a clock signal. It does seem to be a very regular pulse though.

    Any pointers will be appreciated.
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  4. #4
    Join Date
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    I've just pulled out my spare board and a multimeter and traced the connection on the 74LS02. It would seem that the chip performs the same functions on both boards.

    According to the schematics, the output of Pin14 of the PAL is fed into pin 5 of the 74LS02. The output of the gate then being fed into pin 37 of the CPU. Easy to swap out the LS02 just to rule that chip out? Also, is pad X5 still intact? (Just above keyboard encoder)

  5. #5

    Default

    Quote Originally Posted by Moonferret View Post
    I've just pulled out my spare board and a multimeter and traced the connection on the 74LS02. It would seem that the chip performs the same functions on both boards.
    Thanks for doing that.

    Quote Originally Posted by Moonferret View Post
    According to the schematics, the output of Pin14 of the PAL is fed into pin 5 of the 74LS02. The output of the gate then being fed into pin 37 of the CPU. Easy to swap out the LS02 just to rule that chip out? Also, is pad X5 still intact? (Just above keyboard encoder)
    That is very weird. I can't explain why the working board showed no pulse on this pin. I'll have to recheck. Maybe I measured the wrong thing here.

    On the non-working board, the logic on pin 5 and 6 SEEMS ok. No, I can't just swap it out as it's soldered in unfortunately. I'll check the pad.

    SAMs facts does have a section on the Crystal Oscillator and dividers. The chip references are wrong though. However, I can match up the reference code with the actual chip type using a table in SAMs. This should help me identify them on the board. It seems the PAL model might use the same chips for the clock circuit after all.

    I'll do some further work tonight.

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  6. #6

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    Dave (and anyone else who might be interested)

    I continued work on this. I am embarrased to say those strange readings were due to the fact that I was reading the ICs upside down (duh!). In these Apple boards the chips don't point upwards!

    Anyway, it seems that the same ICs are used on these PAL boards, only in a different position. I worked my way through the SAMs manual. Some strange readings on the two 74LS244 so I replaced them. No change (and the strange readings are still there).

    Something is screwed with the data and address lines. The table below shows what SAMs says to expect on certain CPU pins, and where I've found things to be different.

    P=pulse, H=High and L=Low

    Pin on CPU, expected reading, my reading

    22 A12, L, H
    23 A13, L, H
    24 A14, P, H
    25 A15, P, H
    26 D7, P, H
    27 D6, P, H
    28 D5, P, H

    31, D2, P, L

    34 R/W, P, H

    I have a feeling it could be a RAM issue. Certainly ROM is ok, as are all the other Apple proprietry chips as I've swapped them out with a spare board. Unfortunately the RAM is soldered in.

    I'm not going to spend too much time on this board. It's really a spare parts one I thought would be fun to fix. It's proved to be tricker than I expected. I might have to wait until I get access to a scope again before resuming.

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  7. #7

    Default

    I've fixed this now. It was indeed RAM. Actually two or more ICs were at fault.

    Diagnosised in the end by a laborious socketing of the ICs. At least the RAM can easily be replaced if another chip goes
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  8. #8
    Join Date
    Jun 2003
    Location
    Northwest (UK)
    Posts
    389

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    Hi Tez,

    Sorry, been away and missed your posts. Congrats on getting the board working!!
    If it makes you feel any better about looking at the pinouts the wrong way round, I've done the same on more than one occasion and that includes when breadboarding / soldering!

    Dave

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