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Thread: Apple ii europlus

  1. #21
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    Quote Originally Posted by retrogear View Post
    The back of this pdf has some theory how they operate. It covers several models.
    ftp://ftp.apple.asimov.net/pub/apple...lies_Aug82.pdf

    Larry G
    Thanks, I do actually have that document, and I did electronic engineering at uni plus an apprenticeship in control and instrumentation, but I can't quit see how the oscillator works in this circuit, especially with an electrolytic.

  2. #22
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    Quote Originally Posted by Gary C View Post
    Thanks, I do actually have that document, and I did electronic engineering at uni plus an apprenticeship in control and instrumentation, but I can't quit see how the oscillator works in this circuit, especially with an electrolytic.
    Its a switcher... they break the normal rules using voodoo. As with all dark magic you don't ask how it works you just accept that it does.

  3. #23
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    Quote Originally Posted by David_M View Post
    Its a switcher... they break the normal rules using voodoo. As with all dark magic you don't ask how it works you just accept that it does.
    , or doesn't in my case. Need to buy some magic smoke to refill it

  4. #24
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    Quote Originally Posted by David_M View Post
    It could be the small signal transistors and on a rare occasion and open circuit resistor on the primary side. However the most likely problem is that feedback transformer, 70% of the faults I get that are not fixed by a recap are that transformer.
    Attachment 46737

    If it faulty you will read about 68 ohms across these terminals in circuit. To fix it wind on 30-35 turns of enameled copper wire in the same direction as the original windings and terminate the ends on the terminals marked. I've found that 30-35 gives about an output voltage close enough to spec that I dont have to mess about with the adjustment resistors.
    Ok, you might have something.

    T3 (is this the one you mean?) has three coils, two have ~2ohm but the one that connects between R3 and Q1 is giving a dead short.

    So need to fix T3, however it's totally encapsulated and cutting it free to rewind would be a nightmare. The spec sheet doesn't list a generic part number so I imagine this is going to take a bit of finding.

  5. #25
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    Quote Originally Posted by Gary C View Post
    Ok, you might have something.

    T3 (is this the one you mean?) has three coils, two have ~2ohm but the one that connects between R3 and Q1 is giving a dead short.

    So need to fix T3, however it's totally encapsulated and cutting it free to rewind would be a nightmare. The spec sheet doesn't list a generic part number so I imagine this is going to take a bit of finding.
    Normally when they fail they fail open circuit.
    I just pulled one from a working supply.
    The winding in parallel with R7 reads 1.7 ohms 1.75mH
    The winding between R3 and Q1 reads 0.2 ohms
    The winding at the other end reads 1.7 ohms 1.72mH
    All measurements were taken with the transformer out of circuit.

    It looks like yours is OK.

    Now you problem could be either on the primary or secondary side. Do you get a ticking? If so then look at components on the secondary side.
    There are a number of resistors on the primary side that can cause the symptom if they go open circuit. Visually look for a dark spot on one of them. Failing that pull one end from the board and measure them.

    You got lucky on this one, generally the are easier to fix lol

    Just a thought I once accidentally cut a lead on R8 (120 ohm) while cutting the glue off a capacitor when doing a recap. That gave exactly the same symptoms as you are having... you didn't happen to slip with the knife like me did you?
    Last edited by David_M; July 15th, 2018 at 05:48 AM.

  6. #26
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    Well, thanks to your help, the supply is now working, cheers

    However, I'm not totally sure why. Soldered T3 in again and powered it up for a check with the scope and the fuse blew.

    Doh! Using scopes 101, remember the ref is grounded !

    So this morning, checked the tracks for any blobs that might be shorting then powered it up again and, hey presto it's working.

    Now, it might have been a bad connection on T3 that was made good when reinstalled but I may still have a dry joint. Anyway it's working reliably for the moment, thanks for you advice.

    So onto the apple, powers up to a screen full of ? So it's time to reseat the chips as a first gambit.

    At least I'm moving forward (though that's not what the wife thinks)

  7. #27
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    I wrote this ROM to aid in diagnosing RAM problems. It will boot a machine even when RAM is not functional.
    But to use it in a 2+ you will need to use make a small daughter board to adapt the ROM select lines to work for an eprom.

    https://drive.google.com/open?id=0B8...Fd0aEJhNVN6RHM

  8. #28
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    Initial probes look a bit odd.

    Clocks are ok (though not exactly neat square waves) and data bus has activity, irq,nmi,rdy seem to be at the appropriate levels, but the address bus is at 3V steady on every pin and so is rd/wr.

    Would this point to a buffer or CPU problem I wonder. Certainly the cpu isn't doing anything useful.

    Sorry to all for asking for lots of help, hopefully one day I can return the favour.

  9. #29

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    Do you have +12V and -5V ? Those go to the 4116 ram. Do any chips get hot? You could pull all rom except F8 and all ram except the first 16k in row C to see if it runs. Could be the 6502.

    Larry G

  10. #30
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    The clock is never a neat square wave, so that is not unusual.
    What side of the buffer are you getting the low levels on the address bus? If it is the CPU side then it may be a CPU problem.

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