Image Map Image Map
Page 2 of 2 FirstFirst 12
Results 11 to 17 of 17

Thread: Rescueing an Apple III

  1. #11

    Default

    Quote Originally Posted by whizzi View Post
    the CPU looks like it's working. It gets a 4.2v at the 5v line (which I think is a little low, other IC's get the correct 5v, like the memory). RSET is low, IRQ is pulsing, clocks are there..

    This is not a MOS 6502, but a SynerTek 6502A. Not sure if there's any difference in pinout. The CPU does get hot, so it seems to be doing something. Also IC's next to it are getting warm, so my next suspect will be the memory...
    To me these symptoms look like the CPU has issues (you didn't read the previous suggestions, did you?).
    Reset should not remain low, pull the CPU from the socket and check that it still remains low (it means you have to debug the reset circuitry) or it goes high (it means you have to substitute the CPU).
    4.2V is too low but it might be related to the reset issues.
    SY6502 is totally equivalent to a MOS6502 (as it would be a R-6502 and many others).
    (And by the way, did you check the clocks too?)

    Frank IZ8DWF

  2. #12

    Default

    Yes, I've read your previous post and I just noticed a mistake, the CPU gets 4.9v . The RES get a 4.2v signal..

    Current situation (measured with oscilloscope)
    Pin 2 (RDY) High
    Pin 3 (clock 1 out) Pulsating 2.03MHz
    Pin 4 (IRQ) high
    Pin 6 (NMI) high
    Pin 7 (SYNC) Low

    Pin 40 (RES) High
    Pin 39 (Clock 2 OUT) Pulsating 2.03MHz
    Pin 38 (S.O.) Low
    Pin 37 (Clock IN) Pulsating 2.03MHz
    Pin 34 (RW) High
    I have a few computers and only have 612m2 for computers.

  3. #13

    Default

    Quote Originally Posted by whizzi View Post
    Yes, I've read your previous post and I just noticed a mistake, the CPU gets 4.9v . The RES get a 4.2v signal..

    Current situation (measured with oscilloscope)
    Pin 2 (RDY) High
    Pin 3 (clock 1 out) Pulsating 2.03MHz
    Pin 4 (IRQ) high
    Pin 6 (NMI) high
    Pin 7 (SYNC) Low

    Pin 40 (RES) High
    Pin 39 (Clock 2 OUT) Pulsating 2.03MHz
    Pin 38 (S.O.) Low
    Pin 37 (Clock IN) Pulsating 2.03MHz
    Pin 34 (RW) High
    ok that's much better. Can you check if SYNC pulses for a short time after reset? This tells us that the CPU is trying to execute something and more likely gets soon lost into an invalid opcode or something similar.
    Can you obtain or make a 6502 NOP generator?

    Frank

  4. #14

    Default

    20190920_223804.jpg

    Here's the sync signal. It keeps on pulsing continuously.
    I have a few computers and only have 612m2 for computers.

  5. #15

    Default

    Quote Originally Posted by whizzi View Post
    20190920_223804.jpg

    Here's the sync signal. It keeps on pulsing continuously.
    On your previous message, SYNC was "low". I didn't get what's the vertical scale on your scope. Is it 100mV per division (so the signal pulses are roguhly 300mV?).
    If sync is pulsing to valid TTL levels, the CPU is indeed executing something and we can move on other tests.
    Frank

  6. #16

    Default

    That's probably because there's an autoset on the oscilloscope. I did an autoset on the clock, so it'll display 2 clockpulses @ 2 Mhz. The 600 KHz is much slower and showed the part that was low. As far as I know, this means the CPU is working and acting like it should.

    I did put the probe of the oscilloscope on the memory and it showed on one pin a pulsating signal, I guess I have to test them all and figure out which pins are output so I can hopefully get which one(s) are/is broken.
    I have a few computers and only have 612m2 for computers.

  7. #17

    Default

    Quote Originally Posted by whizzi View Post
    That's probably because there's an autoset on the oscilloscope. I did an autoset on the clock, so it'll display 2 clockpulses @ 2 Mhz. The 600 KHz is much slower and showed the part that was low. As far as I know, this means the CPU is working and acting like it should.

    I did put the probe of the oscilloscope on the memory and it showed on one pin a pulsating signal, I guess I have to test them all and figure out which pins are output so I can hopefully get which one(s) are/is broken.
    Ok on the autoset, but what was the vertical volts per division?
    I don't know how you are supposed to identify a bad RAM with an oscilloscope, I've never been able to do that.
    I suggest you to try reading all the ROMs out of circuit with an EPROM reader (if it supports the right pinout) or with an adapter.
    And again the best suggestion is to make a 6502 NOP generator and follow all address signals through all buffers and multiplexers, that will rule out a lot of other problems.
    Usually mT RAM faults can be diagnosed by piggybacking with known good chips, but you still have about 40% chances that a given failed chip can't be identified with this technique.
    Frank

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •