Kaypro II repair - CP/M

This is a discussion on Kaypro II repair - CP/M ; Hello, I am trying to repair a non working Kaypro II, board model 81-240-n, boot eprom 81-232-n. The computer powers up, but the screen gets filled with garbled characters that keep blinking in a dual pattern, always the same, the ...

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  1. Kaypro II repair

    Hello,
    I am trying to repair a non working Kaypro II, board model 81-240-n,
    boot eprom 81-232-n.
    The computer powers up, but the screen gets filled with garbled
    characters that keep blinking in a dual pattern, always the same, the
    reset button has no effect, drives lights stay on. Nothing more happens.
    I've already succesfully tested the Z80 chips (CPU, PIOs, SIO) on
    another Z80 based computer and they all work, RAM ICs (4264) and video
    RAM(2114) seem to be all fine. Same goes for chargen and boot eproms.
    Meanwhile I am desoldering some TTL ICs that I can test on other
    equipents, so to make sure they are fine.
    Does anyone familiar with this computer can give me any hints or
    suggestion on what could be the faulty component?

    Thanks in advance
    Andrea



    --

    http://myretrocomputing.altervista.org

  2. Re: Kaypro II repair

    On Wed, 10 Sep 2008 19:22:38 +0200, myemail@fattimie.i (andrea) wrote:

    >Hello,
    >I am trying to repair a non working Kaypro II, board model 81-240-n,
    >boot eprom 81-232-n.
    >The computer powers up, but the screen gets filled with garbled
    >characters that keep blinking in a dual pattern, always the same, the
    >reset button has no effect, drives lights stay on. Nothing more happens.
    >I've already succesfully tested the Z80 chips (CPU, PIOs, SIO) on
    >another Z80 based computer and they all work, RAM ICs (4264) and video
    >RAM(2114) seem to be all fine. Same goes for chargen and boot eproms.
    >Meanwhile I am desoldering some TTL ICs that I can test on other
    >equipents, so to make sure they are fine.
    >Does anyone familiar with this computer can give me any hints or
    >suggestion on what could be the faulty component?
    >
    >Thanks in advance
    >Andrea


    First make sure all the power supplly voltages ae there. It's common
    for the PS to fail. Seconds if you see two (or more) blinking cursors
    that indicates a likely monitor problem and that will smear the
    characters making them garbled.

    For the life of me I rarely have seen a Z80 fail, but I see people
    test them every time. Must be the sockets.

    I'd look at the reset circuit. Make sure all the clock signals are
    present. before desoldering anything. I've seen way too much
    hardware trashed by soldering damage and substitution of bad
    componenets into the system where good were.

    Another source of failure on old equipment is the sockets themselves.

    Allison

  3. Re: Kaypro II repair

    wrote:

    > First make sure all the power supplly voltages ae there. It's common
    > for the PS to fail.


    First of all, thanks for your help.
    The PSU is fine, all voltages are present.

    > Seconds if you see two (or more) blinking cursors
    > that indicates a likely monitor problem and that will smear the
    > characters making them garbled.


    I've uploaded an image of the screen. Here you are:



    > For the life of me I rarely have seen a Z80 fail, but I see people
    > test them every time. Must be the sockets.
    >
    > I'd look at the reset circuit. Make sure all the clock signals are
    > present. before desoldering anything.


    Unfortunately I can't test clock signals due to lack of both hardware
    and knowledge, I am going the hard way desoldering ICs. I've already
    done some damage in the past on other stuff so I've learned the lesson.
    I am confident I can handle it now without making mistakes with the
    solder machine.

    > Another source of failure on old equipment is the sockets themselves.


    This is something I haven't checked yet, thanks for the advice.

    Regards
    Andrea

    --

    http://myretrocomputing.altervista.org

  4. Re: Kaypro II repair

    andrea wrote:
    >
    > I am trying to repair a non working Kaypro II, board model
    > 81-240-n, boot eprom 81-232-n.
    >
    > The computer powers up, but the screen gets filled with garbled
    > characters that keep blinking in a dual pattern, always the same,
    > the reset button has no effect, drives lights stay on. Nothing
    > more happens. I've already succesfully tested the Z80 chips (CPU,
    > PIOs, SIO) on another Z80 based computer and they all work, RAM
    > ICs (4264) and video RAM(2114) seem to be all fine. Same goes for
    > chargen and boot eproms. Meanwhile I am desoldering some TTL ICs
    > that I can test on other equipents, so to make sure they are
    > fine. Does anyone familiar with this computer can give me any
    > hints or suggestion on what could be the faulty component?


    There is a circuit in there (I forget the details) that configures
    a 2048 byte memory into a 24 (actually 25) by 80 memory. This is
    the video storage. It leaves a few bytes unused. Your symptoms
    sound to me as if something could be wrong with that decoder.

    --
    [mail]: Chuck F (cbfalconer at maineline dot net)
    [page]:
    Try the download section.

  5. Re: Kaypro II repair

    andrea wrote:

    > I am trying to repair a non working Kaypro II, board model 81-240-n,
    > boot eprom 81-232-n.
    > The computer powers up, but the screen gets filled with garbled
    > characters that keep blinking in a dual pattern, always the same, the
    > reset button has no effect, drives lights stay on. Nothing more happens.


    That is usually what is stored in memory, but not cleared
    (by the running software) on power up.

    > I've already succesfully tested the Z80 chips (CPU, PIOs, SIO) on
    > another Z80 based computer and they all work, RAM ICs (4264) and video
    > RAM(2114) seem to be all fine. Same goes for chargen and boot eproms.
    > Meanwhile I am desoldering some TTL ICs that I can test on other
    > equipents, so to make sure they are fine.
    > Does anyone familiar with this computer can give me any hints or
    > suggestion on what could be the faulty component?


    As far as I know, the least likely cause is a bad IC.
    It just happens so rarely, except possibly ones that have
    outside connections (I/O ports) that it should be the last
    thing to check. Other than bad contacts on sockets
    and connectors, maybe most likely are electrolytic
    capacitors. The power supply voltage may be right,
    but have a lot of ripple. Measure the power supply
    outputs with an AC voltmeter.

    An AC voltmeter can tell you if there is a changing
    signal on a pin. A running system should have AC
    signals on the data bus and most of the lines of the
    address bus. You can trace those through buffers and
    to RAM and ROM chips. Measure all the pins on the Z80,
    and be sure that it is supposed to run in that state.
    Check for pins like hold or wait interrupt inputs.
    It is a little easier with a scope, but it is possible
    with an AC voltmeter and a little luck. Bad solder
    joints are also possible, but removing all the soldered
    in ICs and testing them is not likely to help.

    TTL is very tough, unless you really stress it.
    (I knew someone once with a wire wrapped 8080 board,
    where one of the pins went through the electrical tape
    on the power line fuse. That is one of the few ways
    to kill TTL. Vcc above about 7 volts also, but that
    will kill most of the board, not just one or two ICs.)

    -- glen



  6. Re: Kaypro II repair

    On Thu, 11 Sep 2008 15:38:26 +0200, myemail@fattimie.i (andrea) wrote:

    > wrote:
    >
    >> First make sure all the power supplly voltages ae there. It's common
    >> for the PS to fail.

    >
    >First of all, thanks for your help.
    >The PSU is fine, all voltages are present.
    >
    >> Seconds if you see two (or more) blinking cursors
    >> that indicates a likely monitor problem and that will smear the
    >> characters making them garbled.

    >
    >I've uploaded an image of the screen. Here you are:
    >
    >


    This is good as the monitor is working and most of the video logic
    too. It most of the video logic were brioken you would not see that.

    >
    >> For the life of me I rarely have seen a Z80 fail, but I see people
    >> test them every time. Must be the sockets.
    >>
    >> I'd look at the reset circuit. Make sure all the clock signals are
    >> present. before desoldering anything.

    >
    >Unfortunately I can't test clock signals due to lack of both hardware
    >and knowledge, I am going the hard way desoldering ICs. I've already
    >done some damage in the past on other stuff so I've learned the lesson.
    >I am confident I can handle it now without making mistakes with the
    >solder machine.


    A handy test tool if you can find or make one is a logic level probe.
    It lights a let if logic is 1 another for logic 0 and if there are
    pulses a oneshot gets fired to light a third led. RS and otehr used
    to sell them and they are handy for looking in there. Typically cheap
    20-50$.

    Allison




    >
    >> Another source of failure on old equipment is the sockets themselves.

    >
    >This is something I haven't checked yet, thanks for the advice.
    >
    >Regards
    >Andrea



  7. Re: Kaypro II repair

    CBFalconer wrote:

    > There is a circuit in there (I forget the details) that configures
    > a 2048 byte memory into a 24 (actually 25) by 80 memory. This is
    > the video storage. It leaves a few bytes unused. Your symptoms
    > sound to me as if something could be wrong with that decoder.


    I would say more likely that memory was never cleared.

    Static RAM has a fairly high tendency to power up to
    the same value each time. DRAM is somewhat less likely
    to do that, but it may still be possible.

    -- glen


  8. Re: Kaypro II repair

    On Sep 11, 8:49*pm, glen herrmannsfeldt wrote:
    > CBFalconer wrote:
    > > There is a circuit in there (I forget the details) that configures
    > > a 2048 byte memory into a 24 (actually 25) by 80 memory. *This is
    > > the video storage. *It leaves a few bytes unused. *Your symptoms
    > > sound to me as if something could be wrong with that decoder.

    >
    > I would say more likely that memory was never cleared.
    >
    > Static RAM has a fairly high tendency to power up to
    > the same value each time. *DRAM is somewhat less likely
    > to do that, but it may still be possible.
    >
    > -- glen


    Does the screen always display the same text? If so, is it possible
    that the memory being displayed is actually in the ROM and not RAM?
    Maybe a bad address line somewhere?

    Bill H

  9. Re: Kaypro II repair

    Bill H wrote:

    > Does the screen always display the same text?


    Yes, it does.

    > If so, is it possible
    > that the memory being displayed is actually in the ROM and not RAM?


    What I am going to say is surely questionable, due to my lack of
    electronics knowledge, but I have my explaination for this behaviour.
    Since video ram is static unless it get cleared by the system it will
    always send the same output on screen. I can change the display output
    by moving the 2114 ICs on the board, but for a fixed configuration I get
    always the same output.

    Andrea

    --

    http://myretrocomputing.altervista.org

  10. Re: Kaypro II repair

    glen herrmannsfeldt wrote:

    > Other than bad contacts on sockets
    > and connectors, maybe most likely are electrolytic
    > capacitors.


    There are just 2 small electrolytics capacitors and I've already
    replaced them without any luck. Yesterday I also heatened all big ICs
    sockets pins so to "refresh" the welds. Nothing changed yet. I will do
    this also for the small ICs later.

    >The power supply voltage may be right,
    > but have a lot of ripple. Measure the power supply
    > outputs with an AC voltmeter.
    >
    > An AC voltmeter can tell you if there is a changing
    > signal on a pin. A running system should have AC
    > signals on the data bus and most of the lines of the
    > address bus. You can trace those through buffers and
    > to RAM and ROM chips. Measure all the pins on the Z80,
    > and be sure that it is supposed to run in that state.
    > Check for pins like hold or wait interrupt inputs.
    > It is a little easier with a scope, but it is possible
    > with an AC voltmeter and a little luck.


    I am not sure I understand how to make this. As I've already said, I
    have only very basic knowledge of electronics, so this sounds somewhat
    arcane to me. I have a digital tester (multimeter?) that can test AC
    too, after the check I noticed this strange behavior: at power up
    sometimes on the address bus pins (1-5, 30-40) I get 1.6V, while on
    clock pin (6) I get 0V. After pressing the reset button address bus pins
    go to about 6V while clock goes 8.8V. Sometimes instead at power up I
    get 2.4V on the address bus pins and 8.8V on clock. If I press the reset
    none of these values change. I guess this is not normal... is it?

    Andrea

    --

    http://myretrocomputing.altervista.org

  11. Re: Kaypro II repair

    On Sep 12, 7:34 am, myem...@fattimie.i (andrea) wrote:
    >
    > What I am going to say is surely questionable, due to my lack of
    > electronics knowledge, but I have my explaination for this behaviour.
    > Since video ram is static unless it get cleared by the system it will
    > always send the same output on screen. I can change the display output
    > by moving the 2114 ICs on the board, but for a fixed configuration I get
    > always the same output.
    >
    > Andrea


    I think it's admirable that you are working to repair this Kaypro.
    But, the problem is that there is no clear "symptom" to guide your
    repair process. And it's very difficult to unsolder one chip after
    another without damaging the circuit board. So in my somewhat informed
    opinion, I don't think chip-by-chip external testing will result in a
    repair except by pure luck.

    I don't have the schematics in front of me, but typically this kind of
    design has a bunch of logic chips which display memory contents in
    video; and then there's the Z80 side which runs the programs and
    keyboard and all that. Those two sets of electronics share the RAM
    memory.

    Now, the video side is working - you have a display of memory
    contents, the video display is stable. The specifics of what is
    displayed is random and happenstance - if it changes when you change
    RAM chips, that's not informative. But you can in principle consider
    the video display logic as "OK". (So you need to know what chips are
    part of the "video display" because you don't have to test them.)

    But, the Z80 is not running, or not running for very long, or is
    running but crashing. No "logic probe" work will easily determine what
    it is doing or why. A logic probe is not an oscilloscope, it will not
    show you timing and sequence of logic signals. that's what an
    oscilloscope does. It will show if some connection changes state. But
    without knowledge of what it "should" be doing, that information is
    not useful.

    It's hard to say from this point "here is what you should do". The
    problem is you don't have any equipment and you don't have much
    digital circuit knowledge. But that is what it takes to diagnose the
    problems and find what is at fault. The "test each chip" method can
    work if the problem is a failed chip. But it could be a broken PC
    board, or a component like a capacitor or resistor, or a "cold solder
    joint" that does not make a good connection. And you can create
    problems by burning up a circuit trace, since the chips are not
    socketed.

    Again, it's commendable you are working on this Kaypro. But the Kaypro
    does not "care" about your intentions. There is some problem on it,
    which may be signifigant like a bad logic chip, or subtle like bad
    timing, or maybe a ROM which has scrambled its code from age. To fix
    it, someone with knowledge and an oscilloscope or logic analyzer and
    the schematics has to sit down with it and start looking at signals.

    It seems to me, you have an opportunity and a choice. You can learn
    more about digital logic and microprocessor logic - old books are for
    sale or can be borrowed from a library. You can acquire an
    oscilloscope (any hamfest will have them). Then you can start looking
    at the Kaypro and understand what's going on. Or, you can improve your
    soldering skills, test each chip, and hope you get lucky and not only
    find the problem but avoid creating another one.

    Some people may decide these remarks are unfair or discouraging.
    Again, these are chips on a board. The board and chips don't care
    about intentions or hopes. They do what they do. They perform in a
    certain way, by theory and design. Then, in the real world, they
    perform in different ways because of a bad connection, or a component
    like a capacitor changes value, or once in awhile due to a failure of
    a logic gate. And in the real world, THINGS FAIL. This is not Star
    Trek where computers are found running after 100, 200 years.

    So, if you can learn these things, get this equipment, you'll have
    more opportunities to keep other Kaypro's and other microcomputers of
    the era running. For they will ALL FAIL IN TIME.

    Herb Johnson
    retrotechnology.com

  12. Re: Kaypro II repair

    Bill H wrote:

    > Does the screen always display the same text? If so, is it possible
    > that the memory being displayed is actually in the ROM and not RAM?
    > Maybe a bad address line somewhere?


    SRAM has a fairly high chance of powering up to the same
    value each time. Maybe not exactly, but close.

    I don't know specifically the kaypro, but it was fairly
    common to have dedicated SRAM display memory with a mux
    so that the processor could read/write to it. I believe
    that is what others have suggested.

    Not so much later as DRAM became more popular, putting the
    display in main memory, with alternating between processor
    and display. (DRAM was fast enough relative to processor
    speeds of the time.) That is still done on many boards
    with built-in display logic.

    -- glen


  13. Re: Kaypro II repair

    On Fri, 12 Sep 2008 07:24:33 -0700 (PDT), Herb Johnson
    wrote:

    >On Sep 12, 7:34 am, myem...@fattimie.i (andrea) wrote:
    >>
    >> What I am going to say is surely questionable, due to my lack of
    >> electronics knowledge, but I have my explaination for this behaviour.
    >> Since video ram is static unless it get cleared by the system it will
    >> always send the same output on screen. I can change the display output
    >> by moving the 2114 ICs on the board, but for a fixed configuration I get
    >> always the same output.
    >>
    >> Andrea

    >
    >I think it's admirable that you are working to repair this Kaypro.
    >But, the problem is that there is no clear "symptom" to guide your
    >repair process. And it's very difficult to unsolder one chip after
    >another without damaging the circuit board. So in my somewhat informed
    >opinion, I don't think chip-by-chip external testing will result in a
    >repair except by pure luck.


    I would agree having worked on a few.

    >
    >I don't have the schematics in front of me, but typically this kind of
    >design has a bunch of logic chips which display memory contents in
    >video; and then there's the Z80 side which runs the programs and
    >keyboard and all that. Those two sets of electronics share the RAM
    >memory.


    Not the case for a kaypro. Kaypro has a seperate video ram and
    applications (processor ram). However if the Z80 is doing nothing
    the video ram with never be updated.

    >
    >Now, the video side is working - you have a display of memory
    >contents, the video display is stable. The specifics of what is
    >displayed is random and happenstance - if it changes when you change
    >RAM chips, that's not informative. But you can in principle consider
    >the video display logic as "OK". (So you need to know what chips are
    >part of the "video display" because you don't have to test them.)


    Also since the 6845 has to be programmed to get to that state (does
    not power up to that by default) the Z80 must be doing something
    and either getting lost or cant talk to the display ram to update it.

    >But, the Z80 is not running, or not running for very long, or is
    >running but crashing. No "logic probe" work will easily determine what
    >it is doing or why. A logic probe is not an oscilloscope, it will not
    >show you timing and sequence of logic signals. that's what an
    >oscilloscope does. It will show if some connection changes state. But
    >without knowledge of what it "should" be doing, that information is
    >not useful.


    I disagree. A scope is the tool for hunting for logic that is sorta
    there. A logic probe is for those signals that are present or absent.
    The likelyhood of the latter in a known design that did work at one
    time is very high. So the likely circuit fault is maybe only one gate
    that has died.

    >It's hard to say from this point "here is what you should do". The
    >problem is you don't have any equipment and you don't have much
    >digital circuit knowledge. But that is what it takes to diagnose the
    >problems and find what is at fault. The "test each chip" method can
    >work if the problem is a failed chip. But it could be a broken PC
    >board, or a component like a capacitor or resistor, or a "cold solder
    >joint" that does not make a good connection. And you can create
    >problems by burning up a circuit trace, since the chips are not
    >socketed.


    Clearly there is a should do. One is know how the system operates and
    methodically proceed from that understanding to see what is being done
    by the z80 if anything (it is setting up the 6845) and what is not
    happening and then focusing on the logic that leads to that.

    What does operate:

    Power suppply.
    Monitor (you have a stable display)
    Video logic (it starts up and the 6845 is being programmed to
    put out the required timing to drive the monitor.
    The above says the Z80 does start up and so something.

    Do thing happen like the drive lights change?
    Do the drives home to track 00
    Does it appear to read a floppy?
    Do the light on the kayboard light, caps lock chage state?
    Does anything come out of the serial ports?
    Is there a manual out there with a page of troubleshooting hints?

    >
    >Again, it's commendable you are working on this Kaypro. But the Kaypro
    >does not "care" about your intentions. There is some problem on it,
    >which may be signifigant like a bad logic chip, or subtle like bad
    >timing, or maybe a ROM which has scrambled its code from age. To fix
    >it, someone with knowledge and an oscilloscope or logic analyzer and
    >the schematics has to sit down with it and start looking at signals.


    Likely faults in once working equipment is a component failure.
    Possible candidates are bad eproms, or a TTL that has a dead gate.
    Other possible candidates is soemone tried to mod or update it and
    did something wrong like install the wrong rom, or initially installed
    it upside down.

    All it takes is one gate failing to "disconnect" the Z80 from the
    video ram and the screen will never clear or otherwise update.

    >It seems to me, you have an opportunity and a choice. You can learn
    >more about digital logic and microprocessor logic - old books are for
    >sale or can be borrowed from a library. You can acquire an
    >oscilloscope (any hamfest will have them). Then you can start looking
    >at the Kaypro and understand what's going on. Or, you can improve your
    >soldering skills, test each chip, and hope you get lucky and not only
    >find the problem but avoid creating another one.


    Also the schematics and other inforamtion about the kaypro CPU board.
    There is nothing super magical on it. Though later baords did have a
    gate array or two that can fail. My expereince with many dozens of
    machines and board is all it takes is a 74LS00 that developed a stuck
    input or open output on one of the gates n the package to kill a
    system. In short a part that failed with time or some side effect of
    mishandling in it's life before being inserted in the board.

    >Some people may decide these remarks are unfair or discouraging.
    >Again, these are chips on a board. The board and chips don't care
    >about intentions or hopes. They do what they do. They perform in a
    >certain way, by theory and design. Then, in the real world, they
    >perform in different ways because of a bad connection, or a component
    >like a capacitor changes value, or once in awhile due to a failure of
    >a logic gate. And in the real world, THINGS FAIL. This is not Star
    >Trek where computers are found running after 100, 200 years.
    >
    >So, if you can learn these things, get this equipment, you'll have
    >more opportunities to keep other Kaypro's and other microcomputers of
    >the era running. For they will ALL FAIL IN TIME.


    It's an aquired skill for some and for others its' magic and the
    skill eludes them. Troubleshooting is a mystery novel, you know the
    ending and the victums but what happens in between isprocess,
    knowledge and interpreting the clues that are there.

    Allison

    >
    >Herb Johnson
    >retrotechnology.com



  14. Re: Kaypro II repair

    On Sep 10, 1:22*pm, myem...@fattimie.i (andrea) wrote:
    > I am trying to repair a non working Kaypro II, board model 81-240-n,
    > boot eprom 81-232-n.


    > Does anyone familiar with this computer can give me any hints or
    > suggestion on what could be the faulty component?


    Andrea -

    I am more familiar with the BigBoard that the Kaypro is based on
    (having built up several dozen of them in the early 1980's). I
    believe the Kaypro II also works the same way, as does the Xerox 820.

    With the BB the system initializes the DRAM, then copies the ROM to
    DRAM and performs a bankswap. If this doesn't happen properly the
    processor jumps to essentially junk code and crashes and you will see
    the flashing screen.

    Your problem is most likely with the DRAM, or refresh circuitry
    (largely built around a 74164) Since you've swapped the DRAM, I'd
    take a look at the power supply stability and DRAM support IC's
    (74157, 8216, 74164).

    There was a test rom for the big board that ran entirely out of the
    video memory (which as SRAM was much more likely to be working). I
    may still have a copy. It might help in the Kaypro as well. If you
    don't get anywhere let me know and I'll try and dig out a copy and
    burn it for you. It was 2716 based.

    As another try you might swap the 20mhz crystal for a lower value, say
    12-16mhz. It is not critical, but "underclocking" might let you get
    the system working.

    - Gary

  15. Re: Kaypro II repair

    On Sep 13, 9:44*am, gek wrote:
    > On Sep 10, 1:22*pm, myem...@fattimie.i (andrea) wrote:
    >
    > > I am trying to repair a non working Kaypro II, board model 81-240-n,
    > > boot eprom 81-232-n.
    > > Does anyone familiar with this computer can give me any hints or
    > > suggestion on what could be the faulty component?

    >
    > Andrea -
    >
    > I am more familiar with the BigBoard that the Kaypro is based on
    > (having built up several dozen of them in the early 1980's). *I
    > believe the Kaypro II also works the same way, as does the Xerox 820.
    >
    > With the BB the system initializes the DRAM, then copies the ROM to
    > DRAM and performs a bankswap. *If this doesn't happen properly the
    > processor jumps to essentially junk code and crashes and you will see
    > the flashing screen.
    >
    > Your problem is most likely with the DRAM, or refresh circuitry
    > (largely built around a 74164) *Since you've swapped the DRAM, I'd
    > take a look at the power supply stability and DRAM support IC's
    > (74157, 8216, 74164).
    >
    > There was a test rom for the big board that ran entirely out of the
    > video memory (which as SRAM was much more likely to be working). *I
    > may still have a copy. *It might help in the Kaypro as well. *If you
    > don't get anywhere let me know and I'll try and dig out a copy and
    > burn it for you. *It was 2716 based.
    >
    > As another try you might swap the 20mhz crystal for a lower value, say
    > 12-16mhz. *It is not critical, but "underclocking" might let you get
    > the system working.
    >
    > - Gary


    You said that the disk drive light stays on, could the issue be in the
    drives themselves, causing the system to hang? can they be unplugged
    from the motherboard to see what effect that has?

    Bill H

  16. Re: Kaypro II repair

    On Sat, 13 Sep 2008 06:44:34 -0700 (PDT), gek
    wrote:

    >On Sep 10, 1:22*pm, myem...@fattimie.i (andrea) wrote:
    >> I am trying to repair a non working Kaypro II, board model 81-240-n,
    >> boot eprom 81-232-n.

    >
    >> Does anyone familiar with this computer can give me any hints or
    >> suggestion on what could be the faulty component?

    >
    >Andrea -
    >
    >I am more familiar with the BigBoard that the Kaypro is based on
    >(having built up several dozen of them in the early 1980's). I
    >believe the Kaypro II also works the same way, as does the Xerox 820.
    >
    >With the BB the system initializes the DRAM, then copies the ROM to
    >DRAM and performs a bankswap. If this doesn't happen properly the
    >processor jumps to essentially junk code and crashes and you will see
    >the flashing screen.
    >
    >Your problem is most likely with the DRAM, or refresh circuitry
    >(largely built around a 74164) Since you've swapped the DRAM, I'd
    >take a look at the power supply stability and DRAM support IC's
    >(74157, 8216, 74164).
    >
    >There was a test rom for the big board that ran entirely out of the
    >video memory (which as SRAM was much more likely to be working). I
    >may still have a copy. It might help in the Kaypro as well. If you
    >don't get anywhere let me know and I'll try and dig out a copy and
    >burn it for you. It was 2716 based.
    >
    >As another try you might swap the 20mhz crystal for a lower value, say
    >12-16mhz. It is not critical, but "underclocking" might let you get
    >the system working.


    Most of that may not easily apply to the kaypro. the clock derivation
    for video, serial and CPu are different.

    Allison
    >
    >- Gary



  17. Re: Kaypro II repair

    On Sat, 13 Sep 2008 08:40:10 -0700 (PDT), Bill H
    wrote:

    >On Sep 13, 9:44*am, gek wrote:
    >> On Sep 10, 1:22*pm, myem...@fattimie.i (andrea) wrote:
    >>
    >> > I am trying to repair a non working Kaypro II, board model 81-240-n,
    >> > boot eprom 81-232-n.
    >> > Does anyone familiar with this computer can give me any hints or
    >> > suggestion on what could be the faulty component?

    >>
    >> Andrea -
    >>
    >> I am more familiar with the BigBoard that the Kaypro is based on
    >> (having built up several dozen of them in the early 1980's). *I
    >> believe the Kaypro II also works the same way, as does the Xerox 820.
    >>
    >> With the BB the system initializes the DRAM, then copies the ROM to
    >> DRAM and performs a bankswap. *If this doesn't happen properly the
    >> processor jumps to essentially junk code and crashes and you will see
    >> the flashing screen.
    >>
    >> Your problem is most likely with the DRAM, or refresh circuitry
    >> (largely built around a 74164) *Since you've swapped the DRAM, I'd
    >> take a look at the power supply stability and DRAM support IC's
    >> (74157, 8216, 74164).
    >>
    >> There was a test rom for the big board that ran entirely out of the
    >> video memory (which as SRAM was much more likely to be working). *I
    >> may still have a copy. *It might help in the Kaypro as well. *If you
    >> don't get anywhere let me know and I'll try and dig out a copy and
    >> burn it for you. *It was 2716 based.
    >>
    >> As another try you might swap the 20mhz crystal for a lower value, say
    >> 12-16mhz. *It is not critical, but "underclocking" might let you get
    >> the system working.
    >>
    >> - Gary

    >
    >You said that the disk drive light stays on, could the issue be in the
    >drives themselves, causing the system to hang? can they be unplugged
    >from the motherboard to see what effect that has?
    >


    On Kaypros the drive select always stays on, it may change from drive
    to drive B but on is always on with my two (a 4/84 and a K2).

    If the drive was unavaiable or broke it would give a clear screen and
    error message. This never clears the screen if I remember an earlier
    message correctly.

    Allison

    >Bill H



  18. Re: Kaypro II repair

    Herb Johnson wrote:

    > I think it's admirable that you are working to repair this Kaypro.
    > But, the problem is that there is no clear "symptom" to guide your
    > repair process. And it's very difficult to unsolder one chip after
    > another without damaging the circuit board. So in my somewhat informed
    > opinion, I don't think chip-by-chip external testing will result in a
    > repair except by pure luck.


    Herb, that's the way I've repaired most of my failed stuff untill now.
    This is for sure the worst case ever, since usually it's just a
    ROM/EPROM, a leaking capacitor, a failing RAM, burst tantalium
    capacitor, a voltage regulator, a bent IC pin, an inverted connector, a
    track that does not make good connection... pretty basic and simple
    things that I could check and swap without having deep knowledge of how
    computers operate at the CPU signals level. Anyway you are right, this
    is not for sure the best way to repair computers. With the Kaypro when I
    first powered the computer I thought it was a case of failing eprom and
    after replacing all of them and seeing I didn't fix it I had a feeling
    it was going to be a harder job, harder than expected.

    > It seems to me, you have an opportunity and a choice. You can learn
    > more about digital logic and microprocessor logic - old books are for
    > sale or can be borrowed from a library. You can acquire an
    > oscilloscope (any hamfest will have them). Then you can start looking
    > at the Kaypro and understand what's going on. Or, you can improve your
    > soldering skills, test each chip, and hope you get lucky and not only
    > find the problem but avoid creating another one.


    > So, if you can learn these things, get this equipment, you'll have
    > more opportunities to keep other Kaypro's and other microcomputers of
    > the era running. For they will ALL FAIL IN TIME.


    I completely agreee with you, I should go that way.

    Regards
    Andrea


    --

    http://myretrocomputing.altervista.org

  19. Re: Kaypro II repair

    gek wrote:


    > Your problem is most likely with the DRAM, or refresh circuitry
    > (largely built around a 74164) Since you've swapped the DRAM, I'd
    > take a look at the power supply stability and DRAM support IC's
    > (74157, 8216, 74164).


    Thanks for this advice.

    By the way, yesterday I could show the schematics to a person that has
    far much better knowledge of those old computers than me and he told me
    basically the same things that were said by the various posters of this
    thread. In short, it could be anything. So I will finish my plan of
    swapping the last 5 or so chips then, if I get no positive feedback, I
    will start to learn how old computer circuitry operate and to find an
    oscilloscope for this purpose.

    Regards
    Andrea





    --

    http://myretrocomputing.altervista.org

  20. Re: Kaypro II repair

    Bill H wrote:

    > You said that the disk drive light stays on, could the issue be in the
    > drives themselves, causing the system to hang? can they be unplugged
    > from the motherboard to see what effect that has?


    Yes they can, but nothing changes if I unplug them.

    Regards
    Andrea


    --

    http://myretrocomputing.altervista.org

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