On Fri, 12 Sep 2008 07:24:33 -0700 (PDT), Herb Johnson
Post by Herb JohnsonPost by andreaWhat 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.
Post by Herb JohnsonI 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.
Post by Herb JohnsonNow, 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.
Post by Herb JohnsonBut, 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.
Post by Herb JohnsonIt'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?
Post by Herb JohnsonAgain, 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.
Post by Herb JohnsonIt 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.
Post by Herb JohnsonSome 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
Post by Herb JohnsonHerb Johnson
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