Re: Apple II Minus Question

[Eric Smith <eric-no-spam-for-me@brouhaha.com>]

"Bryan Parkoff" <bryan.nospam.parkoff@nospam.com> writes:
>     I would like to know Apple II Minus history that it is the earliest
> model that it starts with Old Monitor Routines rather than AutoStart
> Routines.

There isn't a model called an "Apple II Minus".  The earliest machine in
the family is the "Apple ][".

> Do white horizontal block like GR (low resolution) appears in the
> screen?  Or...Do white horizontal block like TEXT ("@@@@" and "??????")
> appears in the screen?

It's usually somewhat random text, although graphics is also possible.

I say "somewhat" random, because on any given machine the display at
power-up varies a little but is strongly biased toward a particular pattern,
determined by the most probably power-up state of the DRAM.  Each bit in
each DRAM has a tendency to power up in a particular state, but won't do
so 100% consistently.

>     If it is GR, must you always press RESET before it returns to "*"
> prompt?

You must press RESET before you can do much of anything.  This is why
they added an automatic power-on reset on later models.

>     Sometimes, "???>@.?1??<??" show.  Do you suspect that it may be bad RAM?
> Or it may be static leakage?

No, that's just the way it is.

>     Are all "00 00 00 00 00 00 00 00" store in odd pages between $0000 and
> $BFFF and "FF FF FF FF FF FF FF FF" store in even pages between $0000 and
> $BFFF?  If each page do show like "01 01 01 01", "7F 7F 7F 7F", "E1 E1 E1
> E1", "FE FE FE FE", or etc.  I claim that all static RAM chips are bad.

There are no static RAM chips in an Apple ][.  Nor in an Apple ][+.  There
are only dynamic RAM chips.  The power-on state of dynamic RAM chips is
undefined, so the fact that you get patterns does not mean that the RAM
is bad.

>     I bought all 7xxxx chips and RAM chips from electronic store.  I removed
> all old chips out of all sockets.  I threw them in the trash.  I put new
> 7xxxx chips and RAM chips in all sockets.  I did test, but the result is
> still the SAME.  Such as GR and HGR are not working (never turn on, but stay
> in TEXT only).

Why did you throw away the old chips?  Most of them were probably fine.

The first thing I would have suspected with a problem like that would be
the 9334 or 74LS259 chip that implements the "soft switches".  Since you
apparently replaced that without fixing the problem, the next thing that
comes to mind would be the address multiplexers that take the video counter
outputs and produce the DRAM row/column address.  But if you've replaced
those too, I start to wonder if one of the contacts of a socket might
be bad.

> Probably some resistors, capicators, and other electronic parts are bad
> however all chips are working fine on another Apple II+ motherboard.

Although I won't claim that it's impossible, it seems somewhat unlikely
that a passive component failure would cause the symptom you describe.

>     Please suggest which electronic diagnostic tools that is very worth for
> me to buy before I can be able to isoslate defective parts.  If I have found
> defective, I would be ABLE to repair.  Please advise.

The most important electronic diagnostic tool is your brain.  You need to
understand the design, come up with a hypothesis as to what might cause the
observed behavior, and a plan to test the hypothesis.  All the fancy
electronic diagnostic tools in the world are just a waste of money if you
can't do that.

Throwing away all the chips and replacing them with new ones is a "shotgun"
approach suggests that you are not very serious about troubleshooting the
machine.

However, if you ARE serious about it, what you probably need is an
oscilliscope with at least 20 MHz bandwidth.  That will allow you to
examine waveforms of signals.  However, it will require you to figure
out what signals to examine, and what the waveforms should be.

A logic analyzer is another tool that would be useful.  Generally they
cost more money than oscilliscopes.  Logic analyzers are great for
debugging digital problems, but if you need to determine analog signal
levels or waveforms they won't help.  (Unless they're really high-end
ones that have integrated oscilliscope functionality.)  There are,
however, some inexpensive limited-function logic analyzers for PCs, such
as the ones from http://www.usbee.com/

If I had very limited money to spend on such things, I'd probably try
to buy a used Tektronix 465 oscilliscope.  That's a two-channel 100 MHz
analog scope with all of the basic features you need.  They were very
expensive in their day (late 1970s), but can usually be found for under
$250 now.