Re: Weird video on Apple IIc

[dempson@actrix.gen.nz (David Empson)]

Nathan Mates <nathan@visi.com> wrote:

> In article <53m3fi$8vt@news.ycc.yale.edu>,
> Ryan L Macarthur <macrl@pantheon.yale.edu> wrote:
> 
> >where the *'s are the rapidly flashing @ signs, and the ones that scroll
> >up the screen are normal.  Applesoft BASIC seems to work fine, and as the
> >screen is filled with text, text that is scrolled up past the flashing
> >@'s will be changed to a different character.
> 
>    However, this is more confusing. As far as I recall, Applesoft
> should be scrolling the same way that just pressing return does. Does
> 'changed to a different character' mean a 'different, but incorrect'
> character?  If so, it's a sign of which bits are bad. [If it's
> 'differnt from @, but correct', then that pushes the problem to the
> character generator chips.]

The sign bit would only affect inverse/normal display (ignoring
MouseText).  Bits 6 ($40) and 5 ($20) would have to be affected if
spaces were changing into @ symbols.
 
It sounds like an intermittent RAM fault in at least one, possibly two
RAM chips.

> >    When the keyboard is pulled off after power-up (it goes into 
> >self-test otherwise),

It sounds like there is another fault - the keyboard may me reporting
that the Apple keys are held down, there may be a fault in the cable,
the moterboard may have a problem (e.g. the IOU chip may be faulty - it
is responsible for most of the miscellaneous I/O and for generating
video), or you may have a faulty device plugged into the mouse/joystick
port.

> >the problem is still there, which brings me to my final question: what am
> >I supposed to expect from the Apple //c self-test?
> 
>    That first version of the //c may have an incomplete self test. If
> it doesn't stop after a few minutes, then it's not useful.

The self test in the original IIc ROM is useless.  It just displays
pretty patterns.

To get a proper self test, the ROM must be upgraded to the UniDisk 3.5
compatible one (PEEK(64447) reports 0), which may be available from
Alltech Electronics (Apple used to supply it free of charge if you had
bought a UniDisk 3.5 to use on a IIc).  This requires replacing the
socketed ROM, and also making two modifications to the motherboard: The
"W1" modification (to the left of the ROM) must be cut, and the "W2"
modfiication (between the IOU and MMU) must be bridged with a soldering
iron.

The upgraded ROM has a full self test, which should report which RAM
chips are faulty.  It is then a matter of replacing the chips in
question.  The self test displays a RAM fault as a series of eight 0 and
1 digits, with 1s corresponding to the faulty chips.  The leftmost digit
is bit 7 (RAM chip MRD7), the rightmost bit 0 (RAM chip MRD0).

If an asterisk (star) is also displayed next to the numbers, then the
fault is in auxiliary RAM, not main RAM.  The auxiliary RAM is labelled
ARD7 (bit 7) through ARD0 (bit 0).

You will have to remove the disk drive to gain access to MRD7 and ARD0
through ARD7.  All of the RAM chips are soldered onto the board, so to
replace them, you should cut the pins, desolder the remaining part of
each pin, then solder a socket in place of the RAM chip.

Note that if you are getting odd characters appearing on the 40-column
text screen and in normal hi-res modes, then the problem is definitely
in main RAM.

The RAMs are "4264" (1 bit by 64K).  There is no mention of a speed
rating on the chips I can see here.  150 ns or faster would be plenty,
and 250 ns is probably OK.

-- 
David Empson
dempson@actrix.gen.nz
Snail Mail: P.O. Box 27-103, Wellington, New Zealand