28 Sep
2005
28 Sep
'05
3:59 p.m.
The problem
with those diagnostics (and most built-in diagnostics) is
that a fairly large part of the machine has to be operational (at least
the CPU, some memory, ROM, video) for them to be at all useful.
While it is rare that a minor error will surface in, say, the CPU (I have NEVER
seen a CPU go gradually, like "whoops, now I can't ADD or SHL" -- it's
all or nothing), the diagnostics run in something like 1K of
ROM with no memory use
other than video so they are helpful in diagnosing RAM problems. And the
IIRC, there is no separate video RAM on the PCjr, the video circuit just
takes some of the main RAM. And while you can have random bit failures in
RAM, I've found that either complete chips failing or address inputs
failing (so that locations appear to repeat) is much more common.
sound, joystick, video, and floppy drive tests are
helpful in determining if
the machine is functional on a basic level -- I always run them on any new PCjr
I come across to determine if it's worth hauling away or not :-)
Over here, PCjr machines are sufficiently rare that just about any is
worth grabbing.
Apart from a defective gate array chip, I think just about any problem is
repairable.
-tony