13 Jul
2006
13 Jul
'06
9:21 p.m.
On 7/14/06, Jules Richardson <julesrichardsonuk at yahoo.co.uk> wrote:
What's the LED for? It's not lit on this
machine when power's applied - is it
supposed to be, or does it signal some other activity and spends most of its
time unlit?
It's a diagnostic LED. Older ROMs have a built-in diagnostic that you invoke
by grounding a particular pin on the user port. The diagnostic will fail unless
certain user port pins are wired together and certain keyboard pins
are wired together (back in the day, C= technicians had a blue folder
with instructions and a couple of dongles to attach. I have a set at
home, but won't be able to access them for 6 months).
The RAM chips seem to be running at a level best
described as "fairly warm".
Not too hot to touch or anything, but they're still putting out a fair bit of
heat - but then given the age/technology maybe that's normal? (certainly there
don't seem to be any that are running any hotter than the rest)
Throw a scope on the /CS pin of zero page - when the machine is
running, you should see some activity.
I'll put a 'scope on the video lines in a mo
and see if it looks like it's
even generating plausible video signals...
Perfect. One possibility, of course, is that the mobo is fine and the
video is dark.
The largest point of failure for old PET is crappy sockets. There's
no single IC (unlike the PLA in an C-64) that one automatically
gravitates to on a PET. The video is discrete TTL, so there's no
video chip (6545) to replace... as long as the CPU can see ROM and see
RAM, even a lack of I/O shouldn't prevent it from getting as far as
clearing video memory and putting up the normal banner. You won't
have keyboard, tape, IEEE-488 or the user port without the 6520s and
6522s, but at the very least, you should see garbage characters on the
screen if the video circuit and the monitor are functional.
-ethan