29 May
2008
29 May
'08
2:59 a.m.
Got out my ancient oscilloscope and did a bit of probing and got a
little more info:
On power up, the CPU reset line is held low for ~1 sec, then goes high.
At this point if I look at the address/data lines, I can see a very
brief burst of activity and then nothing at all (it's during this
activity that a few characters on screen change). After this activity,
all address lines are high. IRQ & NMI lines are high. CPU is being
clocked throughout all of this, clock rate seems to be correct.
I found an old S-100 static ram board which is populated with TI
TMS4045-45NLs -- from the datasheets I was able to find, these look to
be 2114 compatible, am I correct in this assumption? If they're
compatible I'll try them in the PET and see if it makes any difference...
There's an LED on the PET logic board which does not light up when power
is applied. Is this supposed to be on at powerup, or is it activated by
some hardware/software activity?
Thanks,
Josh
Josh Dersch wrote:
So I've got my "hacked-up" PET powering
up after redoing a bit of
wiring on the power supply. I replaced a couple of obviously bad
74LS157's in the video generator section (legs were corroded and some
had fallen off/cracked) and reflowed a couple of suspicious looking
joints.
The machine now powers up, the display is bright and crisp but I just
get garbage characters on the display. Using my logic probe I've
verified that the CPU is running, voltages appear nominal (4.9V at the
CPU & RAM). The CPU seems to be running something, in that about 1-2
seconds after power up a few characters on the screen will change (not
where you'd expect the COMMODORE BASIC banner to be printed, but
always in the same places). Typing on the keyboard produces no effect.
I've reduced the board to 4K of 2114s and tried swapping around the
2114s used for the display, but the only effect this has is to change
the pattern of garbage characters displayed (interestingly (at least
to me) each RAM chip produces a different pattern, which is
reproducible). If I run without any video RAM installed, I just get a
checkerboard pattern onscreen, which would make sense since character
FF is the checkerboard tile. Unfortunately I have no idea if any of
the 2114s I have are good or not, though I'd wager that they can't ALL
be bad. I don't have any other machines that use them and I don't
have any spares.
Any suggestions on logical places to start looking from here?
Thanks,
Josh