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?
> Date: Sat, 31 May 2008 11:41:15 -0400 (EDT)
> From: der Mouse
> It is probably possible to build a current limiter that provides
> immediate visual feedback. I challenge you, or anyone for that matter,
> to come up with such a design that is as cheap, easy, simple, and
> foolproof as an incandescent bulb. (Well, not cheap once the ban hits,
> which is the problem.)
How's this? Grab a glass jar, fill with water and a salt of your
choice, say, bicarbonate of soda, drop two electrodes into it
(stainless steel or carbon is good; aluminum will tend to polarize
after awhile and form a leaky rectifier). Apply current and watch
for bubbles and/or steam. Vary resistance by varying the distance
Or use a dill pickle instead if you like glowing pyrotechnics.
We will always have appliances with heating elements floating around
that can be pressed into use, particularly where higher-power loads
What I don't like about incandescents used as a load-limiter is the
very low cold resistance. I'd much rather have a "soft" start--but
then carbon-filament incandescents haven't been easy to find for the
last 80 years or so.