18 Aug
2009
18 Aug
'09
3:54 p.m.
On Tue, Aug 18, 2009 at 5:53 PM, Joe Giliberti<starbase89 at gmail.com> wrote:
Okay, thanks for clearing that up.
First off, I suppose I need to check voltages.
Good place to start.
If your PET is mostly working, you are likely to see some form of
stationary video garbage when you power it up, or possibly a TIM
register dump, rather than the ROM banner telling you what version of
BASIC you have and how much RAM. Oh, wait... it's an 8032... in older
PETs, the video circuit is TTL and does not require any CPU activity
to start displaying the contents of video RAM... yours has a 6545 CRTC
"video chip" (similar to what was on the original IBM PC monochrome
video card). It does need to be fed some constants from ROM by the
CPU at startup, so perhaps you'd see a blank screen unless the CPU was
running and able to fetch code from ROM.
Check your voltages for sure, especially the 7905 -5VDC regulator near
the RAM field. Those 4116s are triple-voltage. Pretty much
everything else (except the cassette motor) is +5V-only.
I've only seen one ROM chip go bad ever, so if your PET won't fire up
far enough to reset the video chip, it's probably either bad RAM or
bad address logic or a data or address bus buffer. If you are tracing
that far into it, you will probably want that scope set up.
-ethan