PET problem SOLVED!

From: Dave Dunfield <>
Date: Fri Aug 13 21:07:37 2004

Ethan, Dwight (and anyone else who helped),

Thanks for all the help - I've solved it and it works!

Turns out it was NOT the ROM's -- After my last post I got to thinking that the PET
ROM's are probably fused type, and wouldn't go marginal like an EPROM, so I focused
on the RAM - sure enough, testing again, and I found that bit 1 of location $11 was
stuck at zero (I'm positive I tried it before and it seemed to work - bit the funky
PET monitor with it's exact # of spaces required was causing me some grief!)

Anway, when 01 got incremented, instead of 02, it got 00 - this was hanging the
search for end of RAM, which never incremented the high byte and therefore never
traped at 32k..

Checking the Schematic, bit 1 of the low memory bank is U17 - pulled it, put in a
socket (always!) and stuffed another 4116 - voila!!! it came right up!

Also figured out the startup "noise" mystery (such unanswered questions "bug" me),
turns out the PORTA bit 7, which is used for the DIAG input is also used to control
the speaker (it was right on the schematic once I looked for it - duh) - pulling
this line low disabled the speaker.

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools:
com             Vintage computing equipment collector.
Received on Fri Aug 13 2004 - 21:07:37 BST

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