Re: what the hell is going on with my sys80 CPU!???

"GPE" <GPE_NoSpam@xxxxxxx> wrote in
news:CbVck.19787$oY2.19395@xxxxxxxxxxxx:

The Quickscan80 does fix thing that many of the early System 80 CPU
boards (up to Ice Fever) did not fix. It adds a 3K ohm pullup
resistor to Read/Write line. I have seen one CPU board that acted
eratically like yours for a few minutes then seemingly worked ok.
Plugged in the test fixture and all was fine as soon as power was
applied. Unplugged the test fixture and it went back to doing oddball
stuff again. When I installed the test fixture - it also plugs in a
builtin 3K pullup resistor. When I pulled the test fixture - it
removed this 3K pulllup resistor. This was the only explanation I
could find... So, on the back side of the board, I tack soldered a 3K
pullup resistor between pins 7 and 11 of TC1. Lo and behold... things
fired up properly from then on.

Had another user from Europe email me with a similar issue (Pascal or
Hans... can't remember who). His board would not boot properly unless
test fixture was plugged in (but his board behaved differently from
yours). They also soldered a 3K resistor between pins 7 and 11 --
board fired right up.

This is a fix that Gottlieb used for many boards starting with Ice
Fever. But this fix was always part of that little adapter cable (part
of the watchdog timer reset board) that was plugged into TC1. Some
people pulled this plug and don't use the reset board and have not
suffered any problems. But - I have seen a couple that refused to work
without this resistor.

This may not be your problem but a 5 cent resistor is far, far cheaper
than replacing that 6502. Try tack soldering a 3K resistor between
pins 7 and 11 of TC1. See if that helps.

Thanks, but negative; putting a 3K between 7 & 11 of TC1 didn't change
anything. (Can you tell I'm trying these one by one? :) )

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