CBM 700 Reset circuit

From: A. Fachat <afachat_at_gmx.de>
Date: Sun, 12 Oct 2014 22:12:44 +0200
Message-ID: <1522757.JO8ZKrxZ4F@euler>
Hi there,

the CBM 720 I am trying to repair has the problem that the /RES line of the 
CPU keeps being low. I traced it back to the output pin 9 of a 556 reset timer 
being stuck "in the middle" of TTL, which seems to result in the following 
circuitry not releasing /RES.

Does anyone have had the same experiences? Is it common that the 556 timer 
breaks this way?

BTW: It looks like the first schematics (8256043-...) is a 6x0 schematics, 
while the "8256059-..." is a 7x0 schematics, even though the description only 
says they "seem to be quite identical". But the chip numbers of the 8256059 
match my 720, the 8256043 do not.
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/b/index.html

Thanks
Andr


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