Hello! A. Fachat wrote: > 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? I repaired lots of CBM-II's and have never seen this. It is definitely not common. A most common failure in a CBM-II is broken CIA chip. > 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 The schematics are identical (except the video output circuit) but the chip numbers are different because the 700 has a different motherboard layout than the 600. But everything else is the same. Regards, Michau. Message was sent through the cbm-hackers mailing listReceived on 2014-10-13 12:00:03
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