Thanks for this info. I'll check that over the weekend. I'm wondering what surprises this machine still holds. First check power supply - broken. Second check /RES - broken. I just hope the CPU isn't broken. André Am 13. Oktober 2014 22:10:46 schrieb "Martin Hoffmann-Vetter" <martinHV@arcor.de>: > Hello, > > >> 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. > > At this time, i remember! Yes i've repaired a CBM-II with a broken 556. The > owner try to messure on a connector. But he shorts the reset line with a > power supply line (-12V?). So the 556 was faulty and the CBM-II don't > start. After replacing the 556, all was okay! > > >> 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. > > Yes that's true. But the known schematics aren't from the common pcbs! The > pcb are patched to pass the schematics! So there is a flip flop in the > video circuit without any real function. But the pcb has a cutted wire and > a additional connection. But with this wire you understand the unused > function from the flip flop! > > Have a look at U96 and U54 on 8256059 page 8! On the pcb the wires goes > from pin 9 U54 to pin 1 U96 and there isn't a connection between pin 1 and > pin 2 from U96 ... > > Martin > > > > > Message was sent through the cbm-hackers mailing list Message was sent through the cbm-hackers mailing listReceived on 2014-10-13 21:00:56
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