RE: CBM 700 Reset circuit

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
>
>
>
>
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