RE: CBM 700 Reset circuit

From: Martin Hoffmann-Vetter <>
Date: Mon, 13 Oct 2014 22:10:22 +0200
Message-ID: <001a01cfe721$b8ba5530$>

>> 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.
> 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 ...


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Received on 2014-10-13 21:00:03

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