Re: CBM 700 Reset circuit

From: Michał Pleban <>
Date: Mon, 13 Oct 2014 13:32:54 +0200
Message-ID: <>

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.

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.


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Received on 2014-10-13 12:00:03

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