Re: CBM720 heads up

From: Didier Derny <>
Date: Wed, 22 Oct 2014 10:58:22 +0200
Message-ID: <>
The irq is working correctly ?

in the pre-series 720 I used, the irq was coming from the power supply
when we received it, the 720 was not working

a chip was blown, this chip was used to generate the irq
from the power line frequency

(was fixed by generating the irq from another source)

symptomes:  no keyboard, no cursor


On 19/10/2014 21:08, A. Fachat wrote:
> Hi there,
> just a status on my CBM 720 repair - and maybe you can help as well :-)
> After replacing some capacitors in the original power supply, I checked the
> first logic signals and found /RES continuously low. Traced it back to a 556
> timer chip with a broken output. Replaced it, and found from scope traces that
> the CPU was working.
> Unfortunately after a few minutes the old power supply went south with a small
> "plop"... I decided to not get further with it, and instead replace it with a
> PC power supply.
> That worked well, and got some video output after adjusting the screen
> brightness with the small knob on the back (I found video signals with the
> scope, and then decided to check that knob, which in fact ... :-)
> Unfortunately it's not yet fully working, but much further. The CRTC gets
> initialized, so that means the video output is working, the CPU as well (at
> least to a considerable extent), but other stuff is still broken.
> Anyone knows the video pattern shown here?
> Thanks
> André
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Received on 2014-10-22 09:00:03

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