CBM720 heads up

From: A. Fachat <afachat_at_gmx.de>
Date: Sun, 19 Oct 2014 21:08:28 +0200
Message-ID: <2069923.gsYR05uldV@euler>
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?


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Received on 2014-10-19 20:00:02

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