Re: CBM720 heads up

From: Christian Dirks <>
Date: Sun, 19 Oct 2014 21:21:22 +0200
Message-ID: <>
Hi Andre,
on your pictures it looks like you built something to generate the 50 Hz
square signal, so the signal is there ?

My CBM 700 sometimes shows the cursor and does not start up.
Wiggling a bit at the eprom adapters helps.
The rom sockets are bad, I have to replace them.
Maybe your roms have contact problems too.
Try removing the basic roms, the machine schould then start into the TIM


Am 19.10.2014 21:08, schrieb A. Fachat:
> 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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Christian Dirks

       Message was sent through the cbm-hackers mailing list
Received on 2014-10-19 20:00:43

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