Apple //c+ composite video troubleshoot

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Joined: Apr 24 2012
Posts: 251

I bought an Apple //c plus and the composite video does not work. is strange, the computer boots, but the screen stays black. I know is getting some signal, as it goes blue when there's nothing connected.
Of course I tried with different cables, and TV sets, all the same result. I even tried the pins 12 & 13 of the port directly to the composite, but still black. Using a VGA adapter got image, and the computer self check gives an OK, but of course I want to use it with composite.
Any pointers on how to troubleshoot? Thanks in advance.
Javster

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Joined: Jun 15 2014
Posts: 305
Re: Apple //c+ composite video troubleshoot

From what you tell us apparently there is a hardware problem with your //c+. If you have the skills and an oscilloscope you could check what components of the composite video signal (if any) are present. If you can find a schematics of //c+ more advices could be given.

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Joined: Apr 24 2012
Posts: 251
Re: Apple //c+ composite video troubleshoot

Thanks, unfortunately have no access to oscilloscope, and haven't been able to find //c+ schematics on the net. any help would be appreciated.

ggb
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Joined: Mar 13 2012
Posts: 31
Re: Apple //c+ composite video troubleshoot

Javster, the copy of the schematic I have is difficult to read. it does show that the output of the 74HCT166 (labeled SPI) goes to a module at the back of the video connector (labeled UD1). This module requires both +5V & -5V, sync (IOU pin 39), color ref (TMG pin 3), clear (IOU pin 37), serial data from 74HCT166

GeoffB