Thread: Mk14 vdu
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Old 4th Jun 2020, 9:12 am   #39
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,485
Default Re: Mk14 vdu

I would view 7V anywhere east of the regulators with a great deal of alarm! I was actually already thinking of replacing one of the many decoupling capacitors with a 5.6V zener so that in the event of regulator failure (short from input to output) the zener would at least limit the voltage and more likely fail s/c, which would be bad news for the zener but would hopefully save everything else.

What's the actual supply voltage to the chip supply pins on the VDU? It's possible, I suppose, that somehow the unregulated input has found its way to the +5V - out terminal on the edge connector. Always remember, this was a previously untested PCB so you have to consider things you wouldn't normally consider.

I was aware of the likely problem with 'top page' usually needing to be connected to one of the address select lines in order for the VDU to display two different 256-byte blocks.
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