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64K XOR RAM board

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  by Bill Degnan - 08/14/2017 10:38
I received the following post from Donald Burns. I have not tested my own 64k RAM card to confirm if I have the issue he describs below, but it's probably a good idea to be aware of the potential issue, especially if your machine's RAM card is not working or like in my case the minus voltage shorted out on the CPU board. I don't want to have to repeat the same repairs.

> Good to see this XOR back in use. Mine are down and out until I get the floppy controllers working. One thing to also check is the capacitor on the minus supply on 64K XOR RAM board. Both the component mask and board component stuffing have the capacitor incorrect. Factory stuffed the board to the mask layout. The minus voltage on the board has a cap with the "+" terminal connected to the "-" voltage and the "ground" terminal on the cap is connected to board ground. So the cap's a ticking time bomb and will short out. It takes out the minus voltage needed for the 4116 RAM chips. Replace the cap with a new one and DO reverse the leads!!!
>
> I improved the BIOS programing using Z80 instructions in the wait for character input routine so branch if character is present is faster. I also changed the I/O port mask to test all status lines and stop character overflow at 19200 output to terminal. Plus had a several bank RAM disk working.
>
> If you wish to contact me I have quite a bit interest in getting my systems back up and working.

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