|Xerox 820-II, ca. 1982|
|CPU:||Z80A @ 4MHz|
|Floppy Drives:||2 Shugart SA800 8" single-sided drives|
|Display:||12" White (24x80)|
|Ports:||2 RS-232, parallel printer (internal), floppy/hard drive|
|ROM:||6KB (monitor and ROSR))|
When I received this system, the previous owner pointed out a X-class filter capacitor in the power supply which had exploded when he turned it on. I replaced that cap, along with 3 other X- and Y-class caps for good measure. After that, the unit powered up and displayed the monitor prompt, but the keyboard was mostly non-functional: extra characters, incorrect characters, and at least 2/3 of the keys didn't work at all. I tracked that down to a defective CD4051BE 8-channel analog multiplexer chip on the keyboard PCB. After I replaced the 4051, the keyboard worked, but the system would not boot from diskette. I eventually found the cause to be a defective 4164-20 RAM chip (U7) which had a bit stuck on zero from ED80 - EDFF. After the RAM chip was replaced, the system booted normally from diskette.