Xerox 820-II

Exploded X-class capacitor
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
Model: 820-II
Serial No.:
OS: CP/M 2.2
ROM: 6KB (monitor and ROSR))
Original Price: $3,295

When I received this system, the previous owner pointed out an 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.

Return to the Hardware page.
Return to the Old Technology Collection page.

Last updated on Friday, June 24, 2022