Xerox 820-II

 
Exploded X-class capacitor
 
Xerox 820-II, ca. 1982

CPU: Z80A @ 4MHz
RAM: 64KB
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
Notes:

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.


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Last updated on Monday, December 19, 2016