23

zero bits [moulo?] 4 in each sector.
The self-tester is based on a 16-bit
shift register that shifts once an
18-bit byte time. THus it will
[??way] shift the pattern 4 times in
each sector, and so the check
character will always be its initial
value. This is also true of
almost all test patterns, such as
counting, etc.

Writing the pattern:-

word 1: 200000 00000
all others 0
gives permanent read errors on secter 3 of TRK 0

the pattern:
word 1: 577777 000000
all others 0
gives no read errors.

Dutch found 2 bad modules in check
bit computation circuit. Replaced them.
Now above patterns work. Tried
also