Originally posted in the newsgroup comp.sys.cbm by Michael Parson (firstname.lastname@example.org):
The following was taken from RUN magazine, Sept 1988 Issue, Magic Trick $4CC, page 12.
If your 1541 or 1581 disk drive hasn't been behaving well lately, it may be out of alignment. You could take it to a repair shop, but before you shell out $0 or $50, try "knocking" some sense into it with my Reverse Knocker program.
This 64- and 128-mode program reverse-knocks the drive head 100 times, which may re-align the drive just enough to postpone and expensive realignment. Be forewwarned: have an old work disk in the drive when you run it, and don't worry if running this program makes your disk drive sound like a smoldering Buddy Rich drum solo. It is a noisy program, but if you type it in correctly, it won't hurt the drive or disk at all.
0 REM REVERSE KNOCK YOUR DRIVE - by STEPHEN CHEUNG 10 OPEN 15,8,15,"I" 20 SP=1:FORI=1 TO 100:GOSUB40:NEXT 30 FORI=1 TO 20:SP=-1:GOSUB40:NEXT:PRINT"ALL DONE!":PRINT#15,"I":CLOSE15:END 40 PRINT#15,"M-R"CHR$(0)CHR$(28):GET#15,A$:A=ASC(A$+CHR$(0)):BI=A AND 3 50 BI=BI+SP:BI=BI AND 3 60 R=(A AND 252) OR BI: PRINT#15,"M-W"CHR$(0)CHR$(28)CHR$(1)CHR$(R):RETURN