Oldskooler Ramblings

the unlikely child born of the home computer wars

Archive for September 26th, 2013

The lazy way to work a sticky floppy drive spindle free

Posted by Trixter on September 26, 2013

In preparing for VCFMW, I was dismayed to find that one of my PCjr’s floppy drives (a Qumetrak 142) has a very sticky spindle, or a slightly stretched belt, or both.  The end result is that the diskette spins quite unevenly, like the belt is slipping.  To fix this, there are a lot of things I could do:

  • Order a new belt (someone on the PCjr forums thinks he has a lead on a source)
  • Try to rough up the inside of the belt so it has more friction
  • Partially take apart the drive and adjust two screws that will move the motor away from the spindle which will give the belt more tension

…or, take the lazy way out.  I noticed that, no matter how off the drive speed was, it usually managed to load the boot sector.  So, I created a boot floppy, loaded the boot sector into debug, and patched the boot code to this:

-l 100 1 0 1
-u 013e
1236:013E B402    MOV AH,02
1236:0140 B001    MOV AL,01
1236:0142 0E      PUSH CS
1236:0143 07      POP ES
1236:0144 BB0010  MOV BX,1000
1236:0147 FEC5    INC CH
1236:0149 80E51F  AND CH,1F
1236:014C B101    MOV CL,01
1236:014E B200    MOV DL,00
1236:0150 B600    MOV DH,00
1236:0152 CD13    INT 13
1236:0154 EBE8    JMP 013E

(I know the code can be optimized smaller, no need to tell me — this means you, Peter ;-)  This will proceed to read 1 sector from tracks 0 through 31 and then jump back to track 0 and do it again, endlessly.

I wrote this back to sector 0 and booted the disk.  At first, my drive sounded like this:  Chunk….. chunk chunk…. chunk…. … ….. chunk chunk chunk…..

An hour of it running and now I hear this:  Chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk chunk    Problem solved!  Well, until Saturday anyway, when I’ll likely need to run this again for an hour to work the drive.  But that will be enough for the tutorial diskette to run for the patrons.

PS: That boot sector also makes for a handy exercise to use with a floppy cleaning disk.

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