Troubleshooting Floppy Disk Systems 581

motor, drive belt (if used, and spindle assembly. Be certain that no broken parts or obstructions could jam the spindle. If a belt is between the motor and spindle, be sure that the belt is reasonably tight—it should not slip. You should also examine the floppy drive with a diskette inserted to be certain that the disk’s insertion or alignment is not causing the problem. You can double-check your observations using several different diskettes. Gently remove any obstructions) that you might find. Be careful not to cause any accidental damage in the process of clearing an obstruction. Do not add any lubricating agents to the assembly, but gently vacuum or wipe away any significant accumulations of dust or dirt. Remove any diskette from the drive and reconnect the floppy drive’s signal and power cables. Restore power to the computer and measure drive voltages with your multimeter. Ground your multimeter on pin 2 and measure +12 Vdc on pin 1. Move the meter ground to pin 3 and measure +5 Vdc on pin 4. If either voltage is low or absent, troubleshoot your computer power supply. Once you are confident that the floppy drive is mechanically sound and appropriate power is available, you must determine whether the trouble is in the floppy drive PC board or the floppy drive controller board. Use your logic probe to measure the Motor on signal in the physical interface (pin 16). When drive access is requested, the Motor on signal
582 Floppy Drives should become true (inmost cases an active low. If the Motor on signal is missing, the floppy drive-controller board is probably defective and should be replaced If the Motor on signal is present at the interface, check the signal driving the servo circuit. A missing Motor on signal at the servo circuit suggests a faulty control-circuit IC. If the signal reaches the servo circuit, the servo IC is probably defective. You can replace the servo IC, but your best course is usually to replace the spindle motor/PC board assembly as a unit. If you are unable to replace the spindle motor PC board, you can replace the floppy drive outright. Symptom 16-4. The floppy drive will not read from/write to the diskette All other operations appear normal. This type of problem can manifest itself in several ways, but your computer’s operating system will usually inform you when a disk read or write error has occurred. Begin by trying a known-good, properly formatted diskette in the drive. A faulty diskette can generate some very perplexing read/write problems. If a known-good diskette does not resolve the problem, try cleaning the read/write heads, as described in the previous section. Do not run the drive with a head-cleaning disk inserted for more than 30 seconds at a time, or you risk damaging the heads with excessive friction. When afresh diskette and clean R/W heads do not correct the problem, you must determine whether the trouble exists in the floppy-drive assembly or the floppy-controller IC.
If you cannot read data from the floppy drive, use your logic probe to measure the Read
data signal (pin 30). When the disk is idle, the Read data line should read as a constant logic 1 or logic 0. During a read cycle, you should measure a pulse signal as data moves from the drive to the floppy-controller board. If no pulse signal appears on the Read data
line during a read cycle, use your oscilloscope to measure analog signals from the R/W heads. If there are no signals from the R/W heads, replace the head or head carriage assembly. When signals are available from the R/W heads, the control-circuit IC is probably defective and should be replaced. If you are unable to replace the IC, you can

replace the drive’s control PC board. You can also replace the entire drive outright. If a pulse signal does exist during a read cycle, the floppy-disk controller board is probably defective and should be replaced. When you cannot write data to the floppy drive, use your logic probe to measure the Write gate and Write data lines (pins 24 and 22, respectively. During a write cycle, the Write gate should be logic 0 and you should read a pulse signal as data flows from the floppy controller IC to the drive. If the Write gate remains logic 1 or no pulse is on the Write data line, replace the defective floppy controller board. When the two
Write signals appear as expected, check the analog signal to the R/W heads with your oscilloscope. Ifyou do not find analog write signals, replace the defective control-circuit IC. If analog signals are present to the heads, try replacing the heads or the entire head carriage assembly.
You can also replace the entire drive outright. Symptom 16-5. The drive is able to write to a write-protected disk Before concluding that there is a drive problem, remove and examine the disk itself to ensure that it is actually write rotected. If the disk is not write protected, write protect it appropriately and try the disk again. If the disk is already protected, use your multimeter to check the drive’s write- protect sensor. For an unprotected disk, the sensor output should be a logic 1; a protected disk should generate a logic 0 (some drives might reverse this convention.

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