Famicom Disk System: error 27, a stretched belt and a dirty drive spindle

What the error codes actually mean

Error 27 is a disk speed or seek failure: the drive commanded a read, the head never saw the sector it expected, and the firmware gave up. Error 22 is a disk set or eject problem, usually the eject mechanism not latching. Seeing 27 immediately after pressing start almost always means the belt is slipping.

Listen to the drive before opening it. A healthy FDS spins up to speed in well under a second with a steady whir. A failing belt gives you a slow ramp, a stutter, or the sound of the motor turning while nothing else does.

Confirming the belt before ordering one

Open the drive and look at the belt before buying anything. A belt that has stretched will sit loose on the pulleys and you can usually see it hanging. A belt that has turned to tar will be black and sticky and will have deposited residue on every pulley it touched.

Also check the drive spindle and the head carriage rails. The original lubricant on the rails turns to a sticky varnish after thirty years, and a carriage that drags will produce the same error 27 with a perfectly good belt.

1. Remove the six shell screws and lift the top off the drive assembly. The belt runs from the motor pulley to the drive pulley, visible without further disassembly on most units.
2. Measure the belt. The standard replacement is a square-section belt of 1.5 mm thickness with a 71 mm folded circumference, though kits vary and you should measure the old one.
3. Turn the drive pulley by hand and watch the motor pulley. If the belt slips while you hold the motor pulley still, the belt is finished.
4. Slide the head carriage along its rails by hand. It should move with almost no resistance and no graunching. Sticky rails mean the carriage needs cleaning before the belt goes on.
5. Check the spindle hub where the disk seats. A hub that has been slipping will be polished and shiny where it should be slightly rough.

Belt, rails and spindle

Fit the new belt by rolling it over the motor pulley first, then walking it onto the drive pulley with a plastic tool. Do not stretch it over the pulley with a screwdriver; you will nick it and it will fail within weeks. If you have a choice, buy two belts, because fitting the first one is how most people damage it.

Clean the head carriage rails with isopropyl alcohol and a lint-free swab until the swab comes away clean, then apply a very small amount of light silicone grease. Not oil, and not white lithium grease, both of which attract dust and turn sticky again. A drop the size of a pinhead is enough for the whole rail.

1. Roll the new belt onto the motor pulley, then onto the drive pulley.
2. Clean the carriage rails with isopropyl alcohol and let them dry.
3. Apply a pinhead of light silicone grease to each rail and work the carriage back and forth ten times.
4. Clean the read head with a foam swab and isopropyl alcohol. Let it dry completely before running a disk.
5. Test with a disk you do not mind losing. Write a disk only after read testing passes, because a slipping drive can corrupt a disk's directory.
6. Reassemble and run for ten minutes continuously. A belt that is going to slip usually does so once the drive is warm.