Atari 810 Maintenance Guide

Speed adjustment guide and the location of R142, from page 7-10 of the field service manual

The Atari 810 needs its head cleaned and, from time to time, its belt, spindle speed, radial alignment and track 00 stop checked. Atari's Atari 810 Disk Drive Field Service Manual (FS015854 Rev. 1, July 1980) gives the procedures for the original drive with the MPI mechanism and the single rear board. The later Analog and Tandon drives differ in places, noted below. Faults are diagnosed in the Atari 810 Drive Troubleshooting guide.

Power adaptor

The drive runs from 9 V AC supplied by an external step-down transformer; the rectifier and regulators are inside the drive. Early drives shipped with Atari's 15.3 VA adaptor (C014319, sold as CA014748), and later ones with 31 VA adaptors such as the C017945, which the FAQ lists as suitable for every 810. Sams measured 10.9 V AC at the output plug of a working adaptor.

Unplug the adaptor from the drive before connecting or disconnecting SIO cables. An older edition of the FAQ quotes Rich Mier on the damage that hot-plugging does to the 1050's serial interface chip, and adds that "it is really bad on 810's".

Opening the drive

The drive contains static-sensitive MOS devices, so work on a static-protected surface with a grounding strap. Do not use magnetised tools near the head.

1. Remove the four adhesive covers on the top housing and the four screws under them, then lift the top housing off the base.
2. Disconnect the adaptor and the I/O cables, remove the five screws holding the common baseplate to the base, and lift the baseplate out. Each boss in the base has a rubber washer on top.
3. To remove the side board, disconnect the write protect harness and the stepper motor harness, remove the three screws holding the board and its ground bracket, and lift the board straight up to free its rear jack from the pins of the rear board.
4. To remove the rear board, take out the side board, then the four screws, and lift the board straight up off the pins of the baseplate devices.

Sams describes a different first step: remove the four rubber feet and the four screws from the cabinet bottom, then lift off the top. Atari's torque limits are 6 inch-pounds for screws into plastic and 10 inch-pounds into aluminium.

The internal harnesses end in non-polarised pins and jacks, so note how each plug sits before removing it, and check every plug before switching on. Route the harnesses so that none is pinched when the drive goes back together. Appendix F of the manual shows an alternate wiring harness arrangement.

Head cleaning and demagnetising

Both the head and the pressure pad collect dirt and oxide. Atari's procedure:

1. Inspect the pressure pad with a small mirror for oxide build-up and wear. Replace the head if it is worn or damaged.
2. Clean the head with a cotton swab, or a lint-free cloth such as chamois, moistened with methyl alcohol or 91 % isopropyl alcohol, then dry it. Do not use carbon tetrachloride.
3. Do not lift the pressure pad arm further than the door would lift it in normal use.
4. To demagnetise the head, hold the switched-on demagnetiser about 6 inches above the head, bring it slowly down in a spiral without touching the head, and reverse the movement. Switch it off only when it is at least 6 inches away.

Jason Moore cleans 810 heads with 99 % isopropyl alcohol on a long-stemmed swab, lifting the arm that holds the felt pad. He notes that the felt pad wears and can be replaced. Sams recommends cleaning the heads about once a month or every 100 hours of use with an approved cleaning kit, and carrying the drive with an old disk in the slot and the door closed. Atari's caution sheet also asks for the door to be kept closed when the drive is moved.

Drive belt

The transport has either the older yellow Mylar belt or the newer black neoprene belt. Adjust it with the power off.

• Mylar belt: loosen the two drive motor screws, move the motor until the tension on either leg of the belt reads 17 ±1 grams on a tensiometer, and tighten the screws.
• Neoprene belt: loosen the two drive motor screws, hold the motor as close to the spindle and flywheel as it will go, and tighten the screws.

To take the belt off without stretching it, turn the large flywheel while easing the belt off its outer edge. A belt that has fallen off, broken or stretched leaves the diskette not turning or turning at the wrong speed.

Spindle speed

Atari sets the speed by measuring the drive motor's tachometer. With the drive reading any track, put the oscilloscope on the anode of CR110 (the tachometer output) and adjust R142 on the rear board until the frequency matches the table for the belt and motor pulley fitted.

Tachometer frequencies (FS015854 p. 7-10)

Drive belt	Motor pulley diameter	Tachometer frequency	Period	Drive motor part number
Mylar (yellow)	0.250 in	390 Hz	2.564 ms	3-35003-002
Mylar (yellow)	0.300 in	320 Hz	3.125 ms	3-35003-001
Neoprene (black)	0.300 in	305 Hz	3.279 ms	3-35003-001

A software speed checker does the same job without a scope. Page 6 describes R142 as a plastic disc about 15 mm across with a slot through the centre, usually milky white, in the back left-hand corner of the rear board. Turn it clockwise to slow the drive and anticlockwise to speed it up, in very small steps, while the checker runs. The drive can be run with the cover off for this, touching nothing else. On later drives with an extra board above the mechanism, Page 6 gives the speed control as R104, usually green, to the left of the only chip on that board. The stroboscope rings on the flywheel were made for 300 rpm and are no use for setting an 810.

If the speed will not hold, Atari's troubleshooting guide lists the tachometer lead out of place (connect the green motor harness lead to the centre terminal of the write protect harness), the write protect circuit, belt tension, a stretched belt, the drive motor circuit on the rear board and seizing spindle bearings. The washers between the flywheel, casting and spindle also affect the speed, so keep every one when the flywheel is removed.

Radial track alignment

This check needs an oscilloscope with A+B and channel B invert, a master alignment diskette, the disk alignment cartridge or program, a 0.050 inch hex setscrew driver and a non-conductive tuning wand. Keep the BUSY light off whenever a diskette goes in or comes out.

1. Insert the master alignment diskette and command the drive to read track 16.
2. Set the scope to A+B with B inverted, 20 ms/div and 20 mV/div, and compensate the probes.
3. Ground the scope to the faston lug at the rear of the transport. Put channel A on the rear lead of C154 and channel B on the rear lead of C155. (The step text prints CR154; the figure and the parts list have C154.)
4. The scope shows two cat's-eye lobes. If they differ, back the stepper band pulley setscrew out about a quarter turn, rotate the pulley until the lobes are equal, and tighten the setscrew.
5. Step to track 00 and back to track 16, then to track 32 and back. The lobes must stay within 80 % of each other each time; if not, go back to step 4.

If the result will not hold, Atari suspects a broken or damaged setscrew, a binding or misaligned positioning band, the head carriage binding on its guide rods, or the stepper motor. The positioning band is delicate; do not kink it. On the Tandon mechanism the stepper is adjusted with a cam screw at the rear of the mechanism instead.

Track 00 stop

On the MPI mechanism, read track 00 from the alignment diskette and set the track 00 stop setscrew at the rear of the transport for 0.010 inch clearance to the rear corner of the head carriage moulding, using a feeler gauge. Step out past track 30 and back, and check that the carriage does not butt against the setscrew. The Tandon mechanism is set by watching the waveform amplitude rise and fall.

Capacitors

The rear board has eight aluminium electrolytic capacitors, including three 4,700 µF filters in the power supply, and the side board has one. Values, ratings and checks are on the Atari 810 Capacitor Replacement Guide; see also Capacitor Failure Symptoms.

Checking the result

Atari's floppy quick-check, run before and after any service, needs a computer with BASIC and at least 16 KB, a TV, a write-protected master diskette, a blank scratch diskette and a sample diskette written on a known good drive.

1. Booting test: boot the master diskette; the system should load in about 10 seconds and show READY.
2. Write protect test: try to format the write-protected master; the screen must show ERROR-144.
3. Formatting test: format the scratch diskette (about 45 seconds), then list the directory; it must show 709 free sectors.
4. Write verification: write DOS files to the scratch diskette, list them, delete them, and check that 709 free sectors remain.
5. Compatibility test: boot the sample diskette and load and run a program from it.

Related pages

• Atari 810
• Atari 810 Drive Troubleshooting
• Atari 810 Capacitor Replacement Guide
• Atari 810 Documentation
• Floppy Drive Repair
• Recommended Tools