Atari 1050 Maintenance Guide

Atari 1050 with the top cover removed

The Atari 1050 needs three adjustments to stay compatible with other drives: spindle speed, the track 00 sensor and, on a worn mechanism, the radial alignment of the head. Atari's 1050 Disk Drive Field Service Manual (FD100330) gives the procedures and tolerances, and its diagnostic diskette does most of the measuring. Faults are diagnosed in the Atari 1050 Drive Troubleshooting guide.

Power adaptor

The 1050 runs from an external 9 V AC adaptor; Atari's specification calls for its 31 VA adaptor, C017945. The adaptor has an internal 2 A fuse, and the drive's own bridge rectifier, regulators and voltage doubler are on the main board. Atari specifies the drive's input as 8.52 V AC ±12 %, which is about 7.5 to 9.5 V AC.

• Use an AC adaptor. A DC supply such as the Indus GT's usually blows the 1050's rectifier diodes.
• Unplug the adaptor from the drive before plugging or unplugging the SIO cable. Doing it with the adaptor connected occasionally destroys the LM3086 transistor array at U1, at the rear right of the board.

Opening the drive

Atari's sequence:

1. Unplug the adaptor and the SIO cables.
2. Turn the drive over and remove the four screws in the bottom cover and the two screws holding the lower part of the front bezel.
3. Hold the top and bottom covers together and turn the drive the right way up.
4. Pull the lower part of the bezel forward and lift off the top cover at the same time. The thin plastic tabs joining the top of the bezel to the top cover break easily.
5. The mechanism rests on four dowels above the board. Disconnect its plugs from the board and lift it out.

The board snaps into the bottom cover over three alignment guides. The upper and lower RF shields are held by four bent tabs; note where the cutouts are before removing the top shield, and refit it the same way. Reassemble in reverse order.

Refitting the mechanism plugs

Pin 1 of each connector is marked on the board; ignore any number on the plug itself. Fit each plug by the wire colour order in figure 3-1 of the manual. Plugs fitted to the wrong headers are a common self-inflicted fault: one owner found J10 and J11 swapped on a drive that would not work until they were corrected, and another reports that swapping J1 and J10 killed the track 00 sensor.

Adjustment points on the Tandon mechanism, from figure 2-2 of the field service manual

Cleaning

Clean the read/write head with 91 % isopropyl alcohol; Atari's checklist gives this as the first remedy for intermittent read and write errors and for soft read errors. Check that the write protect sensor hole is free of dirt, because dirt there makes the drive report every disk as write protected.

Sams recommends cleaning the heads about once a month or every 100 hours of use with an approved head cleaning kit, and transporting a drive with an old disk in the slot and the door closed.

Atari's owner's guide asks users to keep the drive at least 12 inches from a television, never to switch the drive on or off with a diskette inside, and never to leave a diskette in the drive while it is off.

The service manual gives no lubrication procedure. Jason Moore's write-up suggests a very small amount of white lithium grease on the head carriage rails, though he notes he had never needed to do it.

Mechanical checks

Before testing, Atari checks that the diskette enable lever moves freely and locks in the perpendicular position, that the eject mechanism clicks into place when a diskette is pushed fully in, and that the diskette is ejected when the lever is returned to the parallel position. A drive that does not spin the disk may simply have its drive belt off the pulleys under the mechanism; refitting the belt fixed one of three drives in Moore's write-up.

Diagnostic diskette

Atari's field test equipment is a 16K computer, a television, a 15 MHz oscilloscope, a digital voltmeter, the 1050 diagnostic diskette (FD100690), the Dymek alignment diskette (TE017575), a work diskette (FD100053) and a preformatted diskette (FD100026). An image of the Atari CPS 1050 diagnostic diskette is on the Internet Archive. Connect only the drive under test to the SIO bus, set it to drive 1, and run the tests with a good single-density formatted disk in the drive; a blank or unformatted disk fails them.

The diskette offers two menus. Option 1 runs the automatic tests (controller, invalid command, write protect, motor start, motor speed, head step and settle, track 00). Option 2 gives the troubleshooting options: speed calibration loop, manual alignment tests, drive select test, a 10-minute single-density burn-in, a 2-hour double-density burn-in, error printout and track zero sensor calibration. The diskette does not work with a drive fitted with a Happy or Speedy board.

Spindle speed

The spindle turns at 288 rpm, one revolution in 208.3 ms. Atari's tolerance for the adjustment is ±1.0 ms.

1. Open the drive and leave the board and mechanism in place.
2. Run troubleshooting option 1, loop on speed calibration.
3. Turn VR2 with a small flat screwdriver until the reading is 208.3 ms.

VR2 is a small blue upright trimmer at the rear left of the board as seen from the front. Its screw is often covered with hard sealant, which has to be chipped away carefully. The board is live while you adjust it, so touch nothing else. A software speed checker can stand in for the diagnostic diskette.

Speed matters for compatibility. A drive that runs fast can format a disk badly enough to give Error 144 later, and a slow drive packs data closer together so that other drives struggle to read its disks, starting with the inner tracks. Drives also run slightly faster from cold. If the speed will not adjust or will not hold, Atari's checklist points to U5, Q4, Q6 and VR2 in the motor and tachometer circuits, an open tachometer winding in the motor, or a slipping belt.

Track 00 sensor

Troubleshooting option 7 reports "forward" or "backward" when the sensor needs moving. Loosen the screw holding the sensor, move it as the screen asks until it reports that the sensor is OK, and hold it in place while tightening the screw. The movement needed is very small. Moore loosened the two screws above and below the sensor to slide it.

The sensor also has an adjustable plastic block behind it that acts as a stop. Set wrongly, it keeps the head's plastic tang out of the sensor.

Head alignment checks

These checks need the Dymek alignment diskette and an oscilloscope set up as in the table, with probes on TP3 and TP4 and ground on TP17.

Oscilloscope set-up for the 1050 alignment checks (FD100330 p. 2-5)

Setting	Value
Channel A	Normal, TP3
Channel B	Inverted, TP4
Vertical mode	Add A and B
Time base	2 µs/div (20 ms/div for cats-eyes, 0.5 ms/div for pad pressure and azimuth)
Vertical	20 mV/div with ×10 probes (50 mV/div for pad pressure and azimuth)
Trigger	Internal, channel A, AC, normal, positive slope

• Track 0: the read signal is a sine wave of about 16.6 µs per cycle, a cross-check of spindle speed.
• Track 16, radial alignment: the two cats-eye lobes should be equal (100 %). Loosen the stepper motor screws and rotate the motor until they are, recheck the track 00 sensor, and repeat until the ratio is within 80-100 %.
• Track 33, head load pad: press very lightly on the pressure pad. If the amplitude rises by more than 20 %, turn the pad a quarter turn at a time; after three-quarters of a turn treat the pad as worn out and replace it.
• Track 34, head azimuth: compare with figure 2-3. Atari replaces the whole mechanism if azimuth is more than 18 minutes of angle out, because the adjustment is too involved for field service.

Burn-in

The single-density burn-in writes and reads every sector of preformatted diskette FD100026 in about 10 minutes, and a failure points to alignment. The double-density burn-in formats a disk, writes and reads it with sequential and back-and-forth seeks for 2 hours, and logs errors to sectors 1 and 3 of each track for the error printout. Disconnect the drive from the SIO bus once it starts.

Capacitors

The drive has twelve electrolytic capacitors, including the three large filter capacitors of the power supply. Values, ratings and post-recap checks are on the Atari 1050 Capacitor Replacement Guide; see also Capacitor Failure Symptoms.

Checklist

1. Check the adaptor is a 9 V AC type in working order.
2. Clean the head with 91 % isopropyl alcohol.
3. Check the diskette enable lever and eject action.
4. Run the diagnostic diskette's automatic tests.
5. Set the speed to 208.3 ms with VR2 if the speed test fails.
6. Calibrate the track 00 sensor if the track 00 test fails.
7. Run the single-density burn-in to confirm compatibility.

Related pages

• Atari 1050
• Atari 1050 Drive Troubleshooting
• Atari 1050 Capacitor Replacement Guide
• Atari 1050 Documentation
• Floppy Drive Repair
• Recommended Tools