Editing Atari 1050 Drive Troubleshooting

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[[File:Atari 1050 main board.jpg|thumb|right|350px|Atari 1050 main board]]
Fault-finding on the '''[[Atari 1050]]''' starts with the drive's own power-up self test, moves to Atari's diagnostic diskette, and then to the symptom checklist and waveforms in the ''1050 Disk Drive Field Service Manual'' (FD100330).<ref name="fsm">Atari, Inc., ''1050 Disk Drive Field Service Manual'' (FD100330). Power supply p. 1-8; serial bus pp. 1-20 and 1-21; diagnostic tests pp. 2-1 to 2-11; disassembly pp. 3-1 and 3-2; symptom checklist pp. 4-1 to 4-7; power supply waveforms p. 4-16; parts list p. 5-11; Tech Tip 21. Hosted here as [[:File:Atari 1050 Disk Drive Field Service Manual.pdf]].</ref> The checklist names parts by Atari's designators; the Sams ''Computerfacts'' for the drive has a full schematic with component values and DC voltages.<ref name="sams">Howard W. Sams & Co., ''Computerfacts'' CD13, ''Atari Model 1050 Disk Drive'' (1987), schematic with voltages and waveforms. Hosted here as [[:File:Sams Computerfacts CD13 Atari 1050 Disk Drive.pdf]].</ref> Adjustments and cleaning are on the [[Atari 1050 Maintenance Guide]].

== Before you start ==

* Use a 9 V AC adaptor. A DC adaptor, such as the Indus GT's, usually blows the rectifier diodes CR17-CR20.<ref name="faq">Michael D. Current (ed.), ''Atari 8-Bit Computers: Frequently Asked Questions'', comp.sys.atari.8bit FAQ, last modified 29 November 2009, sections 3.4 and 6.7. [http://www.faqs.org/faqs/atari-8-bit/faq/ faqs.org]. Community reference.</ref><ref name="fsm" />
* Unplug the adaptor before connecting or disconnecting SIO cables; hot-plugging can destroy U1.<ref name="faq" />
* Disconnect every other SIO device, including disk emulators, set the drive to drive 1, and test with a good single-density formatted disk.<ref name="aa282584">"Attempting to repair a 1050 Disk Drive - no activity light no motor/head spin", AtariAge Forums, thread started by repetto74, 4-5 September 2018, replies by Nezgar and xrbrevin. [https://forums.atariage.com/topic/282584-attempting-to-repair-a-1050-disk-drive-no-activity-light-no-motorhead-spin/ forums.atariage.com]. Community repair report.</ref><ref name="fsm" />
* A drive with a Happy or Speedy upgrade fails Atari's diagnostic diskette whether or not it is faulty.<ref name="ap1050">Jason H. Moore, "Diagnose and Fix an Atari 1050 Disk Drive (30-60 mins)", ''Atari Projects'', 8 August 2021. [https://atariprojects.org/2021/08/08/diagnose-and-fix-an-atari-1050-disk-drive-30-60-mins/ atariprojects.org]. Community repair write-up.</ref>

== Power-up self test ==

Switch the drive on with no diskette. A good drive lights the power LED and the activity LED, starts the spindle motor, steps the head out to track 0, and after several seconds turns the motor and activity LED off.<ref name="fsm" />

During that time the firmware checks the 6532 PIA, tests the 2793 controller's track and sector registers, checksums the ROM, and writes and reads every RAM location. Any failure stops the test and puts the drive into a hard failure state, in which the spindle motor cycles off and on every 2 seconds. The drive also fails if it steps more than 50 tracks without seeing the track 00 sensor.<ref name="fsm" />

{| class="wikitable styled-table" style="width:100%;"
|+'''Power-up behaviour'''
! Symptom !! Probable cause !! Action
|-
| Power LED off, nothing happens || Adaptor, bridge diodes CR17-CR20, 5 V regulator Q7, LED CR21 || Check the adaptor and TP13; see the power supply section<ref name="fsm" />
|-
| Spindle motor cycles on and off every 2 seconds || A self-test failure: PIA, controller, ROM or RAM, or no track 00 signal || Swap the socketed chips one at a time; check the track 00 sensor<ref name="fsm" /><ref name="aa282584" />
|-
| Head steps outwards and bangs against its stop || Track 00 sensor dead, misadjusted or obstructed || See the track 00 section<ref name="aa288046" /><ref name="aa282584" />
|-
| Power LED on, no activity light, no motor, no head movement || Failed chip on the board; one drive had a bad 6810 RAM at U8 || Swap U7, U8, U9, U10 and U13 with known-good parts<ref name="aa282584" />
|}

== Diagnostic diskette ==

Boot Atari's 1050 diagnostic diskette (FD100690) and choose option 1. The tests point to a circuit:<ref name="fsm" />

{| class="wikitable styled-table" style="width:100%;"
|+'''Atari 1050 diagnostic tests (FD100330 pp. 2-3 and 2-4)'''
! Test !! What it checks !! A failure means
|-
| Controller || The FDC2793 recognises a command or data frame || Defective FDC2793
|-
| Invalid command || The FDC2793 rejects an illegal command frame || Controller fault
|-
| Write protect || Write protect sensor and circuit || Sensor or circuit fault; check the sensor hole for dirt
|-
| Motor start || Time from a dead start to valid data || Motor control or read circuit
|-
| Motor speed || Spindle speed in milliseconds || Adjust VR2 with the speed calibration option
|-
| Head step and settle || Read a sector, step, read the next within a time limit || Drive mechanism or read circuit
|-
| Track 00 || Steps out to track 02 and back, then reads the sensor || Adjust with the track zero sensor option
|}

If the drive hangs and ignores the abort command, switch it off, remove the diskette, switch it on, reinsert the diskette and repeat the test.<ref name="fsm" />

=== Burn-in error log ===

The double-density burn-in logs errors on the test disk and option 6 prints them. The pass counter should equal the pass count received; many errors of one type on the same track and sector call for a rerun on a different blank diskette before suspecting the hardware. Atari's guide to the error types:<ref name="fsm" />

{| class="wikitable styled-table" style="width:100%;"
|+'''Error log interpretation (FD100330 p. 2-11)'''
! Error type !! Atari's interpretation
|-
| Data CRC || Several of these: replace the WD2793; troubleshoot the read and write circuits
|-
| Data lost || Problem transferring data between the WD2793 and the 6810 RAM
|-
| Data hard || Data not found after many retries; check the write circuit and the diskette
|-
| Data soft || Read on a later pass, not the first; many in several places suggest speed or the WD2793
|-
| Seek || Many: bad diskette or radial (cats-eye) alignment
|-
| RNF || Many in one place: bad diskette. Many in different places: bad WD2793
|}

== Power supply ==

The power switch feeds the 9 V AC from the adaptor to the bridge rectifier CR17-CR20. Regulator Q7 makes +5 V, and a doubler (CR15, CR16 and C71) feeds regulator Q8, which makes +12 V.<ref name="fsm" />

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Power supply test points and voltages'''
! Point !! Expected !! Source
|-
| TP13 || +5 V DC || FD100330 p. 4-16<ref name="fsm" />
|-
| TP14 || +12 V DC || FD100330 p. 4-16<ref name="fsm" />
|-
| TP15 || Ground || FD100330 p. 4-16<ref name="fsm" />
|-
| Anode of CR21 (power LED) || 1.7 V DC || FD100330 p. 4-16<ref name="fsm" />
|-
| Adaptor output plug, unloaded || 10.9 V AC || Sams CD13 p. 6<ref name="samsts" />
|-
| Cathodes of CR17 and CR18 || About 11.1 V || Sams CD13 p. 6<ref name="samsts" />
|-
| Q7 (µA7805) input || 9.8 V || Sams CD13 schematic<ref name="sams" />
|-
| Q8 (L7812CV) input, pin 1 || 17.8 V on the schematic; 22.4 V in the troubleshooting text || Sams CD13<ref name="sams" /><ref name="samsts" />
|-
| Q6 collector (motor supply) || 12 V, constant || FD100330 p. 4-8<ref name="fsm" />
|}

Atari's waveform page shows a 12 V full-wave ripple of 8.3 ms period at the cathode of CR18, half-wave pulses of about 11 V and 13 V peak with a 16.6 ms period at CR19 and CR20, and a 24 V level with 16.6 ms ripple at the cathode of CR16; these are 60 Hz figures.<ref name="fsm" />

{| class="wikitable styled-table" style="width:100%;"
|+'''Power supply problems (FD100330 p. 4-5)'''
! Symptom !! Possible cause !! Corrective action
|-
| No power or blows power || Defective bridge diodes || Replace all four diodes, CR17-CR20
|-
| || Defective adaptor || Replace the adaptor
|-
| No +5 V at TP13 || Defective 5 V regulator || Replace Q7
|-
| Power LED not lit || Defective 5 V regulator || Replace Q7
|-
| || Defective LED || Replace CR21
|-
| No +12 V at TP14 || Defective 12 V regulator || Replace Q8
|-
| || 12 V doubler circuit || Replace CR15 and CR16, or C71
|}

Sams' own check runs in the same order: measure the adaptor's output, then the rectifier cathodes, then TP13. If +12 V is missing at TP14, look for the doubler's output at pin 1 of Q8; if that is missing, check CR15, CR16, C66 and C68, and if it is present, check Q8, C70 and C65.<ref name="samsts">Howard W. Sams & Co., ''Computerfacts'' CD13, ''Atari Model 1050 Disk Drive'' (1987), p. 6, Troubleshooting (power supply, disk drive inoperative, stepping motor inoperative, no communication). Hosted here as [[:File:Sams Computerfacts CD13 Atari 1050 Disk Drive.pdf]].</ref> The two Sams figures for the Q8 input differ: the schematic marks 17.8 V there, with a 150 mV ripple waveform on an 18 V level, and the troubleshooting text gives 22.4 V.<ref name="sams" /><ref name="samsts" />

The power supply electrolytics are now about forty years old; the [[Atari 1050 Capacitor Replacement Guide]] lists them.

== Drive motor and speed ==

{| class="wikitable styled-table" style="width:100%;"
|+'''Drive motor and speed problems (FD100330 pp. 4-1 and 4-2)'''
! Symptom !! Possible cause !! Corrective action
|-
| rowspan="4" | Motor does not turn when the latch is shut (no activity LED) || Diskette enable switch || Replace the switch assembly on the mechanism
|-
| Bad connection || Check that P1 is properly connected to J1
|-
| Diskette enable circuit (U6) || Troubleshoot (waveforms p. 4-11)
|-
| Motor control circuit (Q4, Q6, U5, VR2) or the motor || Troubleshoot (p. 4-8); replace the motor
|-
| Motor runs sporadically || Connection, motor control circuit (U5, VR2, Q4, Q6), tachometer feedback (U5, VR2, Q6), open tachometer winding || Check P1/J1; troubleshoot; replace the motor
|-
| Speed too slow or too fast || Speed adjustment, tachometer feedback, open tachometer winding || Adjust VR2 for 208.3 ms; troubleshoot (p. 4-9); replace the motor
|-
| Speed unstable || Connection, slipping belt, cone pressure, tachometer feedback (U5, Q6, VR2), motor bearings || Check P1/J1; replace the mechanism (belt, cone); troubleshoot (p. 4-9); replace the motor (bearings)
|}

Sams adds resistance checks at plug P1: about 11 Ω across pins 1 and 2 (the motor) and 186 Ω across pins 3 and 4 (the tachometer); other readings mean a new motor. Pin 11 of the PIA (U7) should read 3.2 V with the motor stopped and 0.1 V with it running; if not, substitute U7.<ref name="samsts" />

The speed loop is measurable: pin 9 of U5 sits at a constant 8 V, and turning VR2 moves the voltage on U5 pin 10, which changes the frequency on the motor control lines.<ref name="fsm" /> One of three drives Moore repaired had simply lost its drive belt, which slips back on to the pulleys under the mechanism.<ref name="ap1050" />

== Stepper and track 00 ==

{| class="wikitable styled-table" style="width:100%;"
|+'''Stepper and drive mechanism problems (FD100330 pp. 4-2 and 4-3)'''
! Symptom !! Possible cause !! Corrective action
|-
| Head will not step || Connection at J15; stepper driver circuit (U2, U3, U7); stepper motor || Check polarity at J15; troubleshoot (waveforms p. 4-10); replace the mechanism
|-
| Skips or steps to the wrong track || J15; driver circuit; band, pulley or carriage binding; track 00 sensor or Q5 || As above; adjust the track 00 sensor
|-
| Diskette will not eject || Eject assembly binding or broken || Replace the mechanism
|-
| Fails the track 00 sensor test || Sensor defective or misadjusted; carriage stop missing or wrongly set; Q5 or U13 || Adjust or replace the sensor; replace the mechanism (carriage stop); troubleshoot (p. 4-11)
|}

Sams checks for 12 V on pins 1 to 6 of plug P15 and about 33 Ω across each stepper winding. An open winding means a new mechanism; with correct voltages, check the waveforms at pins 18, 19, 21 and 22 of the PIA and substitute U7 if any is missing.<ref name="samsts" />

When the sensor is suspect, push the head in and out of it by hand while watching the sensor output, or block the slot with card to see whether the drive's behaviour changes.<ref name="aa288046">"Atari 1050 Track 0 sensor repair", AtariAge Forums, thread started by TGB1718, 8 February 2019, replies by tep392 and kheller2, 22 August 2022. [https://forums.atariage.com/topic/288046-atari-1050-track-0-sensor-repair/ forums.atariage.com]. Community repair report.</ref><ref name="aa282584" /> Check the adjustable plastic stop block behind the sensor before replacing anything: set wrongly, it keeps the head's tang out of the slot.<ref name="aa282584" />

On the Tandon mechanism an Optek OPB840 slotted switch has replaced a failed sensor. The wires to its LED are the other way round from the original, so pins 1 and 2 must be crossed. Mark the sensor's position before removing it, then set it with the diagnostic diskette's track zero calibration.<ref name="aa288046" /> World Storage Technology mechanisms use a sensor in a different package that the OPB840 does not fit.<ref name="aa288046" /> Best Electronics has listed separate track 0 sensors for the Tandon and World mechanisms.<ref name="aa282584" />

== Read and write problems ==

{| class="wikitable styled-table" style="width:100%;"
|+'''Read and write problems (FD100330 pp. 4-3 to 4-5)'''
! Symptom !! Possible cause !! Corrective action
|-
| Intermittent read/write errors || Head load pad pressure; dirty head; burned head || Replace the mechanism; clean with 91 % isopropyl alcohol
|-
| Diskette incompatibility || Radial alignment; azimuth; slipping belt; speed on either drive || Adjust the cats-eyes; replace the mechanism; set VR2 on both drives
|-
| Soft read errors || Diskette; dirty head; slipping belt; noise in the read circuit (U13, U18-U20, U22-U24) || Try the diskette on a known-good drive; clean; replace the mechanism (belt); troubleshoot (p. 4-12)
|-
| Fails to read anything || J6 polarity; read circuit (U13, U18-U20, U22-U24); burned head; speed; pad pressure; alignment || Check J6; troubleshoot; adjust VR2 to 208.3 ms; replace the mechanism
|-
| Fails to write anything || J6 polarity; dirty head; write protect circuit (U11, U13); write circuit (U13, U15-U18, U21, Q1); erase circuit (U11, U15, U17); burned head || Check J6; clean; troubleshoot (pp. 4-13 to 4-15)
|-
| Error 144 during write or format || Write protect circuit (U11, U13) || Troubleshoot (p. 4-13)
|-
| Writes garbled data || Erase circuit (U11, U15, U17); write circuit; dirty head || Troubleshoot (pp. 4-14 and 4-15); clean
|-
| Always or never write protected || Write protect circuit (U11, U13); photo sensor || Troubleshoot; replace the mechanism
|}

A drive that runs fast can also cause Error 144: it formats disks badly, and the errors appear later.<ref name="page6">Garry Francis, "Adjust It!", ''Page 6'' no. 25 (January/February 1987), p. 54. [https://www.page6.org/archive/issue_25/page_54.htm page6.org]. Magazine article.</ref> When replacing the 2793 controller, no adjustment of the variable capacitor or trimmers is needed.<ref name="fsm" />

== Data interface ==

The SIO lines pass through U1 to the PIA, U7.<ref name="fsm" /> U1 is an LM3086 transistor array at the right rear of the board, and the FAQ records it failing after the SIO cable was plugged or unplugged with the power pack connected.<ref name="faq" />

{| class="wikitable styled-table" style="width:100%;"
|+'''Data interface and drive select problems (FD100330 p. 4-6)'''
! Symptom !! Possible cause !! Corrective action
|-
| No response to commands || I/O cable or connections; command signal not reaching U7; U7 || Replace the cable; trace through U1 (waveforms p. 4-17); replace U7
|-
| No data from drive to computer || Cable; data not reaching the I/O connector; U7 || As above
|-
| No data from computer to drive || Cable; data not reaching U7; U7 || As above
|-
| Does not reboot when the computer is switched off and on || VCC/READY signal not reaching the PIA; U7 || Trace through U1; replace U7
|-
| Drive select test fails || Select switch S2; U7 || Replace S2; replace U7
|}

Sams works through the same chain: the drive select switch S2, the cable, then U7 and U1 by substitution. An "I/O failure" message points it to the RAM (U8) and a "boot error" to the controller (U13).<ref name="samsts" />

The +5 V/READY line on SIO pin 10 tells the drive that the computer is on; the manual limits its use as a supply to 50 µA for Atari peripherals.<ref name="fsm" />

== Processor and memory ==

If the other circuits check out, Atari points at the chips of the processor section for power-up failures, boot errors, stepper and drive motor failures, I/O failures and an unrecognised drive code switch: the PIA (U7), 2793 controller (U13), 6507 (U9), ROM (U10), 6810 RAM (U8), 74LS04 (U16), the 4 MHz crystal (Y1), a 74LS74, the 555 timer (U4) and the 74LS00s (U6, U12). The manual notes that the two lists are not matched line for line. It prints the 74LS74's position as U1, but its own parts list puts the 74LS74s at U14 and U18 and an LM3086 at U1.<ref name="fsm" /> The 6507, 6810, 6532, ROM and controller are socketed on the drives community repairers have worked on, so swapping them one at a time from a good drive isolates a dead one quickly.<ref name="aa282584" />

Early drives with ROM revision E or F at U10 fail to boot some protected software that runs on an 810. Atari's Tech Tip 21 replaced them with ROM FC100541 Rev. 1.<ref name="fsm" />

== Related pages ==

* [[Atari 1050]]
* [[Atari 1050 Maintenance Guide]]
* [[Atari 1050 Capacitor Replacement Guide]]
* [[Atari 1050 Documentation]]
* [[Floppy Drive Repair]]
* [[Capacitor Failure Symptoms]]

== References ==

<references />

{{Navbox-AtariPeripherals|state=collapsed}}

[[Category:Atari Systems]]
[[Category:Troubleshooting Guides]]
[[Category:Atari Troubleshooting Guides]]