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Tandon TM100-2 Track 0 Switch Repair

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Tandon TM100 Track 0 switch and adjustment screw (manual figure 6-19)
Tandon's Figure 6-19: the Track 0 switch on its bracket, and the adjustment screw that passes through the chassis wall into the bracket. The inset shows the same parts in perspective.

The Tandon TM100-2 detects track 0 with a mechanical switch inside the drive. As the head carriage moves back towards track 0 it operates the switch, which is activated between tracks 0 and 3. The logic board combines the switch signal with phase 0 of the stepper motor to make the Track 0 signal. That signal stops the drive stepping out past track 0, and goes to the controller on pin 26 of the interface.[1] The switch sits on a bracket at the rear of the chassis, beside the drive motor, and an adjustment screw through the chassis wall sets its position. A separate Track 0 stop screw limits how far the carriage can travel outwards.[2][3][4]

Tandon's manual covers the single-sided Tandon TM100-1 with the same procedures and the same Track 0 parts.[1][2]

Symptoms

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  • The drive gives no Track 0 indication. Tandon's troubleshooting table lists, in order: a seek fault (see "will not seek or restore"), the Track 0 switch not connected (check connector P11), the switch not adjusted, a defective logic board, and a defective Track 0 switch, cured by replacing the switch assembly.[5]
  • The drive seems to be out of alignment. Chuck(G) on the VCFed forum has found that many "alignment" problems on Tandon 100 drives are a dirty track 0 sensor, and the minuszerodegrees.net TM100-2 page quotes him.[6][7] On the cctalk mailing list Chuck Guzis advised cleaning the sensor before any re-alignment, because everything is keyed off the track 0 reference.[8] Tandon's own table sends a drive that will not read to the Track 0 switch adjustment as well as to the cat's eye and index checks, and lists a misadjusted Track 0 sensor as a cause of a failed cat's eye check.[5]
  • The heads run to the far end of their travel. keenmaster486 on the VOGONS forum had a TM100-2A from an IBM 5150 that moved its heads to track 39, or what looked like the end of their travel, during the self test or when reading a diskette, and left them there. Formatting made it step from 37 to 38 to 39 over and over, and the format ended with "Unable to write BOOT". The switch on that drive made contact when pressed but only intermittently when released.[9]
  • The carriage seeks to a track below track 0. Tandon adjusts the Track 0 stop for this.[4]

Thomas Brase of retrocmp.de has found that sometimes only the track 0 setting is wrong on a TM100-2A, and one screw puts it right; he got two drives working again within 30 minutes that way.[10] If the switch tests and adjusts correctly and the drive still will not exchange diskettes with other drives, see Tandon TM100-2 Head Alignment.

A drive that steps by itself can also be picking up noise on an unterminated step line, as mkarcher suggested on the same VOGONS thread; keenmaster486's drive had its terminator fitted and was the only drive on the cable.[9] Termination is covered on the Tandon TM100-2 page.

Tools and parts

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  • A dual-channel oscilloscope, a cat's eye alignment diskette such as the Dysan 224/2A, and an exerciser or software that can step the drive to any track.[11][12]
  • A 0.050-inch Allen wrench or a number 1 Phillips screwdriver for the Track 0 stop screw.[4]
  • Tie wraps and needle-nose pliers for replacing the switch assembly.[13]
  • An ohmmeter and a contact cleaner such as DeoxIT, which keenmaster486 used.[9]
  • Parts: Track 0 Switch Assembly 171006-001 and Track 0 Stop 172266-001.[2]

Procedure

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Cleaning and testing the switch

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Tandon TM100 Track 0 switch mounting screw (manual figure 6-20)
Tandon's Figure 6-20: the Track 0 switch assembly lifted off the chassis, with its single mounting screw and its cable.

The switch cable plugs into connector P11, marked "TRACK 0" on Tandon's drawing of the logic board.[14] keenmaster486 cleaned his switch this way:[9]

  1. Unplug the Track 0 switch connector, P11, from the logic board. With the switch unplugged, keenmaster486's drive sought to track 0 at boot and ran up against the stop.[9]
  2. Check the switch with an ohmmeter. His switch made contact on one pair of its pins when pressed and on another pair when released. The released side was intermittent.[9]
  3. Work contact cleaner into the switch body. He soaked the switch in DeoxIT so that it would seep in through the gaps, and several hours later both sides read under 1 ohm.[9]
  4. Check that the switch has not drooped on its bracket. His had slowly drooped from its original position over the years, and he rotated it back up so that the head carriage would operate it again.[9]

After cleaning or moving the switch, refit the connector and check the switch setting with Tandon's procedure below.[12]

See Switch and Potentiometer Repair for the general method of cleaning switch contacts. A switch that cannot be cleaned is replaced as an assembly.[5]

Checking the Track 0 switch

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Tandon TM100 Track 0 timing (manual figure 4-6)
Tandon's Figure 4-6. The lower trace is the Track 0 adjustment monitor point, IC 4B pin 9: +5 V at tracks 0 and 1, 0 V from track 2 inwards. The upper trace is Test Point 8. The bottom row shows the stepper motor phases.
Tandon TM100 logic board test points (manual figure 5-1)
Tandon's Figure 5-1: the logic board test points, with connector P11 (Track 0) along the bottom edge.
  1. Set up the oscilloscope: channel A on Test Point 1, channel B on Test Point 2, ground on Test Point 10. Read differentially, A plus B with B inverted. Set 20 milliseconds per division, external trigger from Test Point 7 on the positive edge, and 0.1 volt per division.[12]
  2. Apply power to the drive.[12]
  3. Insert a cat's eye alignment diskette and close the front latch.[12]
  4. Step the carriage to the radial alignment track, track 16, and confirm the position by the cat's eye pattern.[12][15]
  5. Set the oscilloscope to monitor the TTL signal at IC 4B pin 9, or at P11 pin 2.[12]
  6. Step the carriage back to track 1. The signal must be at least 2.5 V.[12]
  7. Step the carriage to the radial alignment track again, then back to track 2. The level at IC 4B pin 9 must be below 0.8 V.[12]
  8. If both readings are right, no adjustment is needed. If either is wrong, adjust the switch.[12]

Adjusting the Track 0 switch

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  1. Loosen the retaining screw on the base of the Track 0 bracket.[12]
  2. Retighten it slightly, so that it leaves some friction on the bracket.[12]
  3. Turn the Track 0 adjustment screw, at the rear of the chassis, counterclockwise as far as it will go without forcing it.[12]
  4. Step the carriage back from the radial alignment track to track 1. Do not use a restore to track 0.[12]
  5. Turn the adjustment screw clockwise, very slowly, until the signal at IC 4B pin 9 is at least 2.5 V at track 1 and below 0.8 V at track 2.[12]
  6. Retighten the retaining screw.[12]
  7. Repeat the Track 0 switch check.[12]

If the switch cannot be adjusted, Tandon's table sends a failed Track 0 check back to the "No Track 0 indication" causes listed under Symptoms.[5]

In Tandon's check the alignment diskette confirms that the carriage is at the radial alignment track before the readings at tracks 1 and 2 are taken.[12] Asked how to adjust the Track 0 switch and stop on a TM100-2 without an alignment diskette, Chuck Guzis answered that a known-good diskette, adjusting for the best result, is about all that can be done.[16] Avery's track count check in Tandon TM100-2 Head Alignment uses a bulk-erased diskette formatted on a known-good drive: there should be a signal from track 39 down to track 0, the Track 0 sensor should operate at track 0, and the drive should not step below it.[17]

Adjusting the Track 0 stop

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Tandon TM100 Track 0 stop screw (manual figure 5-13)
Tandon's Figure 5-13: the Track 0 stop screw.

Tandon adjusts the stop screw after the cat's eye pattern has been adjusted, or when the carriage seeks to a track lower than track 0.[4]

  1. Set up the oscilloscope: channel A on Test Point 1, channel B on Test Point 2, ground on Test Point 10, A plus B with B inverted, 20 milliseconds per division, external trigger from Test Point 7.[4]
  2. Apply power to the drive and select it.[4]
  3. Insert an alignment diskette.[4]
  4. Turn the Track 0 stop screw two turns counterclockwise, with a 0.050-inch Allen wrench or a number 1 Phillips screwdriver.[4]
  5. Step the carriage to track 0 and watch the waveform.[4]
  6. Turn the screw slowly clockwise until the amplitude on the oscilloscope begins to fall.[4]
  7. Turn it counterclockwise until the amplitude stops rising.[4]
  8. Turn it a further 1/8 turn counterclockwise.[4]

Replacing the Track 0 switch assembly

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Tandon TM100 Track 0 cable harnessing (manual figure 6-18)
Tandon's Figure 6-18: the cable bundle from connectors 8 to 13 running down past the drive motor, with its two tie wraps.

Removal:[3]

  1. Remove the logic board. Unplug all its connectors, noting where each one goes, remove the two mounting screws, slide the board about half an inch towards the back of the drive and lift it off.[18]
  2. Cut the tie wraps that hold the bundle of cables at the back of the drive motor and attach the cables to the bottom of the chassis (Figure 6-18).[3]
  3. Move the head carriage towards the front of the drive.[3]
  4. Remove the mounting screw that holds the assembly to the chassis (Figure 6-20).[3]
  5. Pull the assembly towards the front of the drive, slipping it off the adjustment screw (Figure 6-19).[3]

Replacement:[3]

  1. Put the new assembly into the drive.[3]
  2. Push it towards the rear of the drive and slip the bracket onto the adjustment screw.[3]
  3. Check that there are no cables underneath the assembly.[3]
  4. Fit the mounting screw that holds the assembly to the bottom of the chassis.[3]
  5. Tie wrap the cable harness to the bottom of the chassis.[3]
  6. Tie wrap the bundle of cables at the back of the drive motor.[3]
  7. Refit the logic board: place it on the drive, fit the two mounting screws into the rails and plug every connector into its proper place.[18]
  8. Adjust the Track 0 switch as above.[3][12]

After replacing the Track 0 switch assembly, Tandon's table of checks calls for the radial track alignment check as well as the Track 0 switch check.[19] The alignment check is in Tandon TM100-2 Head Alignment.

Testing

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  • At IC 4B pin 9 the signal is at least 2.5 V at track 1 and below 0.8 V at track 2.[12]
  • The Track 0 sensor operates at track 0 and the drive will not step below it.[17]
  • Run the drive in the host computer. keenmaster486's drive worked properly once its switch had been cleaned and turned back into position.[9]
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References

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  1. ↑ 1.0 1.1 Tandon Corporation, OEM Operating and Service Manual, TM100-1, TM100-2 Disk Drives, 48 Tracks Per Inch, 179022-001 Rev. B (1983), section 4.3, "Track 0 Switch", p. 4-3: a mechanical switch internal to the drive, activated between tracks 0 and 3; the low-active signal enters on connector P11 pin 1, is debounced and combined with stepper motor phase 0 at IC 4B (Test Point 8), inhibits stepping to track 0 minus 1, and is gated with drive select to give Track 0 on interface pin 26. Hosted as File:Tandon TM100-1 TM100-2 48TPI Operating and Service Manuals 1983.pdf.
  2. ↑ 2.0 2.1 2.2 Tandon, TM100 manual (1983), Appendix A "Recommended Spares and Major Assemblies", pp. A-1 and A-2: 171006-001 Track 0 Switch Assembly and 172266-001 Track 0 Stop, shown at the rear of the drive beside the drive motor.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 Tandon, TM100 manual (1983), section 6.4, "Track 0 Switch Assembly" removal and replacement, pp. 6-22 and 6-23, with Figure 6-18 "Track 0 Cable Harnessing", Figure 6-19 "Track 0 Adjustment Screw" (p. 6-24) and Figure 6-20 "Track 0 Mounting Screw" (p. 6-25).
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 Tandon, TM100 manual (1983), section 5.10 "Track 0 Stop Adjustment", p. 5-13, and Figure 5-13 "Track 0 Stop".
  5. ↑ 5.0 5.1 5.2 5.3 Tandon, TM100 manual (1983), Table 6-1 "Troubleshooting Guide", pp. 6-3 to 6-5: "Will not read", "No Track 0 indication", "Fails Cats Eye alignment check and adjustment" and "Fails Track 0 switch check and adjustment".
  6. ↑ Chuck(G), post of 11 October 2023 in the thread "Floppy drive alignment disks", Vintage Computer Federation Forums: "many of the 'alignment' issues on Tandon 100s" are a dirty track 0 sensor.
  7. ↑ Minuszerodegrees.net, "Tandon TM100-2 in IBM 5150/5155/5160 – Known Problems/Issues", section "'Dirty' track 0 switch".
  8. ↑ Chuck Guzis, "Tandon Track 0 adjustment", cctalk mailing list, 28 August 2021: clean the track 0 sensor before re-aligning, since everything is keyed off the track 0 reference.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 keenmaster486 and others, "Tandon TM-100-2A floppy drive: head moves to track 39 (not 0!) and stays there", VOGONS forum, 25 to 27 September 2023: the symptoms, the unplugged Track 0 connector, the intermittent switch cleaned with DeoxIT to under 1 ohm, the drooped switch turned back into position, and mkarcher's note on termination.
  10. ↑ Thomas Brase, "TM100-2A", retrocmp.de, section "Alignment".
  11. ↑ Tandon, TM100 manual (1983), section 5.2 "Equipment Required", pp. 5-1 and 5-2.
  12. ↑ 12.00 12.01 12.02 12.03 12.04 12.05 12.06 12.07 12.08 12.09 12.10 12.11 12.12 12.13 12.14 12.15 12.16 12.17 12.18 12.19 Tandon, TM100 manual (1983), section 5.9 "Track 0 Switch Check and Adjustment", p. 5-12: oscilloscope settings, the readings at IC 4B pin 9 or P11 pin 2 at tracks 1 and 2, and the adjustment steps; with Figure 4-6 "Track 0 Timing", p. 4-6.
  13. ↑ Tandon, TM100 manual (1983), section 6.4, "Tool Requirements", p. 6-7: feeler gauge, tie wraps and needle-nose pliers in addition to the section 5 tools.
  14. ↑ Tandon, TM100 manual (1983), Figure 5-1 "Logic Circuit Board with Test Points", p. 5-2: P11 marked "TRACK 0".
  15. ↑ Tandon, TM100 manual (1983), section 5.6, "Radial Track Alignment Check", pp. 5-6 to 5-8: track 16 is the radial alignment track.
  16. ↑ Chuck Guzis, "Tandon Track 0 adjustment", cctalk mailing list, 28 August 2021, replying to Mike Loewen's question about the TM100-2 Track 0 switch and stop: use a known-good disk and adjust for best results.
  17. ↑ 17.0 17.1 Philip Avery, "Adjusting the radial alignment a Tandon TM100-2A drive without an alignment disk", published on Terry Stewart's blog, 28 June 2010, step 7: the track count check.
  18. ↑ 18.0 18.1 Tandon, TM100 manual (1983), section 6.4, "Logic Circuit Board" removal and replacement, pp. 6-7 and 6-9, with Figure 6-2.
  19. ↑ Tandon, TM100 manual (1983), Table 6-2 "Guide to Checks and Adjustments", p. 6-6: after replacing the Track 0 switch assembly, the radial track alignment and Track 0 switch checks.


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