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Tandon TM100-2 Write Protect Switch Repair

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Tandon TM100 write protect arm and switch (manual figure 6-23)
Tandon's Figure 6-23: the write protect arm on the guide rail above the write protect switch assembly (left), and the switch, nut plate, washers and adjustment/mounting screws on the side of the chassis (right).

The Tandon TM100-2 senses a diskette's write-protect tab with a mechanical switch screwed to the side of the chassis. A write protect arm on the guide rail above it works the switch, and Tandon's remedy for a broken write protect lever is a new left-hand guide rail.[1][2] When the switch is deactivated it gives a high on J4 pin 5. The logic board buffers and inverts this to disable Write Enable, and gates it with drive select onto the interface, where a write-protected diskette gives a low on J1 pin 28.[3][4]

The single-sided Tandon TM100-1 uses the same switch assembly, part number 171005-001, and Tandon's manual gives one check, adjustment and replacement procedure for both drives.[5][4][1]

Symptoms

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  • The drive will not write. Tandon's troubleshooting table lists, in order: a write-protected diskette, the write enable, select and write data interface lines, alignment, the heads or the write protect switch not connected (connectors P5, P6 and P8), a misadjusted write protect switch, the write logic on the logic board and the heads.[2]
  • The drive fails the write protect switch check below. Tandon gives three causes: a defective switch (replace it), a broken write protect lever (replace the left-hand guide rail) and a switch out of adjustment (adjust it).[2]
  • The drive writes to a write-protected diskette. Check the option jumpers first: with W4 installed the drive responds to the switch, and with W5 installed write protect is inhibited.[6]

For other drive faults see the 5150 troubleshooting guide and Floppy Drive Repair.

Tools and parts

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  • An ohmmeter, or an exerciser or computer that shows the write protect output on J1 pin 28.[4]
  • One diskette with a write-protect tab and one without; Tandon's certified output diskette is the Dysan 104/2D.[7]
  • Phillips screwdrivers; Tandon's equipment list gives sizes 1 and 2.[7]
  • Tie wraps and needle-nose pliers.[8]
  • For a broken lever: a small screwdriver to prise up the rail, a 1/8-inch drill or a soldering iron if a locating tab has broken off in the chassis, two new front panel bushings and a little glue such as Permabond or Superglue.[9][10]
  • Parts: write protect switch assembly 171005-001; left-hand guide rail 172231-001 (aluminium chassis). Tandon lists the front panel with its bushings as 172027-002.[5]

Procedure

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Checking the switch

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Tandon TM100 logic board test points (manual figure 5-1)
Tandon's Figure 5-1: P8, the write protect switch connector, is the left-hand plug in the row along the bottom edge of the logic board; Test Point 9 is just above it.
  1. Connect the drive to an exerciser or computer that shows the write protect output on J1 pin 28. Alternatively, with the power off, unplug Plug 8 and check the switch with an ohmmeter across its two wires.[4]
  2. Insert a diskette without a write-protect tab. There should be continuity between the two wires of Plug 8, or a high (not protected) on J1 pin 28.[4]
  3. Insert a write-protected diskette. There should be no continuity between the two wires of Plug 8, or a low (write protect true) on J1 pin 28.[4]

A faulty logic board can also cause a write protect fault. Test Point 9 should be high with a write-protected diskette and low with an unprotected one; if the switch passes the check and TP9 is wrong, suspect the logic board.[4]

Adjusting the switch

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  1. Loosen the screws that hold the switch to the bracket on the side nearest the front of the drive.[4]
  2. Move the switch up or down as needed, tighten the screws and repeat the check.[4]

If the switch cannot be adjusted to pass, Tandon sends the drive to its troubleshooting table: replace the switch, or the left-hand guide rail if the lever is broken.[4][2]

Replacing the switch assembly

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Tandon TM100 write protect switch mounting screws (manual figure 6-21)
Tandon's Figure 6-21: the switch is held to the side of the chassis by two mounting/adjustment screws with washers, screwing into a nut plate inside.

Removal:[1]

  1. Remove the logic board: unplug all its connectors, noting where each goes, remove the two mounting screws, slide the board about half an inch towards the back of the drive and lift it out.[11]
  2. Remove the mounting/adjustment screws that attach the switch assembly to the side of the chassis.[1]
  3. Cut the tie wrap that holds the cable harness to the bottom of the chassis.[1]
  4. Cut the tie wrap that holds the assembly's cables to the bundle of wires at the back of the drive motor.[1]
  5. Remove the assembly.[1]
Tandon TM100 write protect cable harnessing (manual figure 6-22)
Tandon's Figure 6-22: connectors 8 to 13 at the rear of the drive and the two tie wraps that hold the harness, one at the back of the drive motor.

Replacement:[1]

  1. Fit the new assembly with its mounting/adjustment screws.[1]
  2. Tie wrap the bundle of wires at the back of the drive motor.[1]
  3. Attach the write protect cable harness to the chassis.[1]
  4. Push the write protect arm down and check that the switch clicks.[1]
  5. Refit the logic board: check its option configuration, fit the two screws that hold it to the rails, and plug every connector back into its own place.[11]
  6. Repeat the switch check and adjustment above.[1][12]

Replacing the left-hand guide rail

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A broken write protect lever is cured by fitting a new left-hand guide rail, 172231-001. The front panel has to come off first.[2][9][5]

Tandon TM100 guide rail locating tabs and bushings (manual figure 6-12)
Tandon's Figure 6-12: the locating tabs under each guide rail, and the front panel locating tabs with their two bushings.

Removal:

  1. Remove the logic board as above.[9]
  2. Remove the two bushings on the right- and left-hand sides of the front panel. They are glued on; take care not to break the locating tabs.[10]
  3. Remove the activity LED. Cut the tie wraps that hold the activity LED and write protect switch wiring to the chassis, ease the retaining collar off the LED grommet with needle-nose pliers by pulling it gently away from the front panel, and push the LED out towards the rear of the drive.[13]
  4. Remove the two mounting screws underneath the front of the drive and lift the front panel off the chassis.[10]
  5. Put a screwdriver between the guide rail and the chassis and prise the rail up from both ends.[9]
  6. If a locating tab from the old rail has broken off inside its hole in the chassis, clear the hole completely with a 1/8-inch drill, or melt the plastic out with a soldering iron, and remove the debris. Do not enlarge the hole.[9]

Replacement:

  1. Press the new guide rail evenly into the chassis locating holes.[9]
  2. Place the front latch in its tracks on the front panel and fit the panel on its locating tabs. Refit the two mounting screws underneath.[10]
  3. Refit the activity LED: slip the retaining collar onto the LED, push the LED into the front panel grommet and push the collar over the grommet. Attach the activity LED and write protect switch harness to the chassis using the two holes in the bottom of the chassis, bundle the wires behind the drive motor and tie wrap them.[13]
  4. Fit new bushings on both sides of the front panel over the locating tabs, and put a small drop of glue on top of each.[10]
  5. Check the write protect switch and its adjustment.[9]
  6. Refit the logic board.[9]

Testing

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  • With an unprotected diskette Plug 8 shows continuity and J1 pin 28 is high; with a protected diskette Plug 8 is open and J1 pin 28 is low. TP9 follows: high when protected, low when not.[4]
  • The switch clicks when the write protect arm is pushed down.[1]
  • After a guide rail or the front panel has been replaced, Tandon's Table 6-2 also asks for the cone centring check (section 5.12), and after the front panel the power/drive select check, which confirms that the activity LED lights.[12][14] For the cone centring check, set the oscilloscope to Test Points 1 and 2 (A plus B, B inverted, 100 mV per division, 20 ms per division, ground on TP10, trigger on TP7 positive edge), seek to track 0 and write a 2F pattern on head 0. Measure the signal amplitude with the deepest dip in the signal on the centre vertical graticule. Open the latch, remove and reinsert the diskette and close the latch, repeating until the deepest dip in the waveform appears, then measure the amplitude of the deepest dip. Divided by the first measurement it should be above 85 %, and the signal should not fall below 85 % of its average amplitude anywhere on the track.[15]
  • After the activity LED has been refitted, check with a computer or exerciser that it lights when the drive is selected.[13][14]
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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 Tandon Corporation, OEM Operating and Service Manual, TM100-1, TM100-2 Disk Drives, 48 Tracks Per Inch, 179022-001 Rev. B (1983), section 6.4, "Write Protect Switch Assembly" removal and replacement, p. 6-25, with Figures 6-21 "Write Protect Mounting/Adjustment Screws", 6-22 "Write-Protect Cable Harnessing" (p. 6-26) and 6-23 "Write Protect Arm and Adjustment Screws" (p. 6-27). Hosted as File:Tandon TM100-1 TM100-2 48TPI Operating and Service Manuals 1983.pdf.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Tandon, TM100 manual (1983), Table 6-1 "Troubleshooting Guide", pp. 6-3 and 6-4: "Will not write" and "Fails Write Protect switch check and adjustment".
  3. ↑ Tandon, TM100 manual (1983), section 4.3, "Write Protect Switch", p. 4-3: high on J4 pin 5 when the switch is deactivated, buffered and inverted to disable Write Enable, gated with drive select to the interface.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 Tandon, TM100 manual (1983), section 5.4 "Write Protect Switch Check and Adjustment", p. 5-3, with Figure 5-2 "Write Protect Switch Adjustment", p. 5-4: Plug 8 continuity, J1 pin 28, Test Point 9, and moving the switch on its bracket.
  5. ↑ 5.0 5.1 5.2 Tandon, TM100 manual (1983), Appendix A "Recommended Spares and Major Assemblies", p. A-1: 171005-001 Write Protect Switch Assembly, 172231-001 Left-Hand Guide Rail (Aluminum Chassis), 172027-002 Front Panel with Bushings. One parts list covers both drives.
  6. ↑ Tandon, TM100 manual (1983), section 3, "Write Protect Control (W4, W5)", p. 3-7.
  7. ↑ 7.0 7.1 Tandon, TM100 manual (1983), section 5.2 "Equipment Required", pp. 5-1 and 5-2.
  8. ↑ Tandon, TM100 manual (1983), section 6.4, "Tool Requirements", p. 6-7.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 Tandon, TM100 manual (1983), section 6.4, "Guide Rail" removal and replacement, pp. 6-16 and 6-17, with Figure 6-12 "Guide Rail Locating Tabs and Bushings", p. 6-18.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Tandon, TM100 manual (1983), section 6.4, "Front Panel" removal and replacement, p. 6-16, with Figures 6-10 and 6-11, pp. 6-16 and 6-17.
  11. ↑ 11.0 11.1 Tandon, TM100 manual (1983), section 6.4, "Logic Circuit Board" removal and replacement, pp. 6-7 to 6-9, with Figure 6-2 "Logic Circuit Board Mounting".
  12. ↑ 12.0 12.1 Tandon, TM100 manual (1983), Table 6-2 "Guide to Checks and Adjustments", p. 6-6: write protect switch check after replacing the write protect switch or the logic board; write protect switch and cone centering checks after replacing a guide rail; power/drive select and cone centering checks after replacing the front panel.
  13. ↑ 13.0 13.1 13.2 Tandon, TM100 manual (1983), section 6.4, "Activity L.E.D." removal and replacement, p. 6-13, with Figures 6-8 and 6-9, p. 6-15.
  14. ↑ 14.0 14.1 Tandon, TM100 manual (1983), section 5.3 "Power/Drive Select Check", pp. 5-2 and 5-3: the front panel LED lights when the drive is selected and goes out when it is deselected.
  15. ↑ Tandon, TM100 manual (1983), section 5.12 "Cone Centering Check", p. 5-14.


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