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IBM PC AT (5170) Diskette Drive Speed Adjustment

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IBM PC AT 360 KB diskette drive speed variable resistor (HMR p. 3-21)
IBM's drawing of the underside of a 360 KB drive in the AT, with the speed-adjusting variable resistor marked.

IBM's service procedure for the IBM PC AT (5170) includes a drive-motor speed adjustment for the optional double-sided (360 KB) diskette drive, FRU 8286131. IBM states that the adjustment applies only to 360 KB drives, not to the AT's standard 1.2 MB high-capacity drive (FRU 8286130).[1][2] The speed is read from the SPEED TEST of IBM's Advanced Diagnostics, which measures the time the diskette takes to make one revolution, and set with a variable resistor on the drive's circuit board while the drive runs in the machine.[1][3]

IBM marked 360 KB drives fitted to an AT with an asterisk on the front bezel.[4][5] Why the AT needs such a drive, and the media rules for 360 KB diskettes written in a 1.2 MB drive, are covered in the maintenance guide.

Symptoms

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  • The Advanced Diagnostics' SPEED TEST shows a speed outside the range printed beneath it.[1]

Other diskette faults, including cabling, pin 34 and media problems, are in the troubleshooting guide.

Safety

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  • The adjustment is made with the system unit cover off and the machine running. IBM says to use an insulated screwdriver on the variable resistor.[1]
  • Unplug the power cord whenever the cover is taken off or put back.[6][1]

Tools and parts

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  • IBM's Advanced Diagnostics diskette for the AT.[1]
  • A diskette for the drive under test: the SPEED TEST times one revolution of the diskette.[3]
  • A small insulated screwdriver.[1]
  • A screwdriver for the cover screws and the drive's mounting screws.[6][7]

Procedure

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Checking the speed

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IBM's procedure:[1]

  1. Insert the Advanced Diagnostics diskette in drive A and switch on.
  2. Go to the Diskette Drive Diagnostic menu. It is reached through SYSTEM CHECKOUT, confirming the list of installed devices, RUN TESTS ONE TIME and the diskette drives entry.[3][8]
  3. Select option 4, SPEED TEST, typing the option number and the drive ID as the menu asks.[3]
  4. Read the speed and the range shown under it. IBM's example screen reads "SPEED IS 1660" and "SHOULD BE 1640 TO 1695". If the reading is inside the range on your screen, no adjustment is needed.

Gaining access to the variable resistor

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IBM PC AT diskette drive mounting screws and clips (HMR p. 3-31)
The diskette drive bay from the front, with the mounting screws and clips in the insets.
  1. Switch off and remove the system unit cover.[1][6]
  2. Reconnect the cables that were removed to take the cover off.[1]
  3. Remove the drive's mounting screws and clips.[1][7]
  4. Slide the drive about 76 mm (3 inches) towards the front, leaving its cables connected.[1]

Adjusting the speed

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  1. Switch on, go to the Diskette Drive Diagnostic menu and select SPEED TEST for the drive.[1]
  2. Find the variable resistor on the drive's circuit board, on the underside of the drive.[1]
  3. Carefully insert an insulated screwdriver into the resistor's slot and turn it until the speed on the screen is within the range shown.[1]

Finishing

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  1. Switch off, unplug the power cord and disconnect all cables from the system unit.[1]
  2. Slide the drive back and refit its mounting screws and clips. IBM's adjustment steps go straight to refitting the cover; its general rule is that a part goes back by reversing the removal procedure.[9][7]
  3. Refit the cover.[1]

Testing

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  • Run SPEED TEST again with the drive back in place; the speed should be within the range on the screen.[1]
  • The same diagnostic menu also offers sequential access, random seek and verify tests for the drive.[3]
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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 1.14 1.15 1.16 1.17 IBM, Hardware Maintenance Reference, 6280088 (1986), Repair Information (AT), "Adjustments: Diskette Drive-Motor Speed", pp. 3-19 to 3-21: applies only to 360 KB drives, the SPEED TEST screen, sliding the drive 76 mm forward, the variable resistor on the underside of the drive and the insulated screwdriver. Hosted as File:IBM Hardware Maintenance Reference 6280088 Repair Information (AT).pdf.
  2. ↑ IBM, Hardware Maintenance Service, 6280087, Revised Edition (March 1986), "Parts (AT)", Assembly 3 "Diskette Drives (5-1/4 Inch)", p. 10: 8286130 high capacity, 8286131 double sided. Hosted as File:IBM Hardware Maintenance Service 6280087 Parts Catalog (AT).pdf.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 IBM, Hardware Maintenance Reference, 6280088 (1986), Introduction to Diagnostics (AT), "Diskette Drive and Adapter", pp. 2-17 and 2-18: the diskette diagnostic menu and its options, including SPEED TEST, which measures the time for one revolution of the diskette. Hosted as File:IBM Hardware Maintenance Reference 6280088 Introduction to Diagnostics (AT).pdf.
  4. ↑ Minuszerodegrees.net, "IBM 5170 – Basics, for 5170 beginners".
  5. ↑ IBM, Hardware Maintenance and Service, IBM Personal Computer AT, 1502242, Volume 2, Section 6, "System Setup", p. 6-3: the bezel of a double-sided drive has an asterisk; that of a high-capacity drive is not marked. Scan via minuszerodegrees.net.
  6. ↑ 6.0 6.1 6.2 IBM, Hardware Maintenance Reference, 6280088 (1986), Repair Information (AT), "Cover 1005", pp. 3-27 and 3-28.
  7. ↑ 7.0 7.1 7.2 IBM, Hardware Maintenance Reference, 6280088 (1986), Repair Information (AT), "Half High Diskette Drive 1035", pp. 3-31 and 3-32.
  8. ↑ IBM, Hardware Maintenance Reference, 6280088 (1986), Introduction to Diagnostics (AT), "Select an Option Menu", "Installed Devices Menu", "System Checkout Menu" and "Installed Devices Test", pp. 2-9 to 2-13.
  9. ↑ IBM, Hardware Maintenance Reference, 6280088 (1986), Repair Information (AT), "How to Use the Repair Information", p. 3-3.


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