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Tandy 1400 LT Troubleshooting Guide

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Tandy 1400 LT power PCB (Service Manual Figure 1-4)
The power source PCB: the 5 V switching transistor TR511 and its uPC494 controller, the converter transistor TR508 and transformer T500 for +12 V, -12 V and -15 V, the crowbar thyristor SCR500, and the backlight inverter TR512 and T501.

Fault-finding on the Tandy 1400 LT follows the flowcharts in Part V of Tandy's 1988 service manual, which start from a dead machine and work through the power supply, memory, display, keyboard, clock, ports and drives.[1]

[2]

Quick reference

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Tandy's troubleshooting charts (Part V)
Symptom Chart Page
Will not start Will Not Start 5-3
No output, or wrong voltages DC/DC Converter Check 5-5
Memory errors Memory Check 5-6
Blank or faulty LCD LCD Check 5-8
Backlight dark EL Check 5-10
Internal keyboard dead Internal Keyboard Check 5-11
No beep or distorted beep Buzzer Check 5-13
Clock loses time or settings RTC (Real-Time Clock) Check 5-14
Printer fails Printer Interface Check 5-15
Internal drive fails Internal FDD Troubleshooting 5-16
Serial port fails RS-232C Check 5-21
No colour monitor picture IRGB Check 5-24
No composite picture Composite CRT Check 5-25
Indicator LED faults, low battery warning LED Check 5-26
External drive fails External FDD Troubleshooting 5-28
External keyboard dead External Keyboard Check 5-33

Will not start

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Tandy's first chart:[3]

  1. Check that the relay trips when the power switch is pressed. If not, check the power switch SW500 on the right side of the unit.
  2. Watch the red Low Battery LED as the switch is pressed. If it blinks, charge the battery.
  3. Check for a low pulse at pin 11 of IC2 when the switch is pressed. If it is missing, check IC2, D1, R1 and C1.
  4. Check the 4.77 MHz clock (7.16 MHz in high-speed mode) at pin 8 of IC2. If it is missing, check IC2, crystal CTL1, R78, R80, C2 and C3.
  5. Check the signals from the V20 (IC1) and the ROM select signal at pin 34 of GATE2, then IC2, IC3, the gate array and the ROM (IC15). Tandy asks that the copyright seal on the ROM be moved to any replacement.

An owner reports that his 1400 would not start from the adapter until the battery had taken some charge.[4]

Power supply

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Tandy's DC/DC converter chart:[5]

  1. Check the input: 12-15 V from the battery or 15-20 V from the adapter. If it is wrong, check the battery or adapter.
  2. Check fuse F500 (2 A) and replace it if it has opened.
  3. Check the +5 V output. If it is missing, check the switching transistor TR511.
  4. Check the +12 V, -12 V and -15 V outputs. If they are missing, check TR508.
  5. Otherwise check the rest of the DC/DC converter.

The output limits are on General Maintenance. If TR511 is locked on, the thyristor SCR500 turns on and keeps the input voltage off the +5 V output.[2]

Battery runs down on the adapter

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Owners who fitted a hard disk in the drive B: position had to short resistor R554 on the power supply board, a 27-ohm 2 W part, for the disk controller to work. That removed the protection R554 gave the battery, and owners found the battery ran down while the machine ran from the adapter. Their fix is to refit a 27-ohm resistor of about 2 W in the red battery lead from the disk controller, with a 1N5817-type Schottky diode across it, anode towards the battery; one owner warns that a 1N4001 is not a substitute.[6][7][8] The same owner found R554 at the lower right of the supply board on the 25-3500 and at the upper left on the 25-3500A, viewed from the power switch side.[6] Another installer wrote that with a hard disk spinning the machine draws about 1 A, so Tandy's 750 mA adapter cannot run it and charge the battery at the same time.[7]

Display

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LCD

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Check that the monitor switch is off, then the signal at pin 25 of IC30 (a low pulse from pin 35 of GATE1), then the LCD reference voltages at connector CN2: +5 V at pin 15 and the negative bias at pin 17, given as "Min. -15V". If the voltages and drive waveforms are right and the picture is still wrong, replace the LCD unit.[9]

Backlight

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Check TR15, D10 and pin 40 of GATE3, then TR14, L2, C60, R52 and C61 on the main PCB, and TR512 and C524 on the power PCB. If those are good, replace the EL backlight; the procedure is in General Maintenance.[10] C60 and C524 are electrolytics; see Tandy 1400 LT Capacitor Replacement Guide.

Colour monitor

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The IRGB chart checks the monitor switch SW2 and the 21.4772 MHz crystal CTL2 before the video ICs.[11]

Clock and low battery warnings

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Clock

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Check the +5 V supply, then the nickel-cadmium clock battery BATT1, then the 1 Hz output at pin 3 of IC36 (the frequency the BIOS sets at start-up). If it is wrong, check crystal CTL4 and C52 and adjust trimmer C51.[12] The adjustment is on General Maintenance.

Low battery warning

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The second warning comes at 10.8-11.4 V. Tandy's chart checks the battery voltage at D4 and D6, the voltages at pins 2 and 6 (+5 V) and pin 8 (+12 V) of IC38, and the LOW BATT2 signal at pin 7 of IC38.[13]

Buzzer

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Check TR13 and IC39, then R55-R57, C67 and C68.[14]

Floppy drives

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Before suspecting a drive, check the position of the boot select switch SW3 and that the disk has been formatted, then the drive connectors and cables. Tandy's chart then checks the 16 MHz clock (CTL6, C91, C92) and the drive select and motor enable signals at GATE5, and works through the uPD72065 controller (IC54) and the DMA controller (IC18).[15] Drive-level fault diagnosis, parts replacement and head alignment are in Appendix D.[16]

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References

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  1. ↑ Tandy Corporation, Tandy 1400LT Laptop Computer Service Manual, catalogue no. 25-3500, 1988, hosted as File:Tandy 1400 LT Service Manual (25-3500, 1988).pdf. Part V, "Troubleshooting", pp. 5-1 to 5-33.
  2. ↑ 2.0 2.1 Tandy 1400LT Service Manual, Part IV-20, "Power Supply", pp. 4-55 to 4-58.
  3. ↑ Tandy 1400LT Service Manual, Part V, "Will Not Start", pp. 5-3 to 5-4.
  4. ↑ "Can someone tell me why my TANDY 1110HD computer will not power on?", Vintage Computer Federation forum, thread. Post by mojorific, 16 May 2014.
  5. ↑ Tandy 1400LT Service Manual, Part V, "DC/DC Converter Check", p. 5-5; Power PCB parts list (F500, TR508, TR511), pp. 6-16 to 6-18.
  6. ↑ 6.0 6.1 Stephen Lankton, CompuServe messages "1400LT Hard Disk", 5 October and 8 November 1988, cmshd.txt, TVDog's Tandy 1000 archive (retrieved 2026-10-22).
  7. ↑ 7.0 7.1 Rusty Hill and others, CompuServe messages on the 1400LT, 1400ltdk.txt, TVDog's Tandy 1000 archive (retrieved 2026-10-22).
  8. ↑ Tandy 1400LT Service Manual, Power PCB parts list (R554), p. 6-18.
  9. ↑ Tandy 1400LT Service Manual, Part V, "LCD Check", pp. 5-8 to 5-9.
  10. ↑ Tandy 1400LT Service Manual, Part V, "EL Check", p. 5-10.
  11. ↑ Tandy 1400LT Service Manual, Part V, "IRGB Check", p. 5-24.
  12. ↑ Tandy 1400LT Service Manual, Part V, "RTC (Real-Time Clock) Check", p. 5-14.
  13. ↑ Tandy 1400LT Service Manual, Part V, "LED Check", pp. 5-26 to 5-27; Part IV-20, "Battery Voltage Warning", p. 4-56.
  14. ↑ Tandy 1400LT Service Manual, Part V, "Buzzer Check", p. 5-13.
  15. ↑ Tandy 1400LT Service Manual, Part V, "Internal FDD Troubleshooting", pp. 5-16 to 5-20.
  16. ↑ Tandy 1400LT Service Manual, Appendix D, "3.5 inch FDD Maintenance".


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