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Tandy 1400 LT Capacitor Replacement Guide

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Tandy 1400 LT power PCB (Service Manual Figure 1-4)
The power source PCB, which carries the largest electrolytics, C500 to C533.

Every capacitor in the Tandy 1400 LT is listed in the parts lists of Tandy's 1988 service manual, board by board. The power source PCB carries the largest electrolytics and is the place to start.[1] See Capacitor Failure Symptoms for the signs of a failing part.

[2]

Power source PCB

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Power PCB electrolytics (parts list p. 6-16)
Ref Capacitance Rating Tandy part no.
C500 220 µF 25 V 20308-327
C504, C505 100 µF 25 V 20308-333
C506 47 µF 25 V 20308-328
C507, C508, C509 22 µF 25 V 20308-329
C510 1 µF 50 V 20308-341
C516 470 µF 25 V 20308-331
C520, C521, C522 330 µF 16 V 20308-345
C524 10 µF 25 V 20308-343
C531 1 µF 50 V 20308-346
C533 2200 µF 25 V 20308-347

All are listed as ±20% electrolytics. The other capacitors on the board are film and ceramic parts.[3] C524 is one of the parts Tandy's backlight chart checks, with the inverter transistor TR512.[4] The sub PCB mounted on the power source board has one electrolytic, C536, 47 µF 25 V (20308-367).[5]

Main PCB

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Main PCB electrolytics (parts list pp. 6-9 to 6-10)
Ref Capacitance Rating Tandy part no.
C1 33 µF 25 V 20308-330
C34, C37, C54 10 µF 16 V 20308-340
C55, C56, C58 10 µF 25 V 20308-343
C60, C66 100 µF 25 V 20308-333
C62 100 µF 16 V 20308-342
C64 1 µF 50 V 20308-332
C68 1 µF 50 V 20308-344

Tandy's charts name several of these: C1 with IC2 in the start-up chart, C60 in the backlight drive circuit, and C68 in the buzzer circuit.[6][7] C32 and C51 on the same board are trimmer capacitors for the LCD controller and clock frequencies, not electrolytics; their adjustment is on General Maintenance.

Memory PCB and LCD unit

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  • Memory PCB: C304 and C305, 22 µF 16 V (20308-348).[8]
  • LCD unit: LU-7-7, 4.7 µF 35 V (L23-0080).[9]

Floppy drives

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Each internal drive has a control PCB and a spindle motor board. Tandy notes that the control board's references carry an F prefix in the list that the board drawing leaves out.[10]

Drive polarised capacitors (FDD parts list)
Board Ref Capacitance Rating Type
Control FC102 22 µF 16 V Aluminium electrolytic
Control FC104 33 µF 16 V Aluminium electrolytic
Control FC111 3.3 µF 50 V Aluminium electrolytic
Control FC19, FC60 1 µF 16 V, ±10% Tantalum
Control FC20 1 µF 16 V, ±20% Tantalum
Control FC106, FC108 3.3 µF 20 V Tantalum
Spindle motor SC1, SC10 100 µF 16 V Aluminium electrolytic
Spindle motor SC7, SC8, SC9 4.7 µF 25 V Aluminium electrolytic
Spindle motor SC4 0.47 µF 16 V Tantalum

Replacement procedure

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  1. Remove the battery pack and open the case as described in General Maintenance. The power source PCB comes out of its box after the base frame and drives are lifted out.[11]
  2. Photograph each board and note the polarity of every electrolytic and tantalum before removing it.
  3. Replace one position at a time, matching capacitance and at least the original voltage rating.
  4. Refit the board, reconnect the battery and adapter, and check the outputs against the limits on General Maintenance.
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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 VI-2, "Parts List", pp. 6-9 to 6-31.
  2. ↑ Tandy 1400LT Service Manual, Part IV-20, "Power Supply", p. 4-55.
  3. ↑ Tandy 1400LT Service Manual, Power PCB Assembly parts list, p. 6-16.
  4. ↑ Tandy 1400LT Service Manual, Part V, "EL Check", p. 5-10.
  5. ↑ Tandy 1400LT Service Manual, SUB PCB Assembly parts list, p. 6-19.
  6. ↑ Tandy 1400LT Service Manual, Main PCB Assembly parts list, pp. 6-9 to 6-10.
  7. ↑ Tandy 1400LT Service Manual, Part V: "Will Not Start", p. 5-3; "EL Check", p. 5-10; "Buzzer Check", p. 5-13.
  8. ↑ Tandy 1400LT Service Manual, MEMORY PCB Assembly parts list, p. 6-20.
  9. ↑ Tandy 1400LT Service Manual, LCD UNIT Assembly parts list, p. 6-22.
  10. ↑ Tandy 1400LT Service Manual, FDD parts list, pp. 6-23 to 6-27.
  11. ↑ Tandy 1400LT Service Manual, Part II, "Power Source PCB, Power Source PCB Cover and Power Source PCB Box", p. 2-10.


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