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Tandy 1000 SX Capacitor Replacement Guide

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Tandy 1000 SX 67 W power supply 8790085 silkscreen (Technical Reference Manual)
Silkscreen of the 67 W supply board 8790085, from Tandy's SX technical reference manual.

Tandy published every electrolytic capacitor in the Tandy 1000 SX: the Tandy 1000 SX Technical Reference Manual (catalogue no. 25-1511) has parts lists for the main logic board and for both versions of the 67 W power supply.[1] The supply holds the large electrolytics and is where failures have been reported, so start there.[2] See Capacitor Failure Symptoms for the signs of a failing part.

[3]

Which supply is fitted

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The SX was built with one of two supplies made by Tamura Seisakusho. Both have the same outputs, but the capacitor numbering differs, so read the part number on the supply before ordering.[3]

  • 8790085: single input, 90-135 V AC.
  • 8790084: dual input, 90-135 V or 198-264 V AC, set by a voltage selector jumper.

Single-input supply 8790085

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Values are from Tandy's parts list and positions from the schematic of this supply.[4][5]

8790085 electrolytics
Ref Capacitance Rating Position
C6 680 µF 200 V Primary reservoir
C15 1 µF 50 V +5 V sensing circuit at IC1 and VR1
C16 2200 µF 25 V +12 V, after rectifier D8
C17 470 µF 25 V +12 V, with C16 ahead of series transistor Q4
C18 1000 µF 16 V +12 V output
C19, C20, C21 4700 µF 10 V +5 V, after rectifier D9 and ahead of choke L2
C22 2200 µF 10 V +5 V output, after L2
C23 470 µF 25 V -12 V, after rectifier D10 and ahead of regulator IC2
C24 470 µF 25 V -12 V output, after IC2
C25 1 µF 50 V +12 V control circuit at IC3 and VR2

The other capacitors are film and ceramic: C1 (0.1 µF 250 V AC film) and C2 (0.22 µF 250 V AC film) across the mains, C3A and C11 (2200 pF 400 V AC ceramic), C5 (10000 pF 400 V AC ceramic), C7 to C10 and C12 (film), and C13 (1500 pF 2 kV ceramic).[4] If C1 or C2 is replaced, use an X-class safety capacitor rated for the mains.

Dual-input supply 8790084

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Values are from Tandy's parts list and positions from Tamura's schematic 3P-M1-0165 for this supply.[6]

8790084 electrolytics
Ref Capacitance Rating Position
C6, C7 330 µF 200 V Primary reservoir, two in series with balancing resistors R2 and R3
C16 1 µF 50 V +5 V sensing circuit at IC1 and VR1
C17 2200 µF 25 V +12 V, after rectifier D8
C18 470 µF 25 V +12 V, with C17 ahead of series transistor Q4
C19 1000 µF 16 V +12 V output
C20, C21, C22 4700 µF 10 V +5 V, after rectifier D9 and ahead of choke L4
C23 2200 µF 10 V +5 V output, after L4
C24 470 µF 25 V -12 V, after D10 and ahead of regulator IC2
C25 470 µF 25 V -12 V output, after IC2
C26 1 µF 50 V +12 V control circuit at IC3 and VR2

Apart from the reservoir, each secondary capacitor has the same value and rating as on the single-input supply, with its reference raised by one.[6][4]

Main logic board

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The main logic board (subassembly 8859003A) has fifteen electrolytics, all axial and all rated 16 V. There are no tantalum capacitors in its parts list. The rest are ceramic decoupling and timing parts, monolithic 0.47 µF capacitors and one 0.022 µF polyester capacitor (C118).[7]

Main logic board electrolytics
Ref Capacitance Rating Type Tandy part no.
C100, C119, C120, C145, C148, C151, C157, C181 22 µF 16 V Axial 8316221
C121, C129, C135, C175, C179 10 µF 16 V Axial 8316101
C122, C130 100 µF 16 V Axial 8317101

Replacement procedure

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  1. Unplug the machine, remove the supply, and discharge and measure the reservoir capacitor before touching the board.
  2. Photograph the board and note the polarity of every electrolytic.
  3. Replace the capacitors one position at a time, matching capacitance and at least the original voltage rating. Use 105 °C low-ESR parts on the secondary side (C15 to C25 on 8790085, C16 to C26 on 8790084).
  4. Check the solder joints on the output connectors while the board is out.
  5. Test the supply on a load before refitting it. Tandy's minimum test load is 1.25 A on +5 V (4 ohms, 20 W) and 0.15 A on +12 V (80 ohms, 5 W), and the manual warns that the supply must never run without a load.[3]
  6. Check all three outputs against the limits in General Maintenance, with the machine running.

The main board capacitors are axial. If radial parts are used instead, form the leads to fit the axial holes and keep the polarity the same.

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References

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  1. ↑ Tandy Corporation, Tandy 1000 SX Technical Reference Manual, catalogue no. 25-1511, 1986, hosted as File:Tandy 1000 SX Technical Reference Manual (25-1511, 1986).pdf.
  2. ↑ "Wanted: Someone to repair Tandy 1000SX power supply", Vintage Computer Federation forum, 2018-2019, thread. Post by seat safety switch, 1 June 2019 (vented -12 V capacitor C23).
  3. ↑ 3.0 3.1 3.2 Tandy 1000 SX Technical Reference Manual (25-1511), "1000 SX 67 Watt Single Input Power Supply" and "67 Watt Dual Input Power Supply": troubleshooting caution and parts lists.
  4. ↑ 4.0 4.1 4.2 Tandy 1000 SX Technical Reference Manual (25-1511), parts list for switching power supply unit 8790085.
  5. ↑ Tandy 1000 SX Technical Reference Manual (25-1511), schematic of the single-input supply, hosted as File:Tandy 1000 SX 67 W power supply 8790085 schematic (Technical Reference Manual).png.
  6. ↑ 6.0 6.1 Tandy 1000 SX Technical Reference Manual (25-1511), parts list for switching power supply unit 8790084, and schematic 3P-M1-0165 (PDF p. 114).
  7. ↑ Tandy 1000 SX Technical Reference Manual (25-1511), Main Logic T1000 SX Subassembly 8859003A parts list, pp. 49-53.


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