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TRS-80 Model 100 Capacitor Replacement Guide

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TRS-80 Model 100 main board component side (Service Manual Fig. 7-3)
Main board component side from the service manual (Fig. 7-3), with every capacitor's reference number. The rear panel is along the top edge

All of the TRS-80 Model 100's electrolytic capacitors are on the main board: 17 of them, listed with their values in Radio Shack's service manual. The LCD board uses only ceramic chip capacitors.[1] Owners on the Model 100 mailing list report that these capacitors leak and damage the board around them; Jeffrey Birt, who repairs Model 100s, replaces all of them on every machine he works on.[2]

Which capacitors fail

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  • Birt found C82, C85, C49, C50, C54 and C55 all starting to leak on his first Model 100, together with a bad Ni-Cad memory battery.[3]
  • Josh Malone recommends replacing C82 to C86 on every Model 100, having found one or more of them failed on close to 75 per cent of the machines he had seen, and replacing the rest where they show leakage or the computer has spent its life somewhere hot and humid.[4]
  • Leakage from C82 and C85 has eaten through the trace to the contrast control and the trace between C85 and D15, leaving a blank screen; see the Troubleshooting Guide.[5][6]

For what failed capacitors look like, see Capacitor Failure Symptoms.

Capacitor list

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TRS-80 Model 100 main board electrolytic capacitors
Ref Value (service manual) Notes Position on the board (Fig. 7-3) Size H x D x pitch, mm (Birt)
C49 10 µF 16 V Left of centre, among the modem op-amps 7 x 4 x 1.5
C50 10 µF 16 V Left of centre, among the modem op-amps 7 x 4 x 1.5
C52 1 µF 50 V Non-polarised (Birt) Left of centre, among the modem op-amps 11 x 5 x 2
C54 10 µF 16 V Left of centre, among the modem op-amps 7 x 4 x 1.5
C55 10 µF 16 V Left of centre, among the modem op-amps 7 x 4 x 1.5
C75 47 µF 16 V Non-polarised; RS-232C circuit Rear edge, centre 11 x 5 x 2
C76 47 µF 16 V Non-polarised; RS-232C circuit Rear edge, centre 11 x 5 x 2
C77 47 µF 16 V Non-polarised; RS-232C circuit Rear edge, centre 11 x 5 x 2
C78 3.3 µF 50 V Reset delay Rear right corner, by the RESET switch 11 x 5 x 2
C82 4.7 µF 25 V Converter input Right-hand end, power section 5 x 4 x 1.5
C83 470 µF 10 V Converter input Right-hand end, power section 16 x 10 x 5
C84 470 µF 6.3 V VDD (+5 V) output Right-hand end, power section 12.5 x 10 x 5
C85 33 µF 10 V VEE (−5 V) output Right-hand end, power section 7.5 x 5 x 2
C86 100 µF 6.3 V Right-hand end, power section 11.5 x 6.5 x 2
C90 1 µF 50 V Right-hand end, power section 7 x 4 x 1.5
C92 0.47 µF 50 V VDD feedback Right-hand end, power section 11 x 5 x 2
C103 220 µF 10 V Left end, by the modem transformer and relay 11.5 x 8 x 3.5

Sources: values from the parts list, pp. 6-3 and 6-4;[1] C75 to C77 marked non-polarised on the RS-232C circuit diagram, Fig. 4-25 (p. 4-22);[7] the roles of C78, C82 to C85 and C92 from the power supply and reset descriptions and the "doesn't work at all" flowchart (pp. 4-32 to 4-34, 5-4);[8] positions from the board drawing, Fig. 7-3;[9] sizes and the non-polarised C52 from Jeff Birt's capacitor map, which marks each position on the same drawing.[10]

The parts list gives the tolerance of each capacitor but not its temperature rating.[1] Fit parts of the same capacitance and the same or a higher voltage rating, non-polarised where the original is, and no taller than the original: the keyboard sits over the main board when the case is closed.

Procedure

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  1. Copy off any files you want to keep, then take out the AA cells and disconnect the adaptor.
  2. Open the case and lift out the main board as described in General Maintenance: four case screws, the LCD and keyboard connectors, then seven screws holding the main board.[11]
  3. Note the polarity of each capacitor before removing it.
  4. Remove the capacitors. Birt finds a desoldering tool struggles on this board because the pads joined to the ground and supply planes have no thermal relief. He heats one leg with the iron and pushes the top of the capacitor towards the other side, then does the same on the other leg, walking the part out over three or four goes, and clears the holes afterwards. Corroded solder has to be scraped off first, because the oxides melt at a much higher temperature. A heat gun on a low setting, preheating the board to about 100 °C, helps on a difficult board.[12]
  5. Inspect the pads and traces under each capacitor, especially C82, C85 and C86. Birt's method for corrosion: rub it back to bare copper with a glass-fibre pen; neutralise the residue with a mild alkaline solution such as bicarbonate of soda in water, left for about an hour; wipe and flush with alcohol; flux and tin the bare copper and clean again; check each trace for continuity and plan any wire repairs; coat the tinned areas away from the solder points before fitting the new parts.[13][14] See also Battery Explosion, Capacitor or Corrosion Damage.
  6. Fit the new capacitors, observing polarity on all but C52 and C75 to C77.
  7. Look at the Ni-Cad memory battery while the board is out, and replace it if it shows any sign of leaking; see General Maintenance.
  8. Refit the board and close the case.

Checks after recapping

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Supply checks (Service Manual, p. 5-4 and p. 5-8)
Point Expected
Converter transformer OT2, pin 1 3.6 to 8 V
VDD +5 V
VEE −5 V
VB, power on +5 V
VB, power off 2.0 to 4.0 V

Source: the troubleshooting flowcharts.[15] Then run Radio Shack's check list, which is on the General Maintenance page.[15] A screen that stays blank with correct supplies is covered in the TRS-80 Model 100 Troubleshooting Guide.

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References

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  1. ↑ 1.0 1.1 1.2 Radio Shack, Service Manual: TRS-80 Model 100 Portable Computer, catalogue numbers 26-3801/3802, Section VI, parts lists: main board capacitors C1 to C111 with type, value, voltage and tolerance (pp. 6-3 and 6-4); LCD board capacitors LC1 to LC26, all ceramic (p. 6-11). Hosted as File:Radio Shack TRS-80 Model 100 Service Manual (26-3801, 26-3802).pdf, described at Radio Shack TRS-80 Model 100 Service Manual.
  2. ↑ Jeffrey Birt, "Re: Need help fixing VEE on M100", M100 mailing list, 21 March 2021 (retrieved 2026-10-21). Community source. On the Model 100s he has worked on, most of the capacitors tend to leak and damage the board; although he does not recap vintage equipment as a rule, he recaps every Model 100.
  3. ↑ Jeffrey Birt, "New member - question on 'half' alive Model 100", M100 mailing list, 29 April 2018 (retrieved 2026-10-21). Community source. The Ni-Cad was bad; C82, C85, C49, C50, C54 and C55 had all started to leak; the leakage was small and lifted some silkscreen.
  4. ↑ Josh Malone, "Re: Preemptive recapping", M100 mailing list, 19 August 2020 (retrieved 2026-10-21). Community source. Replace C82 to C86 on all Model 100s; one or more failed on close to 75% of machines; the rest only where leaking or after a hot and humid life; damage from recapping comes from poor parts and poor technique.
  5. ↑ George Hines, "Non working display on my m100", M100 mailing list, 9 July 2019 (retrieved 2026-10-21). Community source. C82 and C85 had leaked and damaged the top of the board; a trace under C82 to the contrast control was patched.
  6. ↑ Nathan Misner, "Model 100 LCD doesn't work", M100 mailing list, 21 July 2018 (retrieved 2026-10-21). Community source. The trace between the negative side of C85 and D15 corroded by a leaking capacitor.
  7. ↑ Service Manual: TRS-80 Model 100, Section IV, Fig. 4-25, "RS-232C Interface Circuit", p. 4-22: C75, C76, C77 47 µF 16 V (N.P).
  8. ↑ Service Manual: TRS-80 Model 100, Section IV, pp. 4-32 to 4-34: VDD and VEE rectified by D13 and D15 and smoothed by C84 and C85; VDD fed back through T13, D4, R121 and C92; R103 and C78 set the reset delay. Section V, p. 5-4: with less than 3.6 to 8 V at the converter transformer, check C82 and C83.
  9. ↑ Service Manual: TRS-80 Model 100, Fig. 7-3, "Main P.C.B. - Component Side", p. 7-3, rendered as File:TRS-80 Model 100 main board component side (Service Manual Fig. 7-3).png.
  10. ↑ Jeff Birt, TRS-80 Model 100 Parts List Updated, capacitor map and order list, 27 May 2018 (retrieved 2026-10-21). Community source. The 17 electrolytics colour-coded on the service manual's board drawing, with values, sizes (height, diameter and lead pitch) and Nichicon equivalents; C52 and C75 to C77 non-polarised.
  11. ↑ Service Manual: TRS-80 Model 100, Section II, pp. 2-1 and 2-2: case screws, connectors, and the seven M3 x 8 screws holding the main board; care with the RESET switch and battery contact spring.
  12. ↑ Jeffrey Birt, "Re: Recap and new battery", M100 mailing list, 16 February 2023 (retrieved 2026-10-21). Community source. No thermal reliefs on pads into the ground and supply pours; walking capacitors out leg by leg; scraping off corroded solder; preheating the board to about 100 °C with a small heat gun.
  13. ↑ Jeffrey Birt, "Re: Need help fixing VEE on M100", M100 mailing list, 11 April 2021 (retrieved 2026-10-21). Community source. Trace damage under leaking capacitors is common and keeps spreading if not dealt with; glass-fibre pen, a mild alkaline solution for about an hour, alcohol, flux and tin, check continuity, coat, then fit the capacitors and wire links.
  14. ↑ Eric LK, "Re: Need help fixing VEE on M100", M100 mailing list, 11 April 2021 (retrieved 2026-10-21). Community source. Pads and traces under C82, C85 and C86 corroded on a board recapped by a previous owner.
  15. ↑ 15.0 15.1 Service Manual: TRS-80 Model 100, Section V, pp. 5-4, 5-8 and 5-12: converter and supply checks, memory protection checks, and the check list.


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