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TRS-80 Model 100 General Maintenance

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TRS-80 Model 100 opened, main board and keyboard (Hustvedt)
A Model 100 opened out: main board in the bottom half (left), LCD board and keyboard in the top half (photo: Hustvedt, CC BY-SA 3.0)

The TRS-80 Model 100 has no mains voltage inside: it runs from four AA cells or a low-voltage adaptor through a DC/DC converter on the main board.[1] Everything the owner has stored is in RAM, held up by a Ni-Cad cell on the main board when the computer is off. Most of the repair notes Model 100 owners share concern two parts: that cell and the main board's electrolytic capacitors, both of which leak.[2]

Before you start

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  • Copy any files you want to keep to another computer over the RS-232 port, or to cassette. The files exist only in battery-backed RAM.[3]
  • Turning the MEMORY POWER switch off, or removing the backup battery, loses the RAM contents.[3]
  • Disconnect the adaptor and any cables, and take out the AA cells.[4]

Opening the case

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  1. Turn the computer over on a soft surface so the key tops are not scratched, and remove the four case screws.[4]
  2. The top half opens to the right like a book. Snaps in the middle of the edges hold the halves together; Club 100's instructions are to stand the computer on its right-hand edge, push in on the middle of the top and front edges of the bottom half, and pull the top half away.[5] Do not pull hard: the LCD and keyboard cables are still connected.[4]
  3. Unplug the LCD and keyboard connectors from the main board, and the buzzer connector from the LCD board.[4]
  4. The keyboard comes off the top case with five screws and the LCD board with four. The main board is held by seven screws; lift it out carefully past the RESET switch and the battery contact spring.[4]

To reassemble, fit the main board with the reset switch knob in its notch, secure the battery contact spring, and use the seven M3 x 8 screws. Lay the top half to the right of the bottom half, reconnect the LCD and keyboard cables, close the case so the tabs engage, and refit the four case screws.[4] Keep the keyboard wires clear as the halves close.[5]

Cleaning

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Tandy's manual calls for a soft, dry, lint-free cloth on the case and LCD, and benzol for stubborn marks on the case, with no other solvent.[6] The polarising film on the face of the LCD scratches easily. For the display surface and its contacts the manual says to use a soft cloth slightly dampened with benzine and to wipe gently, and not to use organic solvents such as alcohol.[7]

Memory backup battery

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The backup cell is part P-36, "Battery Ni-Cad 3-51FT", Radio Shack part ACS-0100.[8] On the board drawing it is marked 3-51FT, beside the MEMORY POWER switch SW3 at the rear edge of the main board.[9] Jeff Birt lists the original as a Yuasa 3-51-FT-A rated 3.6 V and 50 mAh.[10]

  • Check it: with the computer switched off, VB should read 2.0 to 4.0 V. If it does not, the manual's checks are the Ni-Cad battery, D11 and D22.[11]
  • Look at it whenever the case is open. Members of the Model 100 mailing list advise replacing the original cell before it leaks, and cleaning up after it where it already has.[12] Treat any residue as described in Battery Explosion, Capacitor or Corrosion Damage.
  • Replacements used by owners include a new nickel-metal hydride cell, which Brian White fits to his machines, and a supercapacitor, which Birt lists. White points out that a capacitor holds the RAM for a much shorter time than a battery.[13][10]
  • A new or flat cell charges from the AA cells or the adaptor whenever MEMORY POWER is on.[3] The Bitchin100 recovery notes leave the computer on the adaptor overnight with MEMORY POWER on and the power switch off.[14]
  • For long storage the manual says to switch MEMORY POWER off, accepting that the RAM is then not backed up.[3]

Voltage checks

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Supply checks (Service Manual, pp. 4-32 and 5-4)
Rail or point Expected Parts to check if wrong
Converter transformer input, pin 1 3.6 to 8 V C82, C83, battery contacts, adaptor jack
VDD +5 V D13, C84, D4
VEE −5 V D15, C85, D14
VB, power on +5 V T27, T28
VB, power off 2.0 to 4.0 V Ni-Cad battery, D11, D22

The low-battery LED lights at 4.1 V ±0.1 V from the AA cells or adaptor, and the converter is shut down at 3.7 V ±0.1 V.[1][11] The adaptor plug is 5.5 mm, centre negative.[3]

Electrolytic capacitors

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The main board carries 17 aluminium electrolytic capacitors; the LCD board has none.[8] Writing on the Model 100 mailing list, 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 machines, and replacing the others where they show leakage or the computer has lived somewhere hot and humid.[15] Values, positions and the procedure are on the TRS-80 Model 100 Capacitor Replacement Guide.

Adjustments

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The manual states that the Model 100 is aligned at the factory and needs no electrical adjustment.[6] The LCD contrast is set with the display adjustment dial.[3] The one service adjustment is the modem transmit level:[6]

  1. Set the DIR/ACP switch to DIR.
  2. Connect a 600 ohm dummy load between pin 1 (RXMD) and pin 7 (TXMD) of the modem connector CN4, with an AC voltmeter across the same pins.
  3. In BASIC, enter OUT 178,47 and then OUT 168,2 to start the carrier.
  4. Adjust VR2 for −14 to −17 dBm, in both ANS and ORIG modes.

Optional RAM and ROM

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With the case open, the main board has three sockets marked "Optional RAM 26-3801 #1, 2 and 3". Line up every pin of a RAM pack with its socket before pressing it in. The option ROM socket is under a cover in the base, removed with a coin; the board is marked 1, 14, 15 and 28 at the corners of the socket, and pin 1 of the ROM goes to pin 1.[16]

Check list after any repair

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Radio Shack's check list starts with a cold start: hold CTRL and PAUSE and press the RESET button on the back. All user files are erased and the date and time are reset. A warm start, switching off and on or pressing RESET, should leave the files in place.[11] Then:[11]

  1. In BASIC, run the display test. The manual prints it with two lines numbered 20; the first is line 10:


10 FOR I=0 TO 255
20 PRINT CHR$(I)
30 NEXT I


40 ENDThe computer beeps once, clears the screen and prints every printable character.

  1. Set the date, day and time from BASIC, return to the main menu with MENU and check that the calendar shows the new settings.
    2. 

  2. Try every key.
    3. 

  3. Print the screen to a printer with the PRINT key.
    4. 

  4. Save a short program to cassette with CSAVE and read it back with CLOAD.
    5. 

  5. Transfer data between two computers through the RS-232C port and then through the modem.


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References
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  1. ↑ 1.0 1.1 Radio Shack, Service Manual: TRS-80 Model 100 Portable Computer, catalogue numbers 26-3801/3802, Section IV, pp. 4-32 to 4-34: the DC/DC converter that makes VDD (+5 V) and VEE (−5 V), the 4.1 V low-battery LED and 3.7 V shut-down thresholds, and the reset circuit. 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. 

  2. ↑ Jeffrey Birt, "New member - question on 'half' alive Model 100", M100 mailing list, 29 April 2018 (retrieved 2026-10-21). Community source. A bad Ni-Cad battery removed; C82, C85, C49, C50, C54 and C55 replaced because all had started to leak; little Model 100 repair information found online beyond the Ni-Cad and leaking capacitors.

    3. 

  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Service Manual: TRS-80 Model 100, Section I, pp. 1-4 to 1-7: specifications, including the rechargeable memory protection battery (about 40 days with 8 KB, about 10 days with 32 KB, trickle-charged from the adaptor or AA cells), the connectors, and the MEMORY POWER switch, which must be on for the computer to run and which stops RAM backup when off.

    4. 

  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Service Manual: TRS-80 Model 100, Section II, pp. 2-1 and 2-2: disconnecting cables, the four case screws, the snaps holding the case halves, the LCD, keyboard and buzzer connectors, and the screws for the keyboard (five), LCD board (four) and main board (seven, M3 x 8).

    5. 

  5. ↑ 5.0 5.1 "Splitting the Model 100/102/200 case", Bitchin100 DocGarden wiki, from the Club 100 support files, 23 February 2009 (retrieved 2026-10-21). Community source. Where to press to release the case notches, and not pinching the keyboard wires on reassembly.

    6. 

  6. ↑ 6.0 6.1 6.2 Service Manual: TRS-80 Model 100, Section III, Preventive Maintenance, p. 3-1: no electrical adjustments needed; cleaning the body and LCD; modem transmitting level adjustment with VR2.

    7. 

  7. ↑ Service Manual: TRS-80 Model 100, Section IV, "Liquid Crystal Display", p. 4-28: caution on the polarisation plate and cleaning the contacts and display surface.

    8. 

  8. ↑ 8.0 8.1 Service Manual: TRS-80 Model 100, Section VI, parts lists, pp. 6-3, 6-4 and 6-11: the main board capacitors, and miscellaneous part P-36, "Battery Ni-Cad 3-51FT", RS part ACS-0100.

    9. 

  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. 

  10. ↑ 10.0 10.1 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 main board electrolytics with positions, sizes and Nichicon equivalents; the memory battery as a Yuasa 3-51-FT-A, 3.6 V, 50 mAh; a supercapacitor, Mouser part SCMS22H255MRBB0, as an alternative.

    11. 

  11. ↑ 11.0 11.1 11.2 11.3 Service Manual: TRS-80 Model 100, Section V, pp. 5-1 to 5-12: troubleshooting flowcharts, including the DC/DC converter checks (p. 5-4), memory protection (p. 5-8) and the check list (p. 5-12).

    12. 

  12. ↑ Brian White, "Re: Model 100 inspection/repair services", M100 mailing list, 9 February 2019 (retrieved 2026-10-21). Community source. Check inside for obvious corrosion and replace the original Ni-Cad before it leaks.

    13. 

  13. ↑ Brian White, "Re: supercap", M100 mailing list, 4 January 2018 (retrieved 2026-10-21). Community source. Replaces the old batteries with new NiMH cells; a capacitor gives only a fraction of the battery's hold-up time.

    14. 

  14. ↑ "Recovering an Unresponsive Laptop", Bitchin100 DocGarden wiki, last edited 25 September 2013 (retrieved 2026-10-21). Community source. The internal battery soldered to the main board; overnight charging; clearing the RAM with MEMORY POWER off and no power for about 15 minutes; checking the contrast knob.

    15. 

  15. ↑ 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, close to 75% failure of one or more; others only where leaking or after a hot, humid life.

    16. 

  16. ↑ Service Manual: TRS-80 Model 100, Appendix A, "Installation of Optional RAMs and ROM", p. A-1.

    17. 


















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