TRS-80 Model 100 Troubleshooting Guide
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Fault-finding on the TRS-80 Model 100 follows Radio Shack's service manual, which gives a flowchart for each of twelve functions and names the parts to check at each step.[1] Faults that owners have found since, mostly caused by leaking capacitors and the old Ni-Cad memory battery, are added under the symptom they produce. Part numbers M, T, C, D and R refer to the main board drawing (Fig. 7-3) in the manual.[2]
Before you start
[edit | edit source]- Fit fresh AA cells or connect a known-good adaptor (5.5 mm plug, centre negative), and check that both the MEMORY POWER switch and the power switch are on.[1][3]
- After an automatic power-off the switch stays in the ON position. Switch off and on again to restart.[3]
- A computer that has been stored for years may have a flat memory battery and random data in RAM. Before looking for a fault, leave it on the adaptor overnight with MEMORY POWER on and the power switch off; then remove all power with MEMORY POWER off for about 15 minutes, switch MEMORY POWER back on, restore power and do a cold start. Turn the contrast dial through its range afterwards.[4]
- A cold start is CTRL and PAUSE held while pressing RESET on the back; it erases the files and resets the date and time.[1]
- To tell a dead computer from a dead display, do a cold start, press ENTER, type BEEP and press ENTER again. A beep means the computer is running and the fault is in the display or its connection.[4]
Power
[edit | edit source]The DC/DC converter makes VDD (+5 V) and VEE (−5 V). VB is +5 V with the computer on and is held at 2.0 to 4.0 V by the Ni-Cad when it is off.[5][1]
| Check | Expected | If not, check |
|---|---|---|
| Converter transformer OT2, 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 | +5 V | T27, T28 |
| T21 | Oscillating | T22, T13, C81, R126, R127, R140, T20 |
| RESET, high | +2.2 to 5.3 V | T10, T11, T25, T9 |
| RESET, low | 0.8 to 0.3 V | T10, T11, T25, T9 |
| CPU clock | 4.9152 MHz at X1, 2.4576 MHz at CLK | X2, M19 |
| Logic | Runs once the above are right | Try another LCD unit; check the ICs |
- The low-battery LED on the LCD board lights when the supply falls to 4.1 V ±0.1 V; at 3.7 V ±0.1 V the computer switches the converter off.[5]
- Josh Malone found one or more of C82 to C86 failed on close to 75 per cent of the Model 100s he had seen, and Birt found C82 and C85 leaking. Where the converter output is low, replace them and clean the board under them; see the TRS-80 Model 100 Capacitor Replacement Guide.[6][7]
- Leaked electrolyte can leave conductive residue on the top of the board, and Birt has found the flux on the underside of some boards turning conductive; both have caused supply faults on machines he repaired.[8]
Display
[edit | edit source]At power-on the LCD controllers turn every dot on, giving a solid black screen, and the computer clears it as it boots. No black screen at all points to a supply fault; a black screen that never clears means the LCD drivers have been reset but not initialised, and the computer is not completing its start-up.[6][9]
| Symptom | Probable cause | Action |
|---|---|---|
| Blank screen, but BEEP works | Low VEE or a broken contrast line | Measure +5 V and −5 V at the LCD end of the cable.[9] Check continuity from pin 4 of the LCD connector to the base of T23; on one machine a leaking capacitor had eaten through the contrast line.[10] George Hines found a damaged trace under C82 running to the contrast control.[11] |
| Very faint contrast | VEE low | Replace C82 and C85; one owner's VEE returned to −5 V and the contrast to normal.[12] If VEE falls again later, look for corroded pads and traces under the capacitors, and check the RS-232 circuit, which also loads VEE.[8][13] |
| Blank screen after a recap; VEE reads wrong | Corroded trace between C85 and D15 | Nathan Misner found the trace from the negative side of C85 to D15 eaten away and bridged it with a wire on the underside.[14] |
| Solid black screen that never clears | Computer not booting: RAM, ROM or reset | See Will not start up properly. |
| No display, all supplies correct | LCD drive | The manual's order: VDD, VEE and VB; the RESET signal; the LCD drive waveform and its operational amplifier; the interface ICs on the bus including M17 and M25; the LCD connector; the HD44102 and HD44103 drivers on the LCD board, or replace the LCD unit.[1] |
Will not start up properly
[edit | edit source]- Jeffrey Birt's Model 100 powered up with correct supply voltages but left the LCD black and would not run. The soldered-in standard RAM pack was faulty; with one of the option RAM packs in its place, on a socket made from a split DIP socket, the computer worked.[15] In a later thread he gave the same cause for a blank display on a computer that otherwise appeared dead.[16]
- Remove the option RAM packs to see whether one of them is the fault.[6]
- Check the reset circuit: the manual lists T11, T10, T9, T25, D20, D22 and C78, then every IC that uses RESET (M19, M25, M22, M14, M27, M36 and M31), the LCD unit and the RAM.[1] C78 sets the reset delay.[5] One owner has had a failed reset capacitor cause erratic behaviour.[17]
Keyboard and buzzer
[edit | edit source]| Symptom | Check, in order |
|---|---|
| Keys do not work | Keyboard connector seated; key scan signal at M25 PA0 to PA7 and PB1, and if absent, pulses every 4 ms from the TP terminal (pin 10) of M18 and M25; the diodes on the keyboard; the return signal; M15, M13, M16 and pull-up resistor pack MR4; the individual key switches |
| No beep and no melody | The LCD connector and the buzzer connector on the LCD board; the buzzer control logic M26, M34 and M25 (PB5 port and TO terminal); the buzzer |
Source: the keyboard and buzzer flowcharts.[1]
Clock and memory
[edit | edit source]| Symptom | Check | If wrong |
|---|---|---|
| Clock does not keep time | Supply: +5 V on, 2.0 to 4.0 V off | Power supply circuit |
| 32.768 kHz oscillation | X1, C17, C18, M18 | |
| 250 Hz pulse from the TP signal | Clock-setting ICs, given as M14 and M25, and RESET | |
| Files lost when switched off | VB: +5 V on, 2.0 to 4.0 V off | Converter circuit (on); Ni-Cad battery, D11, D22 (off) |
| RAM RST and RESET change level cleanly together at switch-on and switch-off | Reset circuit | |
| CMOS RAM | Replace the faulty RAM |
Source: the clock and memory protection flowcharts.[1] Two Model 100s seen by one owner had poorly soldered joints, with bubble holes, near the clock chip; reflowing them cured the faults.[17] If VB falls with the computer off, the Ni-Cad is the first part the manual lists; replacing it is covered in General Maintenance.
Ports
[edit | edit source]| Port | Check | Parts named in the manual |
|---|---|---|
| Printer | Strobe signals; data signals; connector and cable | M14, T8; M25, M32 |
| Cassette, saving | Modulated waveform at pin 5 of the cassette connector | M34, M19, C68, C64 |
| Cassette, loading | Digital waveform at the CPU's SID pin | D5, D6, M30, M34, M19 |
| Cassette motor | Remote circuit | Relay RY1, T6, M34, M14 |
| Bar code reader | Logic and supply | M34, M25, M19; VDD (+5 V) |
| RS-232C, transmit | ±5 V to ±3.5 V signal at pin 2 while sending; CTS (pin 5) low | M22, M24, M35, C75, C76, C77, C71, C72, C73 |
| RS-232C, receive | Data at M22 pin 20 (RRI); RTSR (pin 4) low | M22, M35, M33, D9, D8, D10 |
| RS-232C, select | PB3 (pin 32) of M25 low | M25, M34 |
| Modem, transmit | Modulated signal at modem connector pin 5 (coupler) or pin 7 (direct); carrier at M31 pin 1 | M22, M30, T4, T7, OT1, RY2 |
| Modem, receive | Modulated signal at M31 pin 1 (Rx Car) | M29, M30, OT1, T2, T3, T5, D1, D2, RY2; then M22 and M31 |
| Auto-dialler | Dialling relays and drivers | RY2, RY3, T7, T24, M36, M26 |
Source: the interface flowcharts.[1] C75 to C77, in the RS-232 circuit, are 47 µF 16 V non-polarised electrolytics.[18] If the modem works but its level is wrong, the transmit level adjustment is on the General Maintenance page.
After the repair
[edit | edit source]Run Radio Shack's check list: a cold start, the display test program, the clock, every key, warm start and cold start, the printer, a cassette save and load, and data transfer through the RS-232C port and the modem.[1] The full list is on the General Maintenance page.
Related pages
[edit | edit source]- TRS-80 Model 100
- TRS-80 Model 100 General Maintenance
- TRS-80 Model 100 Capacitor Replacement Guide
- Radio Shack TRS-80 Model 100 Service Manual
- Capacitor Failure Symptoms
- Battery Explosion, Capacitor or Corrosion Damage
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Radio Shack, Service Manual: TRS-80 Model 100 Portable Computer, catalogue numbers 26-3801/3802, Section V, Troubleshooting, pp. 5-1 to 5-12: power and DC/DC converter (p. 5-4), LCD (p. 5-5), keys and buzzer (p. 5-6), clock and reset (p. 5-7), memory protection and printer (p. 5-8), cassette and bar code reader (p. 5-9), RS-232C (p. 5-10), modem (p. 5-11) and the check list (p. 5-12). 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.
- ↑ 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.
- ↑ 3.0 3.1 Service Manual: TRS-80 Model 100, Section I, pp. 1-4 to 1-7: specifications, connectors (adaptor socket 5.5 mm, centre negative), the automatic power-off after ten minutes without use, and the MEMORY POWER switch.
- ↑ 4.0 4.1 "Recovering an Unresponsive Laptop", Bitchin100 DocGarden wiki, last edited 25 September 2013 (retrieved 2026-10-21). Community source. The overnight charge, clearing the RAM, the cold start and the contrast knob; and the BEEP test for a working computer with a bad display.
- ↑ 5.0 5.1 5.2 Service Manual: TRS-80 Model 100, Section IV, pp. 4-32 to 4-34: the DC/DC converter (transformer OT2, oscillator transistors T21 and T22, rectifiers D13 and D15, smoothing capacitors C84 and C85), VDD feedback through T13, D4, R121 and C92, the 4.1 V low-battery LED and 3.7 V shut-down thresholds, and the reset circuit (R103 and C78 set the delay; T25 shorts C78 at automatic power-off).
- ↑ 6.0 6.1 6.2 Jeffrey Birt, "New member - question on 'half' alive Model 100", M100 mailing list, 29 April 2018 (retrieved 2026-10-21). Community source. Ni-Cad removed and C82, C85, C49, C50, C54 and C55 replaced after leaking; supply voltages restored; solid black LCD with a brief click from the speaker afterwards.
- ↑ Josh Malone, "Re: Preemptive recapping", M100 mailing list, 19 August 2020 (retrieved 2026-10-21). Community source. One or more of C82 to C86 failed on close to 75% of the Model 100s he had seen.
- ↑ 8.0 8.1 Jeffrey Birt, "Re: Need help fixing VEE on M100", M100 mailing list, 21 March 2021 (retrieved 2026-10-21). Community source. Most capacitors on the Model 100 tend to leak and damage the board; the −5 V rail also feeds the RS-232 circuit and can be loaded by a fault there; power supply faults from invisible conductive residue on top of the board and from the factory flux on the underside turning conductive.
- ↑ 9.0 9.1 Jeffrey Birt, "Re: Model 100 LCD doesn't work", M100 mailing list, 21 July 2018 (retrieved 2026-10-21). Community source. The LCD controllers turn every segment on at power-up and the boot clears them; no solid black screen indicates a power supply problem; check +5 V and −5 V on the LCD side of the connector first.
- ↑ Fugu ME100, "Re: M100 display not working, all else seems good", M100 mailing list, 19 August 2019 (retrieved 2026-10-21). Community source. A leaking capacitor had eaten through the contrast control line on another machine; check continuity between pin 4 of the LCD connector and the base of T23.
- ↑ 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 board; the trace under C82 to the contrast control was patched and the screen returned.
- ↑ Eric LK, "Need help fixing VEE on M100", M100 mailing list, 21 March 2021 (retrieved 2026-10-21). Community source. Weak contrast cured by replacing C82 and C85; VEE fell to −3 V again six months later.
- ↑ Eric LK, "Re: Need help fixing VEE on M100", M100 mailing list, 11 April 2021 (retrieved 2026-10-21). Community source. On removing C82, C83, C84, C85, C86, C90 and C92 the parts tested good but the pads and traces under C82, C85 and C86 were corroded.
- ↑ Nathan Misner, "Model 100 LCD doesn't work", M100 mailing list, 21 July 2018 (retrieved 2026-10-21). Community source. A leaking capacitor had corroded the trace between the negative side of C85 and D15; repaired with a jumper wire on the back of the board.
- ↑ Jeffrey Birt, "Re: New member - question on 'half' alive Model 100", M100 mailing list, 13 May 2018 (retrieved 2026-10-21). Community source. The fault was the standard RAM module; an option RAM pack fitted in its place on a socket split from a 30-pin DIP socket brought the computer back, and reflowing the original module did not.
- ↑ Jeffrey Birt, "Re: M100 display not working, all else seems good", M100 mailing list, 19 August 2019 (retrieved 2026-10-21). Community source. No display can be a failure to complete the boot because of bad RAM; on his first Model 100 the boot code overwrote the stack, and swapping the soldered 8K RAM module for an option RAM cured it.
- ↑ 17.0 17.1 Fugu ME100, "Re: New member - question on 'half' alive Model 100", M100 mailing list, 29 April 2018 (retrieved 2026-10-21). Community source. A failed reset capacitor causing odd behaviour; two Model 100s with poor soldering, with bubble holes, near the clock chip, cured by reflowing the joints.
- ↑ 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).