TRS-80 Model 4D Troubleshooting Guide
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The TRS-80 Model 4D has the main board, power supply and monitor of the gate array TRS-80 Model 4; only the drives, their mount and cable, the keyboard and two capacitors differ.[1] Power supply and main board faults are covered in the TRS-80 Model 4 Troubleshooting Guide, which includes Sams' checks for the gate array board. The sections below cover what is particular to the 4D.
 | The power supply rectifies the mains directly, and the CRT holds a charge after switch-off. See the TRS-80 Model 4D General Maintenance guide before opening the case. |
Main board
[edit | edit source]| Symptom | Where to look |
|---|---|
| Each letter typed appears twice and others go missing | The trs-80.com notes put this down to the main board: one or two of the gate arrays, the CRT controller or the video RAM[2] |
| Garbage on the screen at switch-on that clears when Reset is pressed | The power-on reset timing capacitor. Sams shows it as C25, 22 µF, on the gate array board. The trs-80.com notes name C20, which is the reset capacitor on the original Model 4 board; on the gate array board C20 is a crystal load capacitor[3][2] |
| Dead, with no 20.2752 MHz clock at U9 pin 1 | Sams' clock check: C2, C20, R14, crystal Y1 and U9.[3] On the 4D, C2 is 47 pF and C20 is 33 pF[1] |
For reset, CPU, keyboard, video, cassette, printer and RS-232 checks, see gate array main board in the Model 4 guide.
Disk drives
[edit | edit source]TEC's FB-500 Series Service Manual, part of the 4D addendum, has troubleshooting flow charts and a section on errors that are not drive faults.[4]
Errors that may not be the drive
[edit | edit source]TEC lists these "soft errors" before any repair:[4]
| Error | Causes TEC lists | What to do |
|---|---|---|
| Read error | Dust between head and disk; small track offsets not caught when writing; a worn head load pad or disk; poor grounding of the drive's supply in the computer; wrong motor speed | Re-read the track about ten times; if that fails, step the head to track 00, return to the track and try again. Errors that survive this are unrecoverable |
| Write error | Found when the data is read back | Write and read again; if it persists, try another track, then another disk, before blaming the drive |
| Seek error | Stepper motor or its drive circuit; the carriage | Reset the steel band tension and readjust track 00 |
| Interchange error (another drive cannot read the disk) | Head alignment, head output, motor speed, or a damaged centre hole on the disk | Check the alignment, output and speed; if the centre hole is damaged, check the clamp |
Check whether an error repeats with the same disk and with another disk before opening the drive.[4]
Drive repair flow
[edit | edit source]| Symptom | TEC's next step |
|---|---|
| Activity lamp does not light and the drive does nothing | Check the DC supply to the drive |
| Lamp lights but no sound of operation | Media rotation check: motor-on signal, drive motor and its connector, the motor's set screw, disk slipping in the clamp, motor speed and index sensor |
| Stepper turns but the carriage does not move | Tracking check: with the power off, move the carriage by hand both ways to check it runs smoothly; check the steel band for breaks or a loose set screw, the head cable for excessive tension, the track 00 detector, and the head alignment against an alignment disk |
| Drive runs but will not read or write | Read and write circuit checks |
Source: TEC's troubleshooting procedures.[4] The adjustments these checks lead to are listed in the General Maintenance guide. See also Floppy Drive Repair.
Power supply and monitor
[edit | edit source]These are unchanged from the gate array Model 4.[1] See power supply in the Model 4 guide, and the TRS-80 Model 4P Troubleshooting Guide for Tandy's own 65 W supply. Replacement green tubes are listed in the trs-80.com notes under 12VCMP31.[2]
Related pages
[edit | edit source]- TRS-80 Model 4D
- TRS-80 Model 4D General Maintenance
- TRS-80 Model 4D Capacitor Replacement Guide
- TRS-80 Model 4 Troubleshooting Guide
- Tandy TRS-80 Model 4D Service Manual Addendum
References
[edit | edit source]- ↑ 1.0 1.1 1.2 Tandy Corporation, Addendum to the Model 4 and Model 4 Gate Array Service Manual (Cat. No. 26-1067/8/9) for Model 4D (Cat. No. 26-1070), 1985, hosted as File:Tandy TRS-80 Model 4D Service Manual Addendum (26-1070, 1985).pdf. Introduction and parts list differences, PDF pages 5-6.
- ↑ 2.0 2.1 2.2 Ira Goldklang, "TRS-80 Troubleshooting Guide and Repair Q&A", Ira Goldklang's TRS-80 Revived Site (retrieved 2026-10-21). "Model 4" section.
- ↑ 3.0 3.1 Howard W. Sams & Co., Computerfacts CS13, Radio Shack TRS-80 Model 4 Gate Array, 1986, archive.org scan. System board schematic and troubleshooting, pp. 33-35.
- ↑ 4.0 4.1 4.2 4.3 TEC, FB-500 Series Service Manual, document E0-2129-G, December 1984, in the Addendum ... for Model 4D. Troubleshooting chapter, sections 1-3.