TRS-80 Model 4D General Maintenance
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The TRS-80 Model 4D is a gate array TRS-80 Model 4 with TEC double-sided drives and a revised keyboard. Tandy's 4D addendum lists no other differences, so the case, power supply, monitor and main board are looked after as described in the TRS-80 Model 4 General Maintenance guide.[1] The 4D's own differences lie in how the case comes off and in the drives.
 | The power supply rectifies the mains directly, and the CRT holds a charge after switch-off. Follow the CRT Discharge Procedure before working near the tube. |
Opening the case
[edit | edit source]The trs-80.com notes give the same screws for the Model III, Model 4 and 4D: ten underneath and one in the back.[2]
- Underneath: three short machine screws along the front; two long machine screws about 10 cm in, recessed under the base of the drives and the base of the CRT; and five self-tapping screws around the rear, one of which may be under a warranty label.
- At the back: one black screw in the middle.
If the case still will not lift after all eleven are out, something at the back left, such as the video board wiring, may be catching; watch through the top drive opening as the lid comes up. The CRT stands proud of the chassis, and the case has to be angled carefully to clear it.[2] Tandy's own procedure for the Model 4 case is in the Model 4 guide.
Disk drives
[edit | edit source]The 4D's drive assembly is Tandy part 8790133, built by TEC; TEC's FB-500 Series Service Manual is bound into the 4D addendum in place of the drive section of the Model 4 service manual.[1][3]
Head cleaning
[edit | edit source]Tandy's introduction manual for the 4D asks owners to clean the heads regularly, and names the Universal Disk Drive Head Cleaning Kit (catalogue 26-408) as the only Tandy-approved method.[4] For cleaning by hand, TEC's procedure is:[5]
- Dampen a cotton bud slightly with isopropyl alcohol.
- Lift the load arm away from the head without touching the load button.
- Wipe the head gently with the damp part of the bud.
- When the alcohol has evaporated, polish the head gently with a clean bud.
- Lower the load arm back on to the head without letting it strike the head.
TEC warns that any other method may scratch the head, and that on the FB-501 to FB-504 the head is bonded to its carrier and cannot be replaced on its own.[5]
Adjustments
[edit | edit source]TEC's service manual gives these adjustments for the FB-500 series. Most need an alignment disk, an oscilloscope and a drive exerciser; TEC uses a BRIKON tester.[6]
| Section | Adjustment | Method |
|---|---|---|
| 3.1 | Drive motor speed | With a work disk turning under a 50 or 60 Hz fluorescent light, turn VR1 on the drive motor board until the stroboscope pattern on the motor stands still |
| 3.2 | Head radial (CE) alignment | Alignment disk and oscilloscope; let the alignment disk stand at room temperature for at least 20 minutes first |
| 3.3 | Track 00 | Work disk and oscilloscope on the track 00 and step signals while the drive steps between tracks 0 and 2 every 6 ms (48 TPI drives) |
| 3.4 | Index sensor | Alignment disk and oscilloscope |
| 3.5 | Head azimuth | Alignment disk and oscilloscope |
| 3.6 | Head output | Write and read a 2F pattern on track 39 of a work disk and measure the pre-amplifier output |
| 3.7 | VR1 on the drive's main board (TEC's "CPU PC board") | With the 2F pattern on track 39, turn VR1 until the read data waveform stops trembling |
| 3.9 | Steel band tension | Set with TEC's tensioning jig; wear gloves when handling the band |
TEC's acceptance figures for the tester's automatic test (section 3.8) include a motor speed of 200 ± 6 ms per revolution, an index pulse 1 to 5.7 ms wide, a window margin over 750 ns, a step rate of 6 ms on 48 TPI drives and asymmetry under 700 ns.[6] See also Floppy Drive Repair.
Keyboard
[edit | edit source]The 4D keyboard (Tandy 8790555) adds a Backspace key that does the same as the left arrow.[1] The trs-80.com notes mention gate array Model 4s fitted with ALPS membrane keyboards, and link to a separate page on cleaning them.[2]
Memory
[edit | edit source]The 4D was sold with 64 KB. Radio Shack's expansion kit (catalogue 26-1122) took it to 128 KB, and the technician fitting it replaced the 64K label on the case with a 128K one.[7] Radio Shack's M4MEM diagnostic tests the RAM; see the Model 4 guide.
Power supply and monitor
[edit | edit source]Neither is changed for the 4D.[1] For supply identification, output checks and the monitor adjustments, see the TRS-80 Model 4 General Maintenance guide; the Tandy 65 W supply is covered in the TRS-80 Model 4P General Maintenance guide.
Related pages
[edit | edit source]- TRS-80 Model 4D
- TRS-80 Model 4D Troubleshooting Guide
- TRS-80 Model 4D Capacitor Replacement Guide
- TRS-80 Model 4 General Maintenance
- Tandy TRS-80 Model 4D Service Manual Addendum
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 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 III/4" and "Model 4" sections.
- ↑ A&A Japan Ltd., Whole Parts List for catalogue no. 8790133 (manufacturer TEC), 5 August 1985, in the Addendum ... for Model 4D, PDF pages 111-123.
- ↑ Tandy Corporation, Introduction to Your Disk System: Tandy TRS-80 Model 4D, 1985, archive.org scan. Chapter 5, "Drive Maintenance", pp. 23-24.
- ↑ 5.0 5.1 TEC, FB-500 Series Service Manual, document E0-2129-G, December 1984, in the Addendum ... for Model 4D. Chapter 1, 4.1 "Procedure for Cleaning the Read/Write Head".
- ↑ 6.0 6.1 TEC, FB-500 Series Service Manual (E0-2129-G, 1984). Chapter 1, 3 "Adjustment", 3.1-3.9.
- ↑ Matthew Reed, "The TRS-80 Model 4D", TRS-80.org (retrieved 2026-10-21), including the author's reply to a reader's comment.