TRS-80 Model 4D
.png) | |
| Tandy's drawing of the Model 4D from its 1985 introduction manual. No freely licensed photograph of the 4D was found. | |
| Specifications | |
|---|---|
| Manufacturer | Tandy Corporation, sold by Radio Shack |
| Type | Desktop computer |
| Released | Late 1985 |
| Intro price | US$1,199 |
| CPU | Zilog Z80A, 2 or 4 MHz |
| Memory | 64 KB, expandable to 128 KB |
| Storage | Two internal double-sided 5.25-inch floppy drives (TEC) |
| Display | Built-in green monochrome monitor |
| Sound | — |
| OS / Firmware | TRSDOS 6.2.1, with DeskMate |
| Predecessor | TRS-80 Model 4 |
| Model no. | 26-1070 |
The Tandy TRS-80 Model 4D (catalogue 26-1070) was Radio Shack's last machine in the TRS-80 line that began with the Model I in 1977. It was introduced in late 1985 at $1,199 to replace the TRS-80 Model 4, carried a Tandy nameplate where earlier models carried Radio Shack's, came only with two disk drives, and does not appear to have been sold outside the United States and Canada.[1] Electrically it is a gate array Model 4: Tandy's service addendum for the 4D says the two machines are the same apart from the drive mount, the drives and their cable, and the keyboard.[2]
History
[edit | edit source]Radio Shack had been rumoured to be about to drop the Model 4 line, and Ron Stegall, a Tandy senior vice-president, told 80 Micro that the 4D was introduced partly to stop them. The 4D was bundled with DeskMate, Radio Shack's integrated word processor, spreadsheet, database, communications, calendar and mail package; according to Reed, the D stood for DeskMate, though many people took it to mean the double-sided drives. It shipped with TRSDOS 6.2.1. Early announcements promised 128 KB, but the 4D was sold with 64 KB; a Radio Shack kit (catalogue 26-1122) took it to 128 KB, and the technician who fitted it replaced the memory label. The 4D stayed in Radio Shack catalogues until 1990.[1]
Differences from the Model 4
[edit | edit source]| Area | Model 4D | Source |
|---|---|---|
| Drives | Two double-sided drives (TEC, Tandy assembly 8790133) on a new mount and cable | [3][1] |
| Keyboard | Tandy 8790555, with a Backspace key that does the same as the left arrow | [3] |
| Main board | Gate array Model 4 board, with C2 47 pF and C20 33 pF in place of the Model 4 values | [3] |
| Screen | Green phosphor | [1] |
| RS-232 connector | Faces out of the back of the case | [1] |
| Nameplate | Tandy instead of Radio Shack | [1] |
Reed notes that the keyboard and green screen had already appeared on later Model 4s, that is the gate array version (26-1069A).[1][4]
Hardware
[edit | edit source]Main board
[edit | edit source]The main board is the gate array board of the later Model 4, which carries the floppy controller and RS-232 circuits that the original Model 4 had on separate boards.[4] Tandy's manual describing it is the TRS-80 Model 4/4P Technical Reference Manual (26-2119); Howard W. Sams' Computerfacts CS13 has schematics and a troubleshooting guide for the same board.[5][6] The two capacitors the 4D addendum changes, C2 and C20, are the load capacitors on the 20.2752 MHz master clock crystal in the Sams schematic, where they are 33 pF and 22 pF.[3][6]
Disk drives
[edit | edit source]The drives are TEC FB-500 series units, which the archive.org description of the addendum identifies as FB-503s: TEC's double-sided, double-density, 48 tracks-per-inch model, with the spindle at 300 rpm. TEC's service manual for the series makes up most of the 4D addendum, and takes the place of the drive section of the Model 4 service manual.[7]
Power supply and display
[edit | edit source]The addendum lists no changes to the power supply or monitor, so these are as in the gate array Model 4.[2] Sams lists Tandy's 65 W supply, the design used in the TRS-80 Model 4P, as the gate array Model 4's supply, with the two Astec supplies of the original Model 4 as alternatives.[6] See Model 4 power supplies.
Known faults
[edit | edit source]- The power supplies share the Model 4's mains filter capacitor failures; see the TRS-80 Model 4 Capacitor Replacement Guide.
- The trs-80.com repair notes describe a 4D that showed every typed letter twice, with others missing; they put this down to the main board: one or two of the gate arrays, the CRT controller or the video RAM.[8]
- Replacement green tubes are listed in the same notes under the code 12VCMP31.[8]
Documentation
[edit | edit source]- Tandy TRS-80 Model 4D Service Manual Addendum: Tandy's 4D parts differences and TEC's drive service manual
- Radio Shack TRS-80 Model 4 Technical Reference Manual: the 26-2110 and 26-2119 manuals, including gate array board theory
Related pages
[edit | edit source]- TRS-80 Model 4D General Maintenance
- TRS-80 Model 4D Troubleshooting Guide
- TRS-80 Model 4D Capacitor Replacement Guide
- TRS-80 Model 4
- TRS-80 Model 4P
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Matthew Reed, "The TRS-80 Model 4D", TRS-80.org (retrieved 2026-10-21).
- ↑ 2.0 2.1 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, PDF page 5.
- ↑ 3.0 3.1 3.2 3.3 Addendum ... for Model 4D (1985), "Exploded View Parts List Differences" and "Model 4 Gate Array PC Board Parts List Differences", PDF page 6.
- ↑ 4.0 4.1 "TRS-80 Model 4", Wikipedia (retrieved 2026-10-21). Section "Gate Array Model 4".
- ↑ Tandy Corporation, TRS-80 Model 4/4P Technical Reference Manual, catalogue no. 26-2119, hardware part, 1985, hosted as File:Radio Shack TRS-80 Model 4 and 4P Technical Reference Manual, hardware (26-2119, 1985).pdf. Section II, "Model 4 Gate Array Theory of Operation".
- ↑ 6.0 6.1 6.2 Howard W. Sams & Co., Computerfacts CS13, Radio Shack TRS-80 Model 4 Gate Array, 1986, archive.org scan.
- ↑ TEC (Tokyo Electric Co., Ltd.), FB-500 Series Service Manual, document E0-2129-G, December 1984, in the Addendum ... for Model 4D. Introduction; "Reference Document: Specification", chapter 1. Drive model from the description of the archive.org item.
- ↑ 8.0 8.1 Ira Goldklang, "TRS-80 Troubleshooting Guide and Repair Q&A", Ira Goldklang's TRS-80 Revived Site (retrieved 2026-10-21). "Model 4" section.