Tandy 1000 SX
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| A Tandy 1000 SX with two 5.25-inch drives, shown with an IBM monitor and a Northgate OmniKey keyboard (photo: Ben Franske, CC BY-SA 4.0) | |
| Specifications | |
|---|---|
| Manufacturer | Tandy Corporation, sold by Radio Shack |
| Type | Desktop personal computer (IBM PC compatible) |
| CPU | Intel 8088-2, 7.16 MHz or 4.77 MHz |
| Memory | 384 KB, expandable to 640 KB on the main board |
| Storage | One or two 5.25-inch 360 KB floppy drives |
| Display | Built-in colour video with RGBI and composite outputs |
| Sound | Texas Instruments SN76496, three voices |
| Power | Internal 67 W switching supply |
| Dimensions | 16.75 x 13.5 x 5.75 in (425 x 343 x 146 mm) |
| Weight | 18 lb (8.2 kg) with two floppy drives |
| OS / Firmware | MS-DOS 3.2 and DeskMate II |
| Model no. | 25-1051; one-drive versions 25-1052, 25-1054 |
The Tandy 1000 SX is an IBM PC compatible desktop computer in the Tandy 1000 family. It keeps the original 1000's desktop layout but adds a DMA controller on the main board, five 8-bit expansion slots, an 8088-2 processor that runs at 7.16 MHz or 4.77 MHz, and 384 KB of memory.[1][2] The Tandy 1000 TX that followed uses a similar case with an 80286 processor.[1]
History
[edit | edit source]The TVDog model list gives catalogue number 25-1051 and dates the SX to 1987, although Tandy's technical reference manual carries a 1986 copyright.[2][3] Tandy's one-drive instruction sheet covers catalogue numbers 25-1052 and 25-1054.[4] A version sold through Walmart was badged the Tandy 1000 AX.[1]
Specifications
[edit | edit source]| Item | Detail |
|---|---|
| CPU | 8088-2 (U36), 7.16 MHz by default, switchable to 4.77 MHz; socket for an 8087 at U33 |
| Clocks | 28.63636 MHz oscillator (Y1) and 16 MHz oscillator (Y2) |
| RAM | 256 KB system memory in eight 256K DRAMs (U1-U4, U9-U12) plus 128 KB video and system memory in four 64K x 4 DRAMs (U24, U31, U34, U35); eight sockets for 256 KB more |
| Custom chips | CPU support and timing "Light Blue" (U51), video and memory "Big Blue" (U30, 84-pin PLCC), DMA (U29, 68-pin PLCC), keyboard interface array (U22), parallel port array (U55) |
| Floppy controller | µPD765A (U37) with FDC9216 data separator (U38) |
| ROM | 128K ROM at U41 |
| Sound | SN76496 (U37A) |
| Configuration | Four-position DIP switch S2 |
| Power | 67 W switching supply: 8790085 (120 V) or 8790084 (dual input); 120 V AC 60 Hz, 3 A maximum |
| Size and weight | 16 3/4 x 13 1/2 x 5 3/4 in; 18 lb with two drives |
Sources: Tandy's technical reference manual.[2][5][6]
Hardware
[edit | edit source]Main logic board
[edit | edit source]The main logic board (subassembly 8859003A, PCB 8709699A revision A) carries the 8088-2, the custom chips and memory. The "Light Blue" CPU support chip takes the 16 MHz and 28.6 MHz clocks, generates every other clock on the board, produces the system reset and derives the CPU control signals and the floppy controller timing.[6][5] To fill the board to 640 KB, remove jumper E1-E2 and add eight 256K DRAMs in Bank 1; the FAQ specifies 150 ns chips.[6][7]
Interrupts 5, 6 and 7 are wired through the four-position DIP switch S2 to the vertical sync, floppy controller and printer functions on the board. To give one of them to an expansion card, turn its switch off.[8] The Tandy 1000 FAQ gives switch 1 off for a VGA card, and switch 2 off so that a standard hard disk controller can use IRQ 5; as shipped, the SX expects a hard disk controller on IRQ 2.[9]
Power supply
[edit | edit source]Tandy fitted two 67 W supplies made by Tamura Seisakusho: 8790085 for 90-135 V input, and 8790084, which takes 90-135 V or 198-264 V set by a voltage selector jumper. Both are ringing choke converters. The +5 V output is held steady by feedback through photocoupler PHC1, and a crowbar stops the converter if +5 V rises to between 5.8 V and 6.8 V. The supply also runs the 80 mm 12 V cooling fan from its fan-out connector SK2.[10][11]
Known faults
[edit | edit source]- Failed electrolytics in the 67 W supply. One owner found the -12 V capacitor C23 had vented.[12] See Tandy 1000 SX Capacitor Replacement Guide.
- A failing "Light Blue" CPU support chip (U51) lets the machine count memory and then stop with a single beep. The owner who traced it found no easy replacement for the custom chip and fixed the machine with a second-hand main board; another owner replaced U51 with a spare chip.[13][14] See Tandy 1000 SX Troubleshooting Guide.
- A failed supply can take main board chips with it. After one supply lost regulation, an owner had to replace DRAMs, U51 and the Big Blue chip U30; a Big Blue marked 2684-0001 worked in place of a 2684-0002.[13]
Documentation
[edit | edit source]- Tandy 1000 SX Technical Reference Manual (25-1511, 1986): theory of operation, main board parts list and schematic, and the full service information for both 67 W supplies. File:Tandy 1000 SX Technical Reference Manual (25-1511, 1986).pdf
Related pages
[edit | edit source]- Tandy 1000 SX General Maintenance
- Tandy 1000 SX Troubleshooting Guide
- Tandy 1000 SX Capacitor Replacement Guide
- Tandy 1000
- Tandy 1000 TX
- Floppy Drive Repair
References
[edit | edit source]- ↑ 1.0 1.1 1.2 "Tandy 1000", Wikipedia (retrieved 2026-10-21). Sections "Tandy 1000 SX and TX" and "1000 SX": DMA controller, five ISA slots, 7.16 MHz 8088-2, 384 KB, light pen port, volume control, the Tandy AX, MS-DOS 3.2 and DeskMate II, first model whose built-in video can be disabled by an add-on card.
- ↑ 2.0 2.1 2.2 Tandy Corporation, Tandy 1000 SX Technical Reference Manual, catalogue no. 25-1511, 1986, hosted as File:Tandy 1000 SX Technical Reference Manual (25-1511, 1986).pdf. Main Logic Board section: Introduction p. 1 (384K standard, 640K with eight more 256K chips, 67 W supply), Specifications p. 3.
- ↑ Jeff Hayes, "models.txt", TVDog's Tandy 1000 archive, 2005 (retrieved 2026-10-21).
- ↑ Tandy Corporation, Tandy 1000 SX One-Drive System Instructions, Cat. No. 25-1052 and 25-1054, archive.org scan.
- ↑ 5.0 5.1 Tandy 1000 SX Technical Reference Manual (25-1511), Main Logic T1000 SX Subassembly 8859003A parts list, pp. 49-53.
- ↑ 6.0 6.1 6.2 Tandy 1000 SX Technical Reference Manual (25-1511), Theory of Operation: 8087 option (jumper E3-E4, socket U33), memory expansion (jumper E1-E2, Bank 1), CPU clock rates, Light Blue CPU control and timing chip, video interface.
- ↑ Jeff Hayes, "Tandy 1000-series FAQ", version 2.52, 25 October 2005, TVDog's Tandy 1000 archive (retrieved 2026-10-21). Section II.A.1.
- ↑ Tandy 1000 SX Technical Reference Manual (25-1511), Theory of Operation, interrupt assignments (p. 17).
- ↑ Hayes, "Tandy 1000-series FAQ" (2005), sections II.B.1 (VGA: switch 1 off on the SX and TX) and II.D.1 (hard drive controllers: switch 2 off for IRQ 5; factory setting IRQ 2).
- ↑ Tandy 1000 SX Technical Reference Manual (25-1511), "1000 SX 67 Watt Single Input Power Supply" and "67 Watt Dual Input Power Supply" sections: operating characteristics, theory of operation, troubleshooting, parts lists and schematics.
- ↑ Tandy 1000 SX Technical Reference Manual (25-1511), Power Supply Assembly list: fan 80 mm 12 V DC, voltage select switch assembly.
- ↑ "Wanted: Someone to repair Tandy 1000SX power supply", Vintage Computer Federation forum, 2018-2019, thread. Posts by itsvince725 (dead supply, fuse intact, 18-19 September 2018) and seat safety switch (vented C23, 1 June 2019).
- ↑ 13.0 13.1 "Tandy 1000SX "Big Blue" (U30) interchange", Vintage Computer Federation forum, 2023, thread. Posts by seat safety switch, 27 May to 24 August 2023.
- ↑ "Tandy 1000sx quit working.", Vintage Computer Federation forum, 2011-2023, thread. Posts by dabone (12 and 23 April, 3 May and 24 June 2011) and JaketheTech (14 January 2023).