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Tandy 1000A General Maintenance

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Tandy 1000A main logic board component layout (Service Supplement p. 23)
The 1000A main logic board. The custom chips sit at U9, U25, U50 and U64; the 8088 is at U28 with the 8087 socket U29 beside it.

Routine care of the Tandy 1000A follows the original Tandy 1000 for the case, the 54 W power supply and the keyboard, because the 1000A uses the same chassis parts and the same supply. The 1000A main board, its sockets and its other differences are described below. Tandy's 1985 supplement for the 1000A has parts lists, a component layout and schematics, but no disassembly, adjustment or test procedures.[1]

[2]

Opening the main unit

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The supplement lists the same main chassis, drive support, option board panel and front and rear reset buttons, by Tandy part number, as the original 1000, and the same 54 W supply, part 8790070.[3][4] Follow Tandy's order of work on Tandy 1000 General Maintenance, and remove the supply as described under Removing the power supply.

Main board

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Socketed parts

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The parts list gives these sockets on the 1000A board.[5] Reseat them while the board is out, and clean the edge connectors of any option cards with isopropyl alcohol.

Socketed parts on the 1000A board (parts list pp. 16-20)
Socket Positions Parts fitted
68-pin PLCC U64 Custom address array
40-pin U9, U25, U28, U29, U43, U50, U66 Keyboard interface array, timing generator array, 8088, 8087 socket, 8272 floppy controller, video array, printer gate array
28-pin U17, U51 8259A interrupt controller; ROM
24-pin U8 8253-5 timer
20-pin U40, U52, U53, U73 74LS244 and 74LS245 bus buffers
16-pin U1-U4, U10, U11, U18-U21, U32-U35, U45, U70 64K x 1 DRAMs; SN76496 sound chip at U70
8-pin U57 FDC9216 data separator

The DRAM list has sixteen positions, U1-U4, U10, U11, U18-U21, U32-U35, U44 and U45, but U44 is missing from the socket list, so check before trying to lift it. Tandy notes that HCT parts are not an approved substitute for the 74LS373 latches at U54 and U55.[5]

Additions in the parts list

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  • A 10 kΩ resistor added near R33 "to eliminate noise from the light pen circuit".[5]
  • RP3, RP4 and RP12 are 33-ohm 8-pin resistor packs. An owner reported in 1986 that early 1000A boards had a 68-ohm pack at RP12, which blurred the picture on a CM-4 colour monitor; he described the part as an 8-pin single in-line pack under the power supply. The supplement's layout puts RP12 next to U69.[5][6][7]

Memory and expansion

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The 128 KB on the main board is the sixteen 64K x 1 DRAMs, 150 ns.[5] Further memory comes on an expansion card, which also carries the DMA chip that the 1000A board lacks. The memory has to be expanded before a hard disk can be fitted, and a hard disk controller must use IRQ 2 because the slots use IRQ 5 for vertical sync.[8]

Coprocessor

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Socket U29 takes an Intel 8087, which Tandy sold as option 25-1012.[9] The FAQ advises making sure that a second-hand 8087 works, since collectors buy them whether they work or not.[10]

Floppy drives

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The 1000A takes standard 360 KB 5.25-inch drives, though a new drive may need a new cable.[11] Advice given to 1000A owners on CompuServe:[12]

  • The exact replacement drive is a TEAC FD-54B ("54-B"); a TEAC 53-B also works, while a 55-B needs the ribbon cable split between its wires before it reaches the connector.
  • Any IBM-standard 40-track double-sided 5.25-inch drive should work if its connector is on the right-hand side seen from the front and the cable has enough slack. Otherwise a different cable is needed.
  • A 720 KB 3.5-inch drive uses the same interface and fits a 5.25-inch bay in a mounting frame. MS-DOS 3.2 and later support it without a driver; to format 80 tracks double-sided on drive B:, add DRIVPARM=/D:1 to CONFIG.SYS. Earlier versions of DOS need a device driver. Use a 720 KB drive, not a 1.44 MB one.

For cleaning and alignment, see Floppy Drive Repair.

Sound

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The supplement says the 1000A added a volume control for service adjustment, but it does not name the part.[1] The parts list has one variable resistor, R1 (1 kΩ), and the layout puts it beside U7, the LM386 audio amplifier.[5][7] Software can also switch the speaker off: a 1 in bit 4 of I/O location 0061 disables it.[13] If the machine is silent, check software before the hardware; see Troubleshooting.

Power supply

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The supply is the original 1000's 54 W unit, so use the operating limits, the +5 V adjustment and the connector checks on Tandy 1000 General Maintenance.[3] Capacitor values are on Tandy 1000 Capacitor Replacement Guide.

BIOS and clock

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The FAQ gives 01.01.00 as the latest BIOS for the 1000A and the one it shipped with.[14] An owner who fitted a Western Digital WD1002-27X controller and a 30 MB drive wrote that the 1000A needs BIOS 01.01.00 and the controller needs WD's "Super BIOS" ROM.[15]

The main board has no clock. If there is a chip underneath the ROM, it is a Smartwatch clock with its own lithium battery, not a BIOS upgrade; when the battery is dead, the FAQ's advice is to replace the Smartwatch.[14]

Maintenance checklist

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Tandy 1000A maintenance checklist
Task Where
Measure the supply outputs and set +5 V Tandy 1000 General Maintenance#Power supply checks
Reseat the socketed chips and clean card edge connectors #Socketed parts
Check RP12 is a 33-ohm pack on an early board #Additions in the parts list
Clean and test the floppy drives #Floppy drives, Floppy Drive Repair
Check the Smartwatch battery, if one is fitted #BIOS and clock
Inspect the electrolytic capacitors Tandy 1000A Capacitor Replacement Guide
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References

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  1. ↑ 1.0 1.1 Tandy Corporation, Supplement to the Tandy 1000 Service Manual (Cat. No. 25-1000), covering the Tandy 1000A, 1985, hosted as File:Tandy 1000A Service Manual Supplement (1985).pdf. Contents and Section 1, "Introduction", p. 1.
  2. ↑ Radio Shack, Service Manual: Tandy 1000 Computer, catalogue no. 25-1000, 1985, hosted as File:Tandy 1000 Service Manual (25-1000, 1985).pdf. Section 7, "AC Input Circuit", p. 77.
  3. ↑ 3.0 3.1 1000A Service Supplement, Section 3, "Parts List", Main Logic Main Assembly, p. 15.
  4. ↑ Tandy 1000 Service Manual (25-1000), Main Logic Parts List (continued), p. 99.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 1000A Service Supplement, Section 3, "Parts List", Main Logic Sub Assembly, pp. 16-20.
  6. ↑ J. R. Applegate and others, "Tandy 1000 model A Bug Report", February to September 1986, TVDog's Tandy 1000 archive (retrieved 2026-10-22). Entry by Ed Nunez, 14 September 1986.
  7. ↑ 7.0 7.1 1000A Service Supplement, Section 4, "Component Layout", p. 23.
  8. ↑ Jeff Hayes, "Tandy 1000-series FAQ", version 2.52, 25 October 2005, TVDog's Tandy 1000 archive (retrieved 2026-10-22). Sections II.A.1, II.D.1 and II.I.3.
  9. ↑ 1000A Service Supplement, Section 3, "Optional Parts", p. 21.
  10. ↑ Hayes, "Tandy 1000-series FAQ" (2005), section II.F.2.
  11. ↑ Hayes, "Tandy 1000-series FAQ" (2005), section II.C.5.
  12. ↑ Wayne Day and others, CompuServe messages on adding floppy drives to the 1000A, 1ka3-5.txt, TVDog's Tandy 1000 archive (retrieved 2026-10-22).
  13. ↑ 1000A Service Supplement, Section 2, "Custom Keyboard I/F Array", p. 13.
  14. ↑ 14.0 14.1 Hayes, "Tandy 1000-series FAQ" (2005), section III.E.2.
  15. ↑ "Notes on installing a 30M hard drive in a 1000A", t1kahd.txt, TVDog's Tandy 1000 archive (retrieved 2026-10-22).


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