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Tandy 1000 EX General Maintenance

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Tandy 1000 HX and EX side by side (Blake Patterson)
A Tandy 1000 HX (left) and EX (right). The EX's 5.25-inch drive is in the right side of the case (photo: Blake Patterson, CC BY 2.0).

Routine care of the Tandy 1000 EX covers the 28 W power supply, the built-in keyboard, the floppy drives, memory expansion and the video cable. Tandy's Tandy 1000 EX Technical Reference Manual (catalogue no. 25-1510) is the source for the figures here. It gives full service information for both versions of the supply but no disassembly procedure for the case.[1]

[2][3]

Opening the case

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The EX manual has no disassembly procedure. Owners describe the EX as awkward to take apart; one notes that the main board is held by metal clips.[3][4] Note where each cable goes before unplugging it. The keyboard joins the main board through two flat flexible cables in zero-insertion-force connectors, J3 (12 pins) and J7 (13 pins); open the connector latch before pulling a cable out, and seat the cable fully before closing it.[5]

Mains voltage

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Read the rating label before connecting the machine. The single-input supply 8790083 runs from 90-135 V AC only. The dual-input supply 8790086 accepts 90-135 V or 198-264 V, set by a voltage selector jumper on the supply board.[2] One owner bought an EX in Europe that carried multilingual warning labels, connected it to 230 V, and then found the supply board marked 120 V AC; the fuse had blown.[3]

Power supply checks

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The supply connects to the main board at J6:[5]

J6, DC power (6-pin header)
Pins Supply
1, 2 +5 V
3, 4 Ground
5 +12 V
6 -12 V

Tandy's operating characteristics, the same for both supplies:[2]

28 W supply operating characteristics
Output Minimum Typical Maximum Load range Current limit Noise (max)
+5 V 4.85 V 5.00 V 5.15 V 1.25-3.0 A 6.0 A 50 mV p-p
+12 V 11.40 V 12.00 V 12.60 V 0.1-1.25 A 2.5 A 100 mV p-p
-12 V -13.20 V -12.00 V -10.80 V 0-0.1 A 1.0 A 150 mV p-p

If +5 V rises to between 5.8 V and 6.8 V, the thyristor SCR1 shorts the +12 V line and the supply's current limit shuts it down; it restarts once the fault is cleared and power is reapplied.[2] If the supply is out of the machine, load it before switching on: Tandy warns that it must never run without a load, and gives a minimum test load of 1.25 A on +5 V (4 ohms, 20 W) and 0.1 A on +12 V (120 ohms, 3 W).[2] The full test procedure is on Tandy 1000 EX Troubleshooting Guide.

Adjustment

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The +5 V output is set by the 2 kilohm trimmer VR1 in the sensing circuit with the TL431 (IC2). The schematic shows no separate regulator on the +12 V output.[6] Check the connections and capacitors before turning VR1.

Fan

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The main board supplies +12 V to the fan at connector J5.[5]

Keyboard

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The keyboard is built into the case and cannot be swapped for a separate one; the FAQ suggests taking a keyboard from another EX or HX if one fails.[4] It is a plain switch matrix scanned by an 8048 (U6) on the main board. Tandy's keyboard specification allows a contact resistance of up to 500 ohms per key, so a key that reads higher than that when pressed needs cleaning or replacement.[7]

Floppy drives

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  • The internal 5.25-inch drive connects to J11, which has a single drive-select line.[5] The FAQ lists the EX among the models that use standard 360 KB drives.[8]
  • The external drive port J14 carries +5 V and +12 V as well as the drive signals, so the EX powers an external drive itself.[5] Tandy sold a 360 KB external drive (catalogue number 25-1060) and a 720 KB one (25-1061).[8]

General cleaning is covered on Floppy Drive Repair.

Memory and expansion

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  • The EX has 256 KB on the main board. Tandy's Memory PLUS expansion adapter adds a DMA controller and 128 KB, or 384 KB with eight more 256K DRAMs. Fit jumper E1-E2 for 128 KB, or E2-E3 when the extra 256 KB is fitted.[9]
  • Expansion cards use Tandy's PLUS connector, not a standard slot. The FAQ describes an adapter for using standard cards and notes that a hard disk controller also needs the DMA on the memory card.[10]
  • The 8088 can be replaced by a NEC V20; the EX has no maths coprocessor socket.[11]

Video cable

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Pin 7 of the RGBI connector J16 carries green for monochrome monitors, where a CGA connector leaves it unused.[5] One owner found that a straight-through cable to a Commodore 1084 monitor grounded that pin and lost the green signal; leave pin 7 unconnected in an RGBI cable.[12]

BIOS

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The FAQ lists 01.02.00 as the only BIOS for the EX.[13]

Maintenance checklist

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  1. Check the rating label and the supply type before connecting the mains.
  2. Unplug the machine and wait before opening the case.
  3. Remove dust from the supply, fan and drive.
  4. Inspect the supply board for bulging or leaking electrolytics.
  5. Reseat the keyboard flat cables in J3 and J7 and any PLUS cards.
  6. Clean the floppy heads.
  7. Run the machine and measure +5 V, +12 V and -12 V at J6 against the table above.
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References

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  1. ↑ Tandy Corporation, Tandy 1000 EX Technical Reference Manual, catalogue no. 25-1510, 1986, hosted as File:Tandy 1000 EX Technical Reference Manual (25-1510, 1986).pdf.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Tandy 1000 EX Technical Reference Manual (25-1510), "1000 EX 28 Watt Single Input Power Supply" (PDF pp. 89-107) and "1000 EX 28 Watt Dual Input Power Supply" (PDF pp. 108-126).
  3. ↑ 3.0 3.1 3.2 "Hooked a 120V Tandy 1000 EX up to 230V, fuse blew out. Further damage?", Vintage Computer Federation forum, May 2021, thread. Posts by digger (5 May 2021) and kc8eyt (6 May 2021).
  4. ↑ 4.0 4.1 Jeff Hayes, "Tandy 1000-series FAQ", version 2.52, 25 October 2005, TVDog's Tandy 1000 archive (retrieved 2026-10-21). Section II.E.1.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Tandy 1000 EX Technical Reference Manual (25-1510), Main Logic Board connector pin assignments (PDF pp. 12-15) and parts list (PDF pp. 71-74).
  6. ↑ Tandy 1000 EX Technical Reference Manual (25-1510), schematic 3P-M1-0163 of supply 8790083, hosted as File:Tandy 1000 EX 28 W power supply 8790083 schematic (Technical Reference Manual).png.
  7. ↑ Tandy 1000 EX Technical Reference Manual (25-1510), Keyboard section: specification, project 620 non-encoded keyboard matrix, 2-1 contact resistance 500 ohm max at 5 V DC 1 mA, PDF pp. 127-135; Main Logic Board theory of operation, keyboard interface.
  8. ↑ 8.0 8.1 Hayes, "Tandy 1000-series FAQ" (2005), sections II.C.4 and II.C.5.
  9. ↑ Tandy 1000 EX Technical Reference Manual (25-1510), "Memory PLUS Expansion Adapter", jumper settings and parts list, PDF pp. 136-145.
  10. ↑ Hayes, "Tandy 1000-series FAQ" (2005), sections II.D.1 and II.I.1.
  11. ↑ Hayes, "Tandy 1000-series FAQ" (2005), sections II.F.1 and II.F.2.
  12. ↑ "Dim composite video on Tandy 1000 HX", Vintage Computer Federation forum, October 2019, thread. Post by Eudimorphodon, 9 October 2019.
  13. ↑ Hayes, "Tandy 1000-series FAQ" (2005), section III.E.2, BIOS version table.


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