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Tandy 1000 TX Troubleshooting Guide

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Tandy 1000 SX 67 W power supply 8790085 schematic (Technical Reference Manual)
Schematic of the 67 W supply 8790085 from the SX manual. The TX manual describes supplies with the same characteristics and circuit references but does not reproduce a schematic.

Fault finding on the Tandy 1000 TX relies on Tandy's Tandy 1000 TX Technical Reference Manual (catalogue no. 25-1514), which has the theory of operation and full main board schematics but no troubleshooting procedure, and on reports from owners.[1] For the 67 W supply, the procedure in the Tandy 1000 SX manual applies to supplies with the same description.

[2]

Quick reference

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Symptom Check first Section
Completely dead Supply outputs with the main board disconnected; fuse Power supply
Fan or drive runs but no start-up, or start-up only after pressing reset Reset capacitors C135 and C171A/C177A, CPU control chip U52 Will not start
Memory count wrong Jumper E9-E10 and the DRAMs in U54-U57 Memory
720 KB drive formats at 360 KB DOS version and DRIVPARM setting Floppy drives
Keyboard types only full stops Keyboard type; try a Tandy 1000 keyboard Keyboard
No composite picture, RGBI works VR1 (78L05), C258, +12 V Video and sound
Hard disk controller or VGA card not working DIP switch S2 settings Expansion cards

Power supply

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The TX manual gives the same operating characteristics, block diagram and theory of operation for its 67 W supplies as the SX manual, including the same reference numbers for the switching transistor Q1, the crowbar parts D11 and PHC2 and the feedback photocoupler PHC1. It has no test procedure, parts list or schematic for them.[3]

  1. Unplug the supply cable from J8 on the main board, put a test load on the supply and measure the outputs. Tandy's SX manual warns that the supply must never be run without a load, and gives a minimum load of 1.25 A on +5 V (4 ohms, 20 W) and 0.15 A on +12 V (80 ohms, 5 W).[2]
  2. Compare the readings with the limits in General Maintenance.
  3. If there is no output, follow Tandy's No Output sequence on Tandy 1000 SX Troubleshooting Guide: fuse, bridge rectifier and transistors, shorted secondary parts, the crowbar, then the Q1 waveforms. Before using the SX part references, check that the part number on your supply is 8790085 or 8790084.
  4. If the supply is good under load but the machine is dead, look for a short on the main board. On the main board side of J8, the 22 µF filter capacitors C100, C202, C205 and C208 sit across the supply rails.[4]

Will not start

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The "Night Blue" CPU control chip (U52) produces the system reset. At power-on, R24 and C135 hold the reset line low until C135 charges. The reset is then generated a second time: R47A and C171A hold the input of flip-flop U4 high for about 150 ms, and when the 80286 starts, U4 and U67 discharge C135 again. The reset button also works by discharging C135.[5] On the revision C schematic, C135 and the capacitor on R47A (drawn there as C177A) are both 10 µF electrolytics.[6] If the machine starts only after pressing reset, or not at all with a good supply, check these two capacitors and the clock outputs of U52 before suspecting the custom chips.

Memory

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As shipped, jumper E9-E10 is fitted and the board uses 512 KB of system memory. With four 64K x 4 DRAMs fitted in U54-U57, E9-E10 must come out to use them.[7][8] A start-up count of 768 KB with MEM reporting 640 KB is normal: the extra 128 KB is used by the video circuits.[9][10] The system DRAMs are U25-U32 and U40-U47 (256K) and U54-U57 (64K x 4); the video DRAMs are U18, U33, U35 and U48 (64K x 4).[11]

Floppy drives

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  • Tandy warned that the TX and earlier models can treat a 720 KB drive as 360 KB under DOS versions after 3.2. The FAQ's fix is DRIVPARM=/D:0 /F:2 /H:2 /S:9 /T:80 in CONFIG.SYS for drive A:.[12] One owner used the SETBPB utility in AUTOEXEC.BAT to the same end.[9]
  • The 3.5-inch drive takes its power through the data cable. A standard drive connected to the unmodified cable can damage the drive and the computer.[13]
  • One owner's 5.25-inch drive worked only intermittently, and several known good drives behaved the same way in that machine. Recapping the drive did not help. He suspected an interaction with his XT-IDE card but removed the 5.25-inch drive without finding the cause.[14]

Keyboard

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A keyboard that types only full stops has been reported twice on the TX. One owner's original keyboard did it after the machine had been stored for over 20 years; cleaning did not cure it, and the thread has no reply.[15] Another found that a Tandy 2000 keyboard worked for a while and then produced only full stops on both an SX and a TX, and that a Tandy 1000 keyboard worked on both.[16] Try a known good Tandy 1000 keyboard before working on the main board.

Video and sound

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  • The composite video output has its own 78L05 regulator (VR1) fed from +12 V, with a 10 µF electrolytic (C258) on its output. If composite video is missing while RGBI works, check VR1, C258 and the +12 V supply.[17]
  • The internal speaker is driven by an LM386 (U2) through the 100 µF capacitor C105; the headphone and audio output goes through an LM358 (U76) and the 100 µF capacitor C259.[18]
  • The -5 V rail on the expansion bus comes from VR2, a 79M05 fed from -12 V.[4]

Expansion cards

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  • The FAQ gives DIP switch 1 off for a VGA card.[19]
  • As shipped, the TX expects a hard disk controller on IRQ 2. A standard XT controller uses IRQ 5, which needs DIP switch 2 off.[19]
  • Full-length cards do not fit.[20]
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References

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  1. ↑ Tandy Corporation, Tandy 1000 TX Technical Reference Manual, catalogue no. 25-1514, 1987, hosted as File:Tandy 1000 TX Technical Reference Manual (25-1514, 1987).pdf.
  2. ↑ 2.0 2.1 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. 67 W supply troubleshooting: test set-up, Table 1, No Output procedure, Table 2.
  3. ↑ Tandy 1000 TX Technical Reference Manual (25-1514), "1000 TX 67 Watt Single Input Power Supply" and "1000 TX 67 Watt Dual Input Power Supply", PDF pp. 407-416.
  4. ↑ 4.0 4.1 Tandy 1000 TX Technical Reference Manual (25-1514), main logic schematic D-8000288, revision C, sheet 12, PDF p. 99.
  5. ↑ Tandy 1000 TX Technical Reference Manual (25-1514), Theory of Operation, "Reset Circuit", PDF p. 21.
  6. ↑ Tandy 1000 TX Technical Reference Manual (25-1514), main logic schematic revision C, sheet 3 "Control and Clocks", PDF p. 90.
  7. ↑ Tandy 1000 TX Technical Reference Manual (25-1514), Switch Settings and Jumper Pin Configurations, Table 1, PDF p. 12.
  8. ↑ 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.
  9. ↑ 9.0 9.1 "Original Tandy 1000 TX upgrades?", Vintage Computer Federation forum, March-April 2025, thread. Posts by VERAULT, Quagmire and Eudimorphodon, 19 March 2025.
  10. ↑ "Tandy 1000", Wikipedia (retrieved 2026-10-21). Section "1000 TX".
  11. ↑ Tandy 1000 TX Technical Reference Manual (25-1514), main logic schematic revision C, sheets 5 "DRAM Array" and 7 "Video DRAM Array", PDF pp. 92 and 94.
  12. ↑ Hayes, "Tandy 1000-series FAQ" (2005), sections II.C.6 and III.A.2.
  13. ↑ Hayes, "Tandy 1000-series FAQ" (2005), sections II.C.2 and II.C.5.
  14. ↑ "Original Tandy 1000 TX upgrades?", Vintage Computer Federation forum. Posts by VERAULT and Quagmire, 28 March to 9 April 2025.
  15. ↑ "Tandy 1000 TX keyboard only types periods", Vintage Computer Federation forum, December 2022, thread. Post by tinystarship, 24 December 2022.
  16. ↑ "Tandy 1000SX "Big Blue" (U30) interchange", Vintage Computer Federation forum, 2023, thread. Posts by seat safety switch, 24 and 25 August and 12 September 2023.
  17. ↑ Tandy 1000 TX Technical Reference Manual (25-1514), main logic schematic revision C, sheet 6 "I/O Decode and Video Control", PDF p. 93.
  18. ↑ Tandy 1000 TX Technical Reference Manual (25-1514), main logic schematic revision C, sheet 8 "Audio, Joystick Control", PDF p. 95.
  19. ↑ 19.0 19.1 Hayes, "Tandy 1000-series FAQ" (2005), sections II.B.1 and II.D.1.
  20. ↑ "Original Tandy 1000 TX upgrades?", Vintage Computer Federation forum. Post by vwestlife, 28 April 2025.


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