Tandy 1000 HX Troubleshooting Guide
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Fault finding on the Tandy 1000 HX relies on owners' reports, the Tandy 1000 FAQ and the Tandy 1000 EX technical reference manual, since no HX service manual has been found. The HX supply carries the same transformer and part references as the EX's 28 W supply 8790083, so Tandy's EX supply procedure applies.[1][2]
 | The supply rectifies the mains directly. Tandy's test set-up uses an isolation transformer and a variable transformer, and the supply must never be run without a load. Unplug the machine and discharge the reservoir capacitor before working on the board. |
Quick reference
[edit | edit source]| Symptom | Check first | Section |
|---|---|---|
| Completely dead | Supply under load, fuse, primary parts | Power supply |
| Buzz or hiss from the speaker when the screen is idle | Sound chip U19 | Speaker noise |
| Stops at the memory count | EEPROM settings | Stops at the memory count |
| Will not boot a newer DOS, or ignores CONFIG.SYS | Setup program start-up device | DOS and start-up settings |
| 720 KB disks formatted as 360 KB | DRIVPARM in CONFIG.SYS | Floppy drives |
| Dim composite picture | Compare with RGBI output | Video |
Power supply
[edit | edit source]One owner's HX arrived dead, and the fault was in the supply.[3] Test the supply on a load with the main board disconnected, then follow Tandy's start-up test and No Output sequence on Tandy 1000 EX Troubleshooting Guide: fuse, bridge rectifier D1, Q1 and Q2, shorted secondary parts, the SCR1 crowbar, then the Q1 waveforms.[1] Check the supply's part number and rating label first.
Speaker noise
[edit | edit source]One owner's HX made a buzzing, white-noise sound from the speaker whenever the screen was idle, which stopped while the screen was being updated. His fault finding:[3]
- He replaced the capacitors in the audio output stage, with no change.
- He swapped in the supply from his EX, with no change, and found the HX supply's outputs good with little ripple.
- With an oscilloscope, he traced the noise from the LM386 amplifier output back to the output of the 14529 chip that feeds it.
- Removing the 76496-type sound chip at U19 silenced the noise. His board had a NEC 8496 in that position.
- Fitting a new SN76496 in place of the NEC 8496 cured it.
Another member with an HX board noted that the internal speaker picks up much more noise than the headphone output does.[3]
Stops at the memory count
[edit | edit source]On machines with an EEPROM, the FAQ says that a system which shows the memory size and stops, without a beep code, has corrupted EEPROM settings.[4] Its steps:
- Disconnect a hard disk's data cable and remove the expansion cards, then try again.
- If it still stops, take the EEPROM out and leave it pins down on a flat metal surface for about a day.
- Once the machine boots, run the setup program with its /F option, or its restore-defaults option, to rewrite the factory settings.
The FAQ adds that a lithium cell in some Tandy 1000s runs only the clock chip, so removing it does not clear the EEPROM.[4]
DOS and start-up settings
[edit | edit source]- To boot a newer DOS from disk, set the primary start-up device to DISK in the setup program.[5]
- With the HX booting from ROM, CONFIG.SYS and AUTOEXEC.BAT on drive A: are ignored.[5]
- If the machine is set to read CONFIG.SYS and AUTOEXEC.BAT from a hard disk and those files hang it, the FAQ's way out is to disconnect the hard disk's data cable so that the BIOS falls back to its defaults, then correct the settings.[5]
- Wikipedia reports that DOS 4.0 does not work on the HX and that the MS-DOS 6 installer can corrupt the EEPROM.[6]
Floppy drives
[edit | edit source]- Under DOS versions after 3.2, a 720 KB drive may format disks as 360 KB. The FAQ's fix is
DRIVPARM=/D:0 /F:2 /H:2 /S:9 /T:80in CONFIG.SYS for drive A:.[7] - The 3.5-inch drives take power through the data cable. Never connect a standard drive to that cable unmodified.[8]
Video
[edit | edit source]One owner's HX gave a very dim composite picture on a monitor that worked with another HX. Forum members suggested checking whether the RGBI output was good, then the buffer chip ahead of the composite circuit and the composite output transistor; the thread does not record a fix.[9] When making an RGBI cable, leave pin 7 unconnected: on the EX it carries green for monochrome monitors, and a straight-through cable to a Commodore 1084 grounded it.[9]
Related pages
[edit | edit source]- Tandy 1000 HX
- Tandy 1000 HX General Maintenance
- Tandy 1000 HX Capacitor Replacement Guide
- Tandy 1000 EX Troubleshooting Guide
- Floppy Drive Repair
References
[edit | edit source]- ↑ 1.0 1.1 1.2 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. 28 W supply 8790083, PDF pp. 89-107.
- ↑ Blake Patterson, photograph of a Tandy 1000 HX power supply board (T1 marked TO-4349), 2019, CC BY 2.0.
- ↑ 3.0 3.1 3.2 "Tandy 1000 HX noisy speaker with idle screen", Vintage Computer Federation forum, February 2025, thread. Posts by Hamsterbasher (1, 4 and 22 February 2025), Torch and daver2 (2 February 2025), Eudimorphodon (5 February 2025).
- ↑ 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). Sections II.K.8 and III.A.4.
- ↑ 5.0 5.1 5.2 Hayes, "Tandy 1000-series FAQ" (2005), sections III.A.1, III.A.3 and III.A.4.
- ↑ "Tandy 1000", Wikipedia (retrieved 2026-10-21). Section "1000 HX".
- ↑ Hayes, "Tandy 1000-series FAQ" (2005), section II.C.6.
- ↑ Hayes, "Tandy 1000-series FAQ" (2005), sections II.C.2 and II.C.5.
- ↑ 9.0 9.1 "Dim composite video on Tandy 1000 HX", Vintage Computer Federation forum, October 2019, thread. Posts by bladamson, Eudimorphodon (9 October 2019) and Hugo Holden (11 October 2019).
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