Tandy 1000 TX General Maintenance

Sheet 12 of the TX main board schematic: the power connector J8 with its filter capacitors and the -5 V regulator VR2 (bottom left), and the five expansion connectors.

Routine care of the Tandy 1000 TX covers the 67 W power supply, the floppy drives that take their power through the data cable, the memory and configuration settings, and the steel chassis. Tandy's Tandy 1000 TX Technical Reference Manual (catalogue no. 25-1514) is the main source. It has specifications, switch settings, theory of operation and full schematics, but no disassembly procedure and no parts lists.

Opening the main unit

The TX manual does not describe how to take the main unit apart. Wikipedia describes the TX as an SX layout in a slightly modified case, and the SX in turn as using a chassis similar to the original 1000. The original 1000's service manual steps are on Tandy 1000 General Maintenance; check the screw positions on your own machine before following them.

The plastic posts that hold the blank panel over the second drive bay are often broken off the case cover. A forum member suggests double-sided tape or glue to refit the panel. Another notes that full-length expansion cards do not fit in a Tandy 1000.

Cleaning and corrosion

• Blow dust out of the supply enclosure, the drive bays and the expansion slots with the machine unplugged.
• Clean the edge connectors of expansion cards with isopropyl alcohol and reseat them.
• Look for rust on the chassis. Two owners reported corrosion inside their TXs. One found spots of rust and white scaling on the chassis, drive cage and supply enclosure, sanded the affected areas and repainted all three parts.

Power supply checks

The supply connects to the main board at J8; the filter capacitors and the -5 V regulator VR2 (a 79M05) sit beside it on schematic sheet 12. Measure +5 V, +12 V and -12 V there with the machine running, and compare them with Tandy's operating characteristics, which are the same for both versions of the supply:

67 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	1.25-7.0 A	14.0 A	50 mV p-p
+12 V	11.40 V	12.00 V	12.60 V	0.15-2.4 A	4.8 A	100 mV p-p
-12 V	-13.20 V	-12.00 V	-10.80 V	0-0.25 A	1.0 A	150 mV p-p

Input is 90-135 V AC (or 198-264 V AC on the dual-input supply), 47-63 Hz. The crowbar on +5 V fires between 5.8 V and 6.8 V and resets once the fault is cleared and power is reapplied.

The TX manual has no test procedure for the supply. Its description matches the supplies in the Tandy 1000 SX manual, which warns that the supply must never run without a load and gives a minimum test load of 1.25 A on +5 V and 0.15 A on +12 V. The full procedure is on Tandy 1000 SX Troubleshooting Guide.

Floppy drives

The TX's 3.5-inch drive takes +5 V and +12 V through the data cable. The FAQ gives +5 V on pins 3, 5, 7, 9 and 11 and +12 V on pins 29, 31 and 33, all of which are ground pins on a standard drive.

• Replace the drive only with another Tandy-style drive, or modify the cable as the FAQ describes. Connecting a standard 3.5-inch drive to the unmodified cable can damage the drive and the computer.
• The manual's drive specification is for a Sony MP-F63W-72D. The FAQ lists other Tandy-style 720 KB drives from Sony and Teac.
• The cable is straight, not twisted, so each drive's own select jumper decides whether it is A: or B:.
• After a DOS upgrade beyond 3.2, a 720 KB drive may format disks as 360 KB. The FAQ's fix is a DRIVPARM line in CONFIG.SYS, for drive A: DRIVPARM=/D:0 /F:2 /H:2 /S:9 /T:80.

General cleaning is covered on Floppy Drive Repair.

Memory and coprocessor

• The TX ships with jumper E9-E10 fitted, which sets 512 KB of system memory. To raise system memory to 640 KB, fit four 64K x 4 DRAMs in sockets U54-U57 and remove E9-E10. The FAQ specifies 120 ns chips.
• The optional 80287 maths coprocessor goes in socket U15.

Switches and jumpers

Tandy's settings, with the factory position of each:

TX main board switches and jumpers (Table 1)

Setting	Function	Factory setting
S2-1	Monitor: on = colour, off = monochrome or composite	On
S2-2	Interrupt 5 sync source: on = internal sync, off = external	On
S2-3	Interrupt 6: on = on-board floppy controller, off = optional controller	On
S2-4	Interrupt 7: on = printer interrupt enabled, off = disabled	On
E1-E2	Serial port: installed = enabled (interrupt 4)	Installed
E3-E4	Serial port address: installed = COM1, not installed = COM2	Installed
E9-E10	Memory: installed = 512 KB system memory, not installed = 640 KB	Installed

The 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 TX expects a hard disk controller on IRQ 2.

Keyboard

The TX uses Tandy's own keyboard interface, so a replacement has to be a Tandy-style keyboard made for the original 1000, 1000A, HD, SX or TX. One owner found that a Tandy 2000 keyboard typed only full stops after a short time on both an SX and a TX, while a Tandy 1000 keyboard worked on both.

BIOS

The FAQ lists 01.03.00 as the only BIOS for the TX.

Maintenance checklist

1. Unplug the machine and let the supply stand before opening it.
2. Remove dust from the supply, drives and slots, and check the chassis for rust.
3. Inspect the supply board for bulging or leaking electrolytics and cracked joints.
4. Reseat expansion cards and socketed chips.
5. Clean the floppy heads and check each drive's select jumper. Fit only Tandy-style 3.5-inch drives on the original cable.
6. Run the machine and measure +5 V, +12 V and -12 V against the table above.
7. Check that the memory count at start-up matches the installed memory and the E9-E10 setting.

Related pages

• Tandy 1000 TX
• Tandy 1000 TX Troubleshooting Guide
• Tandy 1000 TX Capacitor Replacement Guide
• Tandy 1000 SX General Maintenance
• Recommended Tools