Tandy 1000 General Maintenance

Tandy 1000 main logic board layout from Tandy's service manual.

Routine care of the original Tandy 1000 (catalogue number 25-1000) means opening the main unit, cleaning it, checking the 54 W power supply against Tandy's limits, and looking after the floppy drives, keyboard and memory. The steps follow Radio Shack's 1985 service manual.

Opening the main unit

Tandy's order of work for the main unit:

1. Remove the two screws at the lower front of the case, slide the case forward until it clears the rear panel, and lift it off.
2. Remove the three screws at the back to free the rear panel, and lift it slightly.
3. To remove the drive assembly, take out the two screws nearest the fan on the rear of the chassis, the screw on the right side of the drive structure, and the three screws at the front. Unplug the power and signal cables from the drives and lift the assembly out.
4. To remove an option board, take out the screw at the top of its panel and pull the board up out of the main board.
5. To remove the main logic board, unplug the speaker cable and take out the eleven screws that hold it to the chassis.

Removing the power supply

Layout of the 54 W supply board 8790070. The mains connector SK1 and fuse F1 are at the top left, the output connector SK3 at the right.

The supply is at the right of the main unit and is reachable once the cover and rear panel are off.

1. Unplug the mains cable from the supply.
2. Unplug the DC power cable from the main board and the drive power cable.
3. Unplug the AC fan cable from the supply board (connector SK2, FAN OUT).
4. Remove the four screws that hold the supply and lift it out through the open end of its enclosure.

Refit in the reverse order and check that every cable is back on the right connector.

Cleaning

• Blow dust out of the supply, the fan and the drive bays with the machine unplugged.
• Clean the edge connectors of option cards with isopropyl alcohol and reseat them. Reseat the socketed chips on the main board, which include the 8088, the video array and gate array, the ROM and all sixteen DRAMs.
• Do not bend or scratch the main board while it is out. One owner on the Vintage Computer Federation forum spent months tracing a fault to a single track cut when a drive fell onto the board.

Keyboard

A Tandy 1000 keyboard with the top case removed (photo: OldIsNew, public domain).

Tandy's procedure for opening the 90-key keyboard:

1. Unplug the keyboard from the main unit and lay it upside down on a soft surface with the cable towards you.
2. Remove the three screws nearest the front. Keep them apart: they are shorter than the rest and must go back in the front positions, or they damage the keyboard plastic.
3. Remove the remaining six screws from the sides and back.
4. Hold the two halves together, turn the keyboard right side up and lift the top cover off.
5. Lift the cable out of its moulded channel, unplug it from the right rear of the board and lift the board out.

When reassembling, make sure the keyboard supports sit on their springs before the top cover goes back. The Tandy 1000 FAQ says the keyboards can be dismantled and cleaned, and advises letting a washed keyboard dry for three days before use. The original 1000 uses a non-standard keyboard interface, so a replacement has to be a Tandy-style keyboard.

Power supply checks

Tandy's operating limits for the 54 W supply:

54 W supply operating characteristics

Output	Minimum	Typical	Maximum	Load range	Noise (max)
+5 V	4.85 V	5.05 V	5.25 V	2.0-7.5 A	50 mV p-p
+12 V	11.40 V	12.00 V	12.60 V	0.2-1.25 A	100 mV p-p
-12 V	-11.00 V	-12.00 V	-15.00 V	up to 0.1 A	150 mV p-p

Input is 100-135 V AC, 47-63 Hz. The over-voltage crowbar fires between 5.8 V and 6.8 V on the +5 V output (6.25 V typical), and the supply then stays off until the mains is disconnected for a short time.

Measure the outputs at the main board connector with the machine running. If the supply is out of the case, put a load on it first: Tandy warns that it must never run without a load, or damaging oscillation can result. The minimum load in Tandy's test table is 2.0 A on +5 V (2.5 ohms, 30 W) and 0.2 A on +12 V (60 ohms, 5 W).

Setting the +5 V output

After any repair, Tandy sets the supply at maximum load and 120 V AC input, adjusting VR1 for 5.05 V on the +5 V output. The manual then runs the supply at 105 V and 135 V input, at maximum and minimum load, and watches for instability while the input is swept. Check the connectors before turning VR1 to correct a low reading at the main board. A poor joint drops voltage between the supply and the board, and raising VR1 to make up for it pushes the supply's own output up.

Connector joints

The drive power connector on the supply board is worth inspecting whenever the supply is out. One owner found the +12 V pin of that connector cracked free of its solder, which left both drives without power; resoldering it cured the fault. Another owner reported bad solder joints on a Tandy 1000 supply as the cause of a dead drive.

Floppy drives

The service manual's drive is the TEAC FD-54B, a double-sided 48 tpi 5.25-inch drive. General drive cleaning is covered on the Floppy Drive Repair page. Points that are particular to the Tandy 1000:

• Each drive is set as drive 0 or drive 1 by a jumper, and the cable has no twist.
• Forum members report that the original short cable has its stripe on the pin 34 side and was made for the TEAC drives, so a replacement drive of another make may need a different cable.
• The FAQ warns that the data connector on a replacement 5.25-inch drive may be upside down compared with Tandy's, and that the drive must be the right density for the machine.

Memory and clock

The 128 KB on the main board is sixteen socketed 64K DRAMs. More memory comes on Tandy's memory expansion card, which also carries the DMA controller that the main board lacks. To find a failed DRAM, replace the chips one at a time with known good parts; one owner found a single bad chip this way.

The main board has no clock. Tandy's Mouse/Clock/Calendar board keeps time on a 3.0 V lithium coin cell, type 2320 (Radio Shack 23-163), for which Tandy gave a life of one year. Some machines have a SmartWatch clock chip fitted underneath the ROM, which has its own internal lithium cell and is replaced as a unit. See Battery Explosion, Capacitor or Corrosion Damage for cell leakage.

BIOS version

The FAQ gives 01.01.00 as the last BIOS for the original 1000 and says it was needed for most hard disk controllers. A 2026 forum write-up compares versions 1.00 and 1.01 and identifies a DMA setting in 1.00 that corrupts data with Tandy's Xebec-based hard disk controller; other owners report running hard drives on 1.00 without trouble.

Maintenance checklist

1. Unplug the machine and let the supply stand before opening it.
2. Remove dust from the supply, fan, drives and slots.
3. Inspect the supply board for discoloured parts, bulging electrolytics and cracked joints at the connectors.
4. Reseat option cards and socketed chips.
5. Clean the floppy drive heads and check the drive jumpers and cable orientation.
6. Run the machine and measure +5 V, +12 V and -12 V against the table above.
7. Check that the BIOS memory count matches the installed memory.

Related pages

• Tandy 1000
• Tandy 1000 Troubleshooting Guide
• Tandy 1000 Capacitor Replacement Guide
• Tandy 1000 Documentation
• Tandy 1000A General Maintenance
• Recommended Tools