Tandy 1000 Troubleshooting Guide

Schematic of the 54 W supply from the service manual. The +5 V output runs through D9 and L2, +12 V through IC1 and -12 V through IC2.

Fault-finding on the original Tandy 1000 (catalogue number 25-1000) starts at the 54 W power supply and the start-up self-test. Tandy's 1985 service manual gives the self-test sequence and its messages, a power supply test procedure and a table of collector waveforms for shorted parts; owners on the Vintage Computer Federation forum have added the faults they meet now.

Quick reference

Common symptoms

Symptom	Likely cause	Action
Nothing at all, fan silent	Fuse F1 (3.15 A) open, or supply failed	Check F1. If it has blown, check bridge D1, Q1 and D4 for shorts before fitting a new fuse.
Machine starts, floppy drives dead and silent	No +12 V reaching the drives	Measure +12 V at the drive connector; inspect the joints on the supply's drive power connector.
Supply starts, then shuts down	Over-voltage crowbar tripping	Check D10, R16 and photothyristor PHC2.
Blank screen and a steady tone from the speaker	BIOS not running, so the sound chip has not been silenced	Check supply rails, clocks, reset, ROM and the data bus.
Memory count shows only 128K with a memory card fitted	DMA refresh not working	Check the DMA controller on the memory card.
Drive light on and motor running from power-on	Drive cable fitted backwards	Reverse the cable at the board or drive.
Self-test passes, then no boot	Floppy controller or DMA fault	Check the 8272 and FDC9216 circuit and the drive cable.

Start-up self-test

At power-on or reset the 8088 fetches its first instruction from the ROM at U9, so much of the main board has to work before anything appears. The BIOS then sets up the video circuits, which should show a blank screen with a cursor in the home position, silences the sound chip, and resets the floppy controller. Tandy's notes on reading the result:

• A stable display means the 6845 has been programmed and the ROM can be read, so the memory and I/O buses are basically working.
• The BIOS tests a 180h-byte area of video and system RAM at BBE80 to BBFFF for its stack. A fault produces VID ADDR: BASE: BBE8 OFF: xxxxH and DATA FAULT: WRITTEN xxxx READ: xxxx. An offset at the top of the range (BFFFE) means no RAM is responding at all. A bad bit in the low (even) byte corrupts characters; a bad bit in the high (odd) byte shows as blinking or colour changes.
• FAULTY WRITE AND READ OF 8253 CHANNEL 2 REG points at the 8253 timer (U114).
• MEMORY SIZE = xxx K reports the RAM found. If it shows only the base 128K, the most probable fault is the DMA refresh; any other short count means faulty memory.
• FAULTY WRITE AND READ OF 8259 MASK REG points at the 8259A interrupt controller (U63).
• The full memory test then lists up to 15 errors, with the data written, the data read and the address. It makes one pass and stops if it finds an error.
• If every test passes and the machine still does not boot, the manual names the floppy controller or the DMA subsystem.

IC positions are from the main logic parts list.

No display, no start

Clocks and reset

All system timing comes from a 28.63636 MHz oscillator. U39 (74F161) divides it, and its 14.31818 MHz output clocks both the video array and the 8284A clock generator (U45). The 8284A divides by three to give the 8088 its 4.77 MHz clock. Check each frequency with an oscilloscope or frequency counter. One forum owner found 12 MHz where 14.3 MHz should have been; the eventual cause was a track cut when a drive fell onto the board.

The reset signals start at the 8284A, which synchronises the RES* input. RES* comes from capacitor C26 (10 µF), which the reset button or diode CR6 discharges. If the machine behaves differently each time it is reset, check RES* at U45 as well as the clock.

Steady tone from the speaker

A continuous noise from the speaker with no picture means the BIOS has not run far enough to silence the SN76496. Work back from the supply rails to the clocks, reset and ROM. Many chips are socketed, including the 8088, 8259A, 6845, video array, gate array, ROM, DRAMs and 74LS245 transceivers, so known good parts can be swapped in.

Memory faults

The sixteen 64K DRAMs (150 ns) on the main board are socketed. One owner with start-up memory errors found the fault by replacing the motherboard DRAMs one at a time; the memory card on that machine used HM50256 chips and the main board 4264-15s. Remember that the expansion memory depends on the DMA controller on the memory card for refresh, so a dead DMA chip makes all of the expansion memory disappear.

Power supply faults

Test set-up

Tandy's bench set-up uses an isolation transformer, a 0-140 V variable transformer, two digital meters, an oscilloscope with x10 and x100 probes, and a load board. Never run the supply without a load; Tandy warns that this causes damaging oscillation.

Tandy load board values (Table 1)

Output	Minimum load	Resistor	Rating	Maximum load	Resistor	Rating
+5 V	2.0 A	2.5 ohms	30 W	7.5 A	0.67 ohms	50 W
+12 V	0.2 A	60 ohms	5 W	1.25 A	9.6 ohms	30 W
-12 V	none	-	-	0.1 A	120 ohms	30 W

With the minimum load connected, bring the input up slowly on the variable transformer. The supply should start at about 40-60 V AC and regulate by 100 V AC.

No output

Tandy's sequence:

1. Check fuse F1. If it has blown, replace it, but do not apply power until the cause is found.
2. Check the bridge D1, the switching transistor Q1 (2SC2938) and the catch diode D4 for shorted junctions.
3. With the loads disconnected, check the outputs with an ohmmeter for shorted rectifiers or capacitors.
4. Check the over-voltage protection: raise the input from 0 V while watching +5 V. If it appears and then drops back to 0 V, check D10, R16 and PHC2.
5. Look at the collector and base waveforms of Q1 with a x100 probe and compare them with Figures 18 to 22.

Q1 collector waveform against shorted secondary parts (Table 3)

Waveform	Shorted parts to check
Figure 18	C11, C12, C14, C16, C17, C18, C19, IC1-3, "IC21-3" (as printed), R15
Figure 19	D7, D8, transformer T1 windings 11-12 and 7-8
Figures 20 to 22	D9, transformer T1 winding 9, 10-13

The table is reproduced as printed. The manual's text says Figures 18 and 19 are the normal collector and base waveforms and that Figures 20 to 22 show the shorted cases, while the table also lists Figures 18 and 19 against shorted parts, so compare a suspect waveform with all five figures. Q1 and Q2 are listed as 2SC2938 and 2SD1207 in the parts list.

+12 V missing

A dead +12 V output stops the drive motors while the rest of the machine runs, because the main board uses little +12 V. One owner found the +12 V pin of the drive power connector on the supply board cracked free of its solder. Another, describing a Tandy 1000 with dead drives, found the +12 V rail itself dead, and a third traced the same symptom to bad solder joints in the supply. On this supply +12 V comes from its own transformer winding through rectifier D7, reservoir capacitors C11 and C12 and regulator IC1, with C13 on the output.

Adjustment after repair

Set the input to 120 V AC at maximum load and adjust VR1 for 5.05 V on the +5 V output. Then check the supply at 105 V and 135 V input, at maximum and minimum load, and sweep the input between 105 and 135 V while watching for instability. The limits are in the General Maintenance guide.

Floppy drive faults

• Drive light on and motor running from power-on: the cable is on backwards.
• Both drive lights on and neither spinning: also a reversed cable, at the other end.
• Drives respond but motors do not turn: check +12 V first, then the drives themselves.
• The drives are set by jumpers and the cable has no twist; a PC-style twisted cable does not work.

For drive mechanics and head cleaning, see Floppy Drive Repair.

Hard disk controllers

The FAQ gives BIOS 01.01.00 as the version needed for most hard disk controllers on the original 1000. A 2026 forum write-up shows a DMA mode setting in BIOS 1.00 that corrupts data with Tandy's Xebec-based controller; other owners have run hard drives on 1.00 with other controllers. The controller also needs the DMA controller on a memory card.

Related pages

• Tandy 1000
• Tandy 1000 General Maintenance
• Tandy 1000 Capacitor Replacement Guide
• Tandy 1000 Documentation
• Capacitor Failure Symptoms
• Floppy Drive Repair