Tandy 1000 SX Troubleshooting Guide

Schematic of the 67 W single-input supply 8790085, from Tandy's SX technical reference manual.

Fault finding on the Tandy 1000 SX starts with the 67 W power supply, for which Tandy's Tandy 1000 SX Technical Reference Manual (catalogue no. 25-1511) gives a full test procedure. The main board has fewer documented faults. The one most often reported is a failed custom "Light Blue" chip, which stops the machine after the memory count.

Quick reference

Symptom	Check first	Section
Completely dead, fan still	Fuse F1, then the supply's No Output checks; disconnect the main board to separate the two	Power supply
Supply output appears, then drops to zero	Over-voltage crowbar: zener D11 and photothyristor PHC2	No output
Memory count, one beep, then hang	Light Blue chip U51; floppy controller U37 and data separator U38	Hangs after the memory count
Memory errors at start-up	DRAMs in banks 0 and 1, and the four 64K x 4 video DRAMs	Memory
Several chips dead after a supply failure	DRAMs, U51, U30	After a supply fault
Keyboard types only full stops	Keyboard type; try a Tandy 1000 keyboard	Keyboard
No video on a VGA card	DIP switch S2 switch 1	Expansion cards
Hard disk controller not working	IRQ setting and S2 switch 2	Expansion cards
Vented or bulging capacitor in the supply	Replace the supply electrolytics	Tandy 1000 SX Capacitor Replacement Guide

Power supply

First decide whether the fault is in the supply or on the main board. Unplug the supply's DC cable from J6 on the main board, put a test load on the supply, and measure the outputs. Tandy warns that the supply must never be run without a load, or damaging oscillations may result. When one owner's SX would not power on, a forum member suggested inspecting the main board for a shorted tantalum capacitor. The owner then ran the supply with nothing connected, and the fan did not turn.

Tandy's test loads for the 67 W supply:

Table 1, load board values (67 W)

Output	Minimum load	Resistor	Rating	Maximum load	Resistor	Rating
+5 V	1.25 A	4 ohms	20 W	7.0 A	0.7 ohms	60 W
+12 V	0.15 A	80 ohms	5 W	2.4 A	5 ohms	50 W
-12 V	none	none	none	0.25 A	48 ohms	5 W

Start-up test

With the minimum load connected, bring the input up slowly on a variable transformer while watching +5 V on a meter and an oscilloscope. The supply should start at about 40-60 V AC and regulate once the input reaches 90 V AC. On the dual-input supply 8790084, check the voltage selector jumper first; the figures for 240 V operation are 80-120 V AC to start and 180 V AC to regulate. If +5 V reaches 5 V, check the outputs against the operating characteristics in Tandy 1000 SX General Maintenance. If there is no output, work through the next section.

No output

Tandy's sequence:

1. F1 is a 3 A 250 V fuse. If it has blown, replace it, but do not apply power until the cause is found.
2. Check the bridge rectifier D1, the switching transistor Q1 (2SC2833) and the drive transistors Q2 and Q3 for shorted junctions, and replace any that are shorted.
3. With the loads disconnected, measure with an ohmmeter from each output to common, looking for a shorted rectifier or capacitor.
4. To test the over-voltage protection, raise the input from zero while reading +5 V. If the output appears and then falls back to 0 V, check zener D11 and photothyristor PHC2.
5. With a x100 probe, compare the Q1 collector and base waveforms at 90 V AC and minimum load with the manual's figures. A collector waveform like Figure 4 means a shorted part on the secondary side. For the single-input supply 8790085, Tandy's Table 2 lists the candidates as D8, D9, D10, C16, C17, C19, C20, C21, C22 and C23. The dual-input supply 8790084 numbers its capacitors differently, and its Table 2 lists D8, D9, C17, C18, C20, C21, C22 and C23.
6. On the dual-input supply only, check resistor R26. If it is open, check D10, C24 and IC2 on the -12 V output.

An intact fuse does not clear the supply. One owner's SX supply was silent, with no visible damage and the fuse intact, but gave 0 V on both +5 V and +12 V. Another found that the -12 V capacitor C23 had vented and decided to replace every capacitor in the supply. On the single-input supply, C23 is one of the parts in Tandy's Table 2.

Replacing the supply

One forum member fitted a modern supply inside the Tandy enclosure, which keeps the original power switch. Another, unable to repair a Tandy 1000 supply with no +12 V output, fitted what he believed was a Mean Well RT-85B inside the old aluminium case. Any replacement has to meet the loads in Table 1 and feed the 12 V fan.

Hangs after the memory count

The symptom: the screen shows the memory size, the machine beeps once and stops. The keyboard lock lights respond for a few seconds after power-on and then stop, and the floppy drives are never accessed.

In the forum case, the owner reseated every socketed chip, cleaned the pins of the 40-pin parts, replaced the DRAMs, removed all cards and tried other floppy cables and drives without success. He then bought a second SX and swapped chips between the two machines. The custom CPU support chip U51 ("Light Blue", Tandy part 8075306) was bad in both: one was completely dead, the other reproduced the memory-count-and-beep fault in a good machine. He found no easy replacement and repaired the machine with a tested second-hand main board. Another owner later replaced a dead U51 with a spare chip. In 2023 another owner with the same fault asked whether anyone had found a substitute for U51; no answer is recorded in the thread. Light Blue takes the 16 MHz and 28.6 MHz clocks, generates the other clocks on the board and the system reset, and produces the floppy controller's timing and chip select.

Earlier in the same thread, another member suggested the FDC9216 data separator (U38). He had traced the same symptom on a Tandy 1000 in the 1980s to that chip by cooling it with freezer spray until the drive worked, and he added that an Intel 8272 is a pin-compatible substitute for the µPD765 floppy controller (U37). Both U37 and U38 are socketed, so they are quick to swap before suspecting U51.

After a supply fault

A supply that loses regulation can damage the main board. In one SX, a corner had broken off the supply board and the owner believed the supply had lost +5 V regulation. The DRAMs became very hot and had to be replaced. The Light Blue chip (U51) gave no clocks or reset and was replaced with a spare, and the Big Blue video and memory chip (U30) was loading the 28 MHz master clock. A Big Blue marked 2684-0001 worked in place of the original 2684-0002; both carry Tandy part number 8040684. The machine then booted after the owner also replaced the 8088 with a NEC V20, replaced a failed video DRAM and refitted 256K DRAMs where 64K parts had been substituted. After any supply failure, check the supply under load before reconnecting it, then check the main board chip by chip.

Memory

System memory is eight 256K DRAMs at U1-U4 and U9-U12, with eight more sockets for expansion; the 128 KB shared by video and the system is four 64K x 4 DRAMs at U24, U31, U34 and U35. If chips are missing, the start-up test reports RAM errors. Swap suspect chips with known good ones of the same type. When Bank 1 is fitted, jumper E1-E2 has to be removed, so check it whenever the memory count is wrong.

Keyboard

One owner found that a Tandy 2000 keyboard worked for a while and then produced only full stops on an SX, and that a Tandy 1000 keyboard worked correctly. The SX needs a Tandy 1000-style keyboard.

Expansion cards

• The FAQ says DIP switch 1 on the main board must be off for a VGA card.
• As shipped, the SX expects the hard disk controller on IRQ 2. A standard XT controller uses IRQ 5, which needs DIP switch 2 off. IRQ 5, 6 and 7 are shared with the vertical sync, floppy controller and printer circuits until their switches are turned off.
• The FAQ reports that the SX tends to lock up often with Tandy's "286 Express" card.
• In one owner's machine, a hard card stopped the SX from powering on until it was removed.

Main board supplies

The main board makes -5 V from the -12 V rail with VR2, a 79M05 regulator. A missing -12 V output from the supply therefore removes -5 V as well.

Related pages

• Tandy 1000 SX
• Tandy 1000 SX General Maintenance
• Tandy 1000 SX Capacitor Replacement Guide
• Tandy 1000 Troubleshooting Guide
• Floppy Drive Repair