Tandy 1000 EX Troubleshooting Guide

Schematic of the EX's 28 W single-input supply 8790083, from Tandy's technical reference manual.

Fault finding on the Tandy 1000 EX starts with the 28 W power supply. Tandy's Tandy 1000 EX Technical Reference Manual (catalogue no. 25-1510) gives a complete test procedure for both versions of the supply, which is set out below. Owners' reports suggest that the Tandy 1000 HX uses the same supply.

Quick reference

Symptom	Check first	Section
Dead after connection to the wrong mains voltage	Fuse F1, primary parts	Wrong mains voltage
Completely dead	Supply under load with the main board disconnected	Power supply
Supply output appears, then drops to zero	Crowbar: zener, Q3, SCR1	No output
Supply good on its own, dead when connected	Short on a main board rail	Short on the main board
Missing green on an RGB monitor	Pin 7 of the RGBI cable	Video
Hard disk controller not detected	Memory PLUS card with DMA fitted	Expansion cards

Power supply

Unplug the supply from J6 on the main board and from the drive, connect a test load, and measure the outputs. Tandy warns that the supply must never be run without a load, or damaging oscillation can result.

Table 1, load board values (28 W)

Output	Minimum load	Resistor	Rating	Maximum load	Resistor	Rating
+5 V	1.25 A	4 ohms	20 W	3.0 A	1.7 ohms	50 W
+12 V	0.1 A	120 ohms	3 W	1.2 A	10 ohms	30 W
-12 V	none	none	none	0.1 A	120 ohms	3 W

Start-up test

With the minimum load connected, raise the input 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 8790086, check the voltage selector jumper first; for 240 V operation the figures are 80-120 V AC to start and 180 V AC to regulate. If +5 V comes up, check the outputs against the limits in General Maintenance; if not, work through the next section.

No output

Tandy's sequence, with the part references for each supply:

1. Check the fuse F1 (1.6 A 250 V on both supplies). 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 and the drive transistor Q2 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 crowbar, raise the input from zero while reading +5 V. If the output appears and then falls back to 0 V, check the zener diode (D12 on 8790083, D14 on 8790086), transistor Q3 and thyristor SCR1.
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. Table 2 lists the candidates:
   • 8790083: D6, D9, D10, D11, C9, C10, C11, C12, C13, C14, C15, IC1 and SCR1.
   • 8790086: D9, D11, D12, C12, C13, C14, C15, C16 and SCR1.
6. On 8790086 only, the manual adds a sixth step: check resistor R13 in the -12 V winding, and if it is open, check D13, C17 and IC1. The text names R12 in the second half of the step; the schematic shows R13 in series with D13, so R13 is the part meant.

Wrong mains voltage

One owner connected a 120 V EX to 230 V mains. The machine started, flashed at the back after a few seconds and went dead, with the fuse blown. One forum member pointed out that a fuse opens on current, not voltage, and expected a failed part on the primary side; another suggested that the mains filter capacitor might have shorted. A third suggested replacing the fuse and testing the supply disconnected from the computer. Test it with a load, as Tandy requires, and follow the No Output steps above, starting with D1, Q1 and Q2.

Replacing the supply

Forum members have suggested a small ATX-style supply as a replacement, noting that the EX draws only 28 W and needs no -5 V; the original connectors are standard parts. An open hardware project publishes a drop-in replacement supply board for the EX and HX. Any replacement must supply -12 V at up to 0.1 A as well as +5 V and +12 V.

Short on the main board

One owner restored a dead supply by replacing its capacitors, then found that it would not start when connected to the computer, although it worked with a test load. The +12 V rail on the main board was shorted to ground; the thread has no reply and no cause was posted. The main board's electrolytics are listed on Tandy 1000 EX Capacitor Replacement Guide; disconnect the drive and any PLUS cards and measure each rail at J6 before suspecting the chips.

Video

Pin 7 of the RGBI connector J16 carries green for monochrome monitors. One owner lost the green signal when a straight-through cable to a Commodore 1084 grounded that pin; leave pin 7 unconnected.

Expansion cards

The EX has no DMA controller on its main board. A hard disk controller needs the DMA on Tandy's Memory PLUS card, and the FAQ says it must be set to IRQ 2. On the Memory PLUS card, jumper E1-E2 must be fitted with 128 KB and E2-E3 with the extra 256 KB.

Related pages

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