Tandy 1000A Troubleshooting Guide

Fault-finding on the Tandy 1000A starts where it does on the original Tandy 1000: at the 54 W power supply, which the two models share. Tandy's 1985 supplement for the 1000A has no troubleshooting section, so the supply procedure comes from the original service manual, and the 1000A-specific checks below come from the supplement's circuit descriptions and parts lists and from owners' reports.

Quick reference

Tandy 1000A symptoms

Symptom	First checks	Section
Dead; fan and drive lights off	Mains fuse and the 54 W supply	#Power supply faults
Fan runs, no picture, no start	Supply outputs, the 28 MHz clock, timing generator U25, socketed chips	#No display or no start
Memory errors at start-up	The sixteen DRAMs	#Memory faults
No sound, especially after using GW-BASIC	GW-BASIC 1.01, the speaker control bit, R1, U7 and C6	#No sound
Blurred picture on a CM-4 colour monitor	Resistor pack RP12	#Blurred colour picture
Printer double-spaces	Printer settings and cable	#Printer problems
Hard disk controller not recognised	Memory card with DMA, IRQ 2, BIOS	#Hard disk will not work
Fails to boot after a processor upgrade	NEC V20 compatibility	#Processor upgrades
Drives respond but motors do not turn	+12 V supply	Tandy 1000 Troubleshooting Guide#+12 V missing

Power supply faults

The 1000A's assembly list names the same 54 W supply, Tandy part 8790070, that the original 1000's service manual covers. Tandy's test set-up, the no-output checks and the waveform table for that supply are on Tandy 1000 Troubleshooting Guide, and its operating limits are on Tandy 1000 General Maintenance. Tandy warns that the supply must never run without a load.

No display or no start

On the 1000A the custom timing generator array at U25 does the work of the original board's 8284 clock generator and memory timing logic. The 28.63636 MHz oscillator is Y2. The supplement's pin description gives these U25 signals:

Custom timing generator array U25: pins to check

Pin	Signal	Description in the supplement
22, 23	XTALIN, XTALOUT	28 MHz crystal input and output
25	CLK14M	14 MHz clock
26	CPUCLK	System CPU clock
27	CLK358M	3.58 MHz clock
3	RESET	Synchronised reset output
5	READY	Synchronised system ready
40, 20	VDD, VSS	Power and ground

1. Measure the supply outputs at the main board connector with the machine running.
2. Check for the 28 MHz signal from Y2 at U25 pin 22, then the 14 MHz clock at pin 25 and the CPU clock at pin 26.
3. Watch RESET at U25 pin 3 while pressing a reset button; it should change state and then settle.
4. Reseat the socketed custom chips (U9, U25, U50 and U64), the 8088 at U28 and the ROM at U51.
5. If an NEC V20 has been fitted in place of the 8088, refit an 8088 before going further; see #Processor upgrades.

Memory faults

The 128 KB on the main board is sixteen 64K x 1 DRAMs, 150 ns, at U1-U4, U10, U11, U18-U21, U32-U35, U44 and U45. The socket list covers all of these except U44. Change suspect chips one at a time for known-good 150 ns parts, or faster. Memory above 128 KB, and the DMA chip, are on the expansion card, not the main board.

No sound

Software

Owners found in 1986 that GW-BASIC version 1.01 disabled all sound on the 1000A, and that the sound stayed off after leaving BASIC. Their fixes:

• BASIC starts in beep-on mode. Type BEEP OFF:SOUND ON as soon as BASIC starts, or put those commands in a file called INIT.BAS and start BASIC with BASIC INIT.
• For a permanent fix, patch one byte in a copy of BASIC.EXE:

RENAME BASIC.EXE BASIC.TMP
DEBUG BASIC.TMP
- E 6AA9 74
- W
- Q
RENAME BASIC.TMP BASIC.EXE

The 1000A can also silence its speaker under software control: a 1 in bit 4 of I/O location 0061 disables the internal speaker. A program that leaves that bit set gives a silent machine with nothing wrong in the hardware.

Hardware

The SN76496 tone generator is at U70 (socketed) and the LM386 amplifier at U7. On the schematic, the LM386 output at pin 5 drives the 8-ohm speaker at J1 through C6, a 100 µF non-polarised capacitor. The volume control added on the 1000A is a service adjustment; the parts list's only variable resistor is R1 (1 kΩ), beside U7 on the layout. Check R1, then C6 and the speaker connector, then U7 and U70.

Blurred colour picture

An owner reported in September 1986 that early 1000A boards had a 68-ohm resistor pack at RP12 instead of 33 ohms, and that fitting the right value could improve the picture on a CM-4 colour monitor. He took the fix from the July 1986 "Dave's MS-DOS Column" in 80 Micro and wrote that Tandy had told its repair centres about it. The parts list gives RP12 as a 33-ohm 8-pin SIP pack, and the layout places it next to U69.

Printer problems

• Extra line feeds on Epson printers: an owner found that some programs added line feeds even with LF.COM loaded and MODE LFOFF run, and cured it by covering pin 14 of the parallel cable with tape. Tandy's Printer Interface Selector adds the line feeds back, so its output cables need the same treatment.
• Double spacing on Tandy DMP printers: a DIP switch on the printer chooses whether a line feed also returns the carriage. Tandy's LPINST program asks about the printer and writes the matching LF and MODE commands to AUTOEXEC.BAT. The wrong printer cable can also cause double spacing.

Hard disk will not work

• The 1000A has no DMA controller on the main board. A hard disk needs a memory expansion card fitted first, because the DMA chip is on that card.
• The slots use IRQ 5 for vertical sync, and the 1000A has no switch to free it, so the controller must use IRQ 2. Western Digital WD1002-WX1, WD1002-WX2, WD1004A-WX1, WDXT-GEN2 and WD1004-27X controllers could be modified for IRQ 2, sometimes with a special Tandy ROM; Tandy's own hard cards for the 1000 and 1000A came with the changes made.
• An owner who fitted a WD1002-27X controller wrote that the 1000A needs BIOS 01.01.00 and the controller WD's "Super BIOS" ROM. He got the jumper settings for his drive from Western Digital, and started the controller's low-level format from DEBUG with G=C800:5.

Processor upgrades

An NEC V20 in place of the 8088 works in about 70% of original 1000s and 1000As. In the rest the machine fails to boot or behaves erratically, and there is no way of telling in advance. The PC Sprint daughterboard for the 1000 and 1000A came with both a V20 and an 8088-2 for this reason. The 286 Express card sold by Tandy was incompatible with some programs and expansion cards.

Related pages

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