TRS-80 Model 6000 Troubleshooting Guide

The 6000's major assemblies (service manual Figure 1-1).

Fault finding on the Tandy 6000 follows Tandy's 1985 service manual. The boot ROM on the Z80 I/O processor runs its own tests at switch-on and reports failures as two-letter codes; the 68000 CPU and memory boards are checked with Tandy's diagnostics after the Model II diagnostics have cleared the I/O side; and the 140 W power supply has its own bench procedure. Tandy's procedure isolates faults to field-replaceable parts such as the power supply, the I/O processor board and socketed ICs.

Setting up

Tandy's set-up for fault finding: unplug the unit, remove the top cover (see TRS-80 Model 6000 General Maintenance), check that the monitor is still connected to the video/keyboard board and the power supply, take any disks out of the drives, check that the power switch under the left lip of the base is off, and then reconnect the mains.

Start-up sequence

At switch-on the Z80 runs the boot ROM, whose tests run in order, each only if the previous one passed:

1. Interrupts are disabled and all RAM except page 0 is deselected.
2. The CRT controller is set up and the whole screen is turned white.
3. The ROM checksum is checked ("BOOT ERROR CK" on failure).
4. The Z80 registers are checked by passing a bit pattern between them ("BOOT ERROR Z8").
5. Lower RAM, from 1000H to 7000H, is checked.
6. The keyboard buffer is flushed.
7. The hard disk (if fitted) and floppy controllers are reset; the heads seek to track 5 and then restore to track 0.

If the machine is to boot from floppy, "INSERT DISKETTE" appears in the middle of the white screen. When a disk is inserted and the latch closed, the screen blanks with a blinking cursor in the middle while the system loads. The boot program read from the disk then tests the DMA, PIO and the memory from 2000H to FFFFH, and shows "32K MEMORY" or "64K MEMORY" when it passes, followed by "LOADING" or "INITIALIZING" at the top left.

Boot error codes

Floppy disk errors (manual pp. 48-50)

Code	Meaning
DC	Controller or drive error: busy not reset, not at track 0, or seek error
DO	Drive not ready
TK	Record not found reading track 0 (no readable formatted data)
LD	Lost data on read
SC	CRC error: data read incorrectly
RS	Not a Radio Shack format disk ("BOOT" and "DIAG" not found)

Hard disk errors (manual p. 50)

Code	Meaning
HT	Timeout waiting for ready
HC	CRC error in data
HI	CRC error in ID
HN	ID not found
HA	Aborted command
HO	Track 0 error
HM	Data address mark not found
HD	Any other error

Other errors (manual p. 50)

Code	Number	Meaning
CK		Bad ROM checksum
Z8		CPU failure
MF		RAM failure
ML	01	Low RAM failure (0-3FF)
MH	02	High RAM failure
DM	03, 13, 23	DMA data failure; DMA gave no interrupt; DMA gave an extra interrupt
PI	04, 14, 24	PIO data failure; PIO gave no interrupt; PIO gave an extra interrupt
CT	05, 15, 25, 45	CTC gave no interrupt on channel 0, 1 or 2; CTC gave an extra interrupt

Source: The boot program's memory test also reports "BOOT ERROR LM" for lower memory and "BOOT ERROR HM" for higher memory.

Hangs at "LOADING"

TRSDOS 4.0 and later need 16 KB of memory above the first 64 KB, from Z80 memory or the 68000's memory. The boot diagnostics do not test it, so if it is missing or faulty the machine stops with "LOADING" at the top left of the screen and no error message.

I/O processor tests

Tandy test programs for the I/O processor system (manual p. 51)

Program	What it tests
SYSTEM (29 November 1982)	Serial channels A and B, PIO (printer and external floppy interfaces), DMA, Z80 interrupts and bus; needs loop-back test cables
MEMII (version 1.3)	Video refresh RAM, shadow ROM checksum, checkerboard, modified address, coincidence and bank select
DISKDG8	Thinline floppy drives
DOVIDAL	Video alignment and centring

Source:

CPU and memory boards

To isolate a fault on the 68000 side, Tandy's order is:

1. Run all current Model II diagnostics. If any fail, refer to the Model II Technical Reference Manual; see the TRS-80 Model II Troubleshooting Guide.
2. If the 68000 CPU or memory board is still suspected, remove both and run the Model II diagnostics again.
3. Once the fault is placed on the 68000 CPU board or the 512K/1 MB memory board, use the charts below.

6000 memory test (uses the Z80A to test the 68000 memory)

Result	Possible fault	Action
Test locks up	Z80A to 68000 interface	Check interface controller U36 and its select logic
Test locks up	Bus arbitration controller U6	Check latch U7 and the bus arbitrator
Test locks up	Refresh logic	Check all the refresh circuit logic
Bad RAM reported	RAM	Work out which chips from the report and replace them
Bad RAM reported	Z80A to MC68000 interface	Check the interface circuit
Bad RAM reported	Memory board	See the memory board chart below
No errors		The chart refers to the memory board section; the Model 16 manual sends the technician to the CPU board charts at this point

6000 interrupt and offset/limit tests (CPU board)

Test	Result	Possible fault	Action
Interrupt	Locks up on a software interrupt	Interrupt circuit; MC68000; clock logic	Check interrupt controller U15 and U34, U40, U26; replace the MC68000; check the clock circuit
Interrupt	Locks up on a hardware interrupt	Interrupt circuit; MC68000; clock logic	Check U15 and every hardware interrupt input to it; replace the MC68000; check the clock circuit
Interrupt	Software interrupt received was not the one generated	Interrupt controller	Replace U15
Interrupt	Hardware interrupt received was not the one generated	Interrupt circuit	Check the interrupt input lines for shorts
Offset/limit	OBERROR in the user range	Offset/limit registers	Check the output address lines for shorts
Offset/limit	Protected memory modified	U15, U41	Check those ICs
Offset/limit	OBERROR not generated	4-bit full adders	Check U23, U24, U38
Offset/limit	Random errors	I/O decoding and strobes	Check the I/O and strobe circuit

6000 memory modified address test (headed "Mother Board Troubleshooting" in the manual)

Result	Possible fault	Action
Bad RAM reported	RAM	Work out which chips and replace them
Bad RAM reported	Data buffers	Check buffers U5 to U8
Bad RAM reported	Address buffers	Check U3, U4, and U9 for A17
Bad RAM reported	Memory control	Check U9 and the signal control logic and multiplexers

Source: Tandy's charts, pp. 52-54. The CPU board's clocks come from a 48 MHz crystal oscillator, divided down to 24, 12, 8 and 4 MHz; the charts' "check clock circuit" means this logic.

Power supply

The supply is an Astec AA11082 (Tandy 8790047), 140 W. Bench test it only with at least the minimum load on every output; Tandy warns that running it unloaded can cause damaging oscillation. Load values and output pins are on the TRS-80 Model 6000 General Maintenance page.

Start-up test

1. Inspect the board for broken, burnt or damaged parts, and check the fuse with an ohmmeter if in doubt.
2. With the minimum load connected, bring the mains up slowly on a variac while watching the +5 V output on an oscilloscope and a meter. The supply should start at about 40-60 V AC and regulate by 95 V AC.
3. If +5 V appears, run the performance test below. If not, follow the no-output steps.

No output

1. If the fuse has blown, replace it but do not power up again until the cause is found.
2. Check the diode bridge DB1, power transistor Q7 and its catch diode for shorted junctions, and replace any that are shorted.
3. With an ohmmeter, check each output for a short. If +5 V is shorted, check the crowbar SCR (SCR1) and zener Z1 as well.
4. Check B+: with the X10 probe's ground on the cathode of D5, turn the input up slowly and measure at the cathode of D9. At 95 V AC input it should be 130-170 V DC. If not, check DB1 and the fuse F1, then R5, D12, D13 and the input capacitors C5, C6, C7 and C8.
5. Check that Q7 is switching, with the X10 probe on its TO-3 case (collector). If not, check Q7 for shorts, then look at its base (under the board, the centre pin nearest the board edge). A correct base waveform means the fault is on the secondary side; a wrong one means the control section, and Tandy's instruction is to return the supply to the repair depot.

The manual's figures 7-16 and 7-17 show the expected collector and base waveforms.

Performance test

Test conditions (manual p. 200)

Test	Input	Loads on all outputs
1	95 V AC	Maximum
2	135 V AC	Maximum
3	135 V AC, then varied over the full range to look for instability	Maximum
4	135 V AC	Minimum
5	95 V AC	Minimum

Voltage and ripple limits (manual Table 7-10)

Output	Minimum	Maximum	No load	Ripple (max.)
V01, +5 V	4.90 V	5.10 V		50 mV p-p
V02, +12 V	11.40 V	12.60 V		100 mV p-p
V03, +12 V	See note	See note		100 mV p-p
V04, +24 V	21.20 V	26.40 V	30.0 V	250 mV p-p
V05, −12 V	−11.40 V	−12.60 V		50 mV p-p

V03 must stay within 100 mV of its starting value when the +24 V load steps from 0 to 2.5 A (decaying to 0.92 A within 350 ms) or from 0.55 A to 3.05 A (decaying to 1.45 A), and within 5% under all other conditions. The same switching-supply approach, for the Model II's AA11080, is on the TRS-80 Model II Troubleshooting Guide.

Floppy drives

• Of the boot errors, DC and DO come from the drive status, and TK, LD and SC from reading track 0. Tandon's manual for the TM848 drives lists the causes of soft errors as airborne contaminants, electrical noise, excessive temperature and other external causes, and its first checks are grounding between supply, drive and host, freedom from conducted and radiated noise, temperature, and repeating the fault on a second diskette.
• Tandon also says to check the supply voltages are within specification before working through its troubleshooting table.
• If drives have been changed, or the controller has drifted, the data separator and pulse width adjustments on the I/O processor board may need resetting; see the TRS-80 Model 6000 General Maintenance page.

Related pages

• TRS-80 Model 6000
• TRS-80 Model 6000 General Maintenance
• TRS-80 Model 6000 Capacitor Replacement Guide
• TRS-80 Model 16 Troubleshooting Guide
• TRS-80 Model II Troubleshooting Guide
• Tandy 6000 Service Manual