TRS-80 Model II Troubleshooting Guide

The AA11080 power supply schematic from the Model II manual (p. 124), with the input capacitors C5 and C6, switching transistor Q1, control module EP-1053 and the four outputs on connector S2.

Fault-finding on the TRS-80 Model II display console. Radio Shack's TRS-80 Model II Technical Reference Manual traces faults to a board or subassembly; owners' experience on the VCF forums fills in what the manual leaves out. Read the safety section of TRS-80 Model II General Maintenance before opening the console; the supply's heat sink can sit 330 V above line common and the CRT's anode lead carries about 12,000 V.

Before you start

• Remove any diskette from the drive. Without a Disk Expansion Unit, the manual expects the terminator on the disk expansion connector, although owners report the internal drive working without it.
• With the cover off, enable test mode on machines with an interlock switch by pulling up its plunger.
• Never power the AA11080 supply without a load.
• Not every symptom is a fault. The picture moving slightly while the drive is working is normal on the Model II, which is not fully shielded.

Start-up sequence and boot errors

At switch-on the Z80 runs the bootstrap ROM on the CPU board, which works through these steps and stops with a message when one fails:

Bootstrap sequence and error messages (manual p. 9)

Step	Message on failure	Meaning
Set up the CRT controller and fill the screen with 0A0H, giving a solid white screen	None
Verify the ROM checksum	CK ERROR	Bootstrap ROM bad: replace the ROM or the CPU board
Test data transfers between the Z80's registers	Z8 ERROR	CPU fault: replace the Z80 or the CPU board
Test RAM from 1000H to 7FFFH (read, complement, write, compare)	MF ERROR	RAM error in the lower 32K: replace the memory card
Flush the keyboard, show INSERT DISKETTE and wait for a disk and a closed door	None
Clear the screen, send a track seek, wait three seconds and check the disk status	DC ERROR	Disk controller still busy, seek error, or drive not back at track 0
Disk status check	D0 ERROR	Drive reports not ready
Read the track ID	SC ERROR	CRC error in the track ID field
Read the track into RAM	TK ERROR	Program not found on track 0
Read the track into RAM	SC ERROR	CRC error in the record ID
Read the track into RAM	LD ERROR	Lost data
Check the loaded data	RS ERROR	Data loaded is not in Radio Shack boot record format
Call the system diagnostic routine and jump to TRSDOS	None

In owners' experience, a BOOT ERROR DC can come from a dirty or sluggish drive; Chuck(G) suggests cleaning the track 0 sensor and asks whether the fault depends on where the head starts. RS ERROR means the drive read the disk but it is not a TRSDOS boot disk. With a non-bootable disk inserted and the door shut, a healthy drive gives a click, a pause, another click and then two more in quick succession before the error appears.

Radio Shack's troubleshooting steps

Follow these in order (manual pp. 9-10):

1. Switch on and wait for the CRT to warm up. Set the brightness and contrast controls on the front of the console. If any picture appears, go to step 10.
2. If the pilot light is on, go to step 4. If it is off, check that the pilot light and reset switch connector (J2) is seated on the CPU board, then go to step 3.
3. Measure +5 V on one of the boards. If it is outside 4.8-5.2 V, go to step 5. If it is correct, the pilot LED has failed: replace it and start again.
4. Check whether the CRT filament is lit. If it is, go to step 7. If not, check for +12 V at the monitor board's power plug, pin 7 (pin 1 is ground). If +12 V is there, go to step 6; if not, go to step 5.
5. Switch off. Check the power supply fuse and replace it if necessary. Check for shorts across the supply rails; if there are any, remove cards from the card cage until the short disappears, and replace the offending card. Otherwise the power supply has probably failed and should be replaced.
6. The CRT filament has burned out. Replace the CRT (with its matched video board).
7. Look at pins 6 and 9 on the monitor board and compare them with the sync timings below. If they are correct, go to step 9. If not, switch off and try a known good video board.
8. If the computer still does not work, Radio Shack's instruction was to return it for factory service.
9. Replace the CRT electronics. If there is still no picture, go to step 8.
10. A white screen with INSERT DISKETTE in the centre means the hardware has passed its start-up tests; go on to diagnostics. A white screen with another message is decoded in the table above. Otherwise, swap in a known good video card, and then a known good CPU card, to find the faulty one.

Signals at the monitor board (manual pp. 10 and 92)

Signal	Where	Expected
+12 V supply	Monitor plug pin 7 (pin 1 ground)	About 12 V DC
Horizontal drive	Monitor plug pin 6	About 2 V p-p at about 15,750 Hz; sync period 64.1 µs, width 27 µs
Video	Monitor plug pin 8	About 1.5 V p-p
Vertical drive	Monitor plug pin 9	Period 16⅔ ms, width 1.0256 ms

Measure the signals with an oscilloscope; a meter shows only the average of a pulse train.

Power supply

Missing +5 V at the card cage is often caused by a short on one of the cards. On the VCF forums pski advises measuring at the cage while removing one card at a time, and testing the supply only under load. Owners have found the +5 V rail missing or as low as 4.4-4.6 V, and the +24 V rail as high as 27.8 V.

To work on the supply out of the machine, Radio Shack used an isolation transformer of at least 500 VA (the supply draws its power at the peaks of the mains waveform), a variac, two digital meters, an oscilloscope with ×10 and ×100 probes, and a load board. Start with every output at its minimum load and bring the mains up slowly while watching +5 V: the supply should start at about 40-60 V AC and regulate from 95 V AC.

If there is no output:

1. Check the fuse. If it has blown, replace it, but do not apply power until the cause is found.
2. Check bridge BR1, switching transistor Q1 and catch diode D3 for shorted junctions.
3. With the loads disconnected, check from common to each output for shorted rectifiers or capacitors. If +5 V is shorted, also check crowbar SCR1 and zener Z1.
4. Check for B+ at the jumper marked F2 near the power transistor, with the ×100 probe's ground on the negative terminal of the large input capacitor nearest Q1. With 95 V AC in, it should read 280-300 V DC. If not, check BR1, the fuse, R5, D1 and D2, and the input capacitors C5 and C6.
5. Check the collector waveform on the case of Q1 with the ×100 probe; it should be switching. If it is not, check Q1's junctions and then its base waveform. If the base shows no clock pulses (spikes of about 2 V every 50 µs), replace the control module.

AA11080 performance limits (manual pp. 121-122)

Output	Minimum	Maximum	Ripple	Current limit, min./typ./max.
+5 V	4.90 V	5.10 V	50 mV p-p	9.0 / 11.0 / 14.0 A
+12 V	11.40 V	12.60 V	100 mV p-p	4.6 / 5.5 / 7.0 A
+24 V	21.20 V	26.40 V (30 V unloaded)	250 mV p-p	1.8 / 2.5 / 3.5 A
−12 V	−11.40 V	−12.60 V	50 mV p-p	typ. 1.0, max. 2.0 A

Check the outputs at 95 V AC and 135 V AC, at full and minimum load, and sweep the mains across the range at 135 V AC full load to look for instability. The +5 V crowbar fires between 5.94 and 7.00 V, and the −12 V output comes from a thermally protected IC regulator. Some owners have replaced the supply with a modern four-output unit wired to the original output connector.

Video monitor

The monitor takes +12 V and three TTL signals from the video card, so a missing picture can be the power supply, the video card, the monitor board, the CRT, the yoke or the high-voltage leads. Radio Shack's troubleshooting table for the Motorola board:

Motorola monitor faults (manual pp. 92-93)

Symptom	Possible cause	Remedy
No video at all	Video generator card, monitor board or power supply	Check about 12 V at pin 7 of the monitor plug (pin 1 is towards the back); no voltage means a power supply fault. Check pin 6 for about 2 V p-p at about 15,750 Hz and pin 8 for about 1.5 V p-p; if either is missing, replace the video generator card.
Thin horizontal or vertical line, no picture elsewhere	Deflection yoke	Replace the yoke.
Thin horizontal line	Q7 on the monitor board	Replace Q7. With power off and the green and blue wires removed, check continuity across the yoke winding; open or high resistance means a faulty yoke.
Thin vertical line	Q4 on the monitor board	Replace Q4. With power off and the red and black wires removed, check continuity across that winding.
Picture off centre	Centring magnets on the yoke	Adjust the magnets, remembering the set is on.
Poor focus	R30	Adjust R30 for the sharpest focus without shorting the tool to other parts.
Picture too dim	R11	Set the front-panel brightness to mid-travel, then adjust R11.
No sync	Video generator card	Replace U11, the MC6845 CRT controller; if that does not cure it, replace the card.
Picture jitters, dots drop out and reappear	High-voltage leak, or a failing CRT	With power off and the capacitors discharged, check the high-voltage leads for breaks and kinks, and route them towards the monitor board and away from the motherboard brackets.
No video after replacing the monitor board	Video generator card	Replace U4 (74LS86) and U15 (74LS11) on the video card.

In some cases the video card and the monitor board both have to be replaced. To check sync, the manual suggests turning brightness and contrast to maximum and looking for thin horizontal or diagonal lines across the screen.

Machines with the RCA monitor board need its own schematic and parts list (manual pp. 105-109); the Motorola table's designators do not apply. After recapping a monitor board, check every replacement for value, voltage rating and polarity before blaming anything else; daver2 on the VCF forums has seen a monitor board whose silkscreen polarity markings were wrong.

CPU, memory and keyboard

• The diagnostic ROM by Dave Giller, Frank IZ8DWF and Adrian Black has a Model II version that tests all 2 KB of video RAM and up to 512 KB of system RAM in a loop. It goes in a 2716 in the CPU board's ROM socket. Because the video may not work, it signals that it is running by working the floppy drive three times (the activity light comes on and the head solenoid clicks), and it reports bad video RAM bits by blinking the drive light, long for a bad bit and short for a good one, from bit 7 to bit 0. The authors warn that the Model II's CRT can be damaged if it runs without a valid signal from the video board, citing roughly three seconds before damage: be ready to switch off. Remove any bootable disk and disconnect hard disks, or the ROM may try to boot instead of looping its tests. Radio Shack's manual gives the same warning: never leave the CRT off for more than three seconds during a test.
• Radio Shack's own diagnostic diskette, AXX2012, tested memory, the printer port, the floppy drive and video alignment. Tandy diagnostic disks are in the Model II community archive.
• An MF ERROR or failures in the diagnostics with known good RAM chips can be the sockets or the RAM chips' own legs: one owner found blackened, broken legs and corroded sockets on a memory board.
• If the keyboard does nothing (no key lights), check the cable: one owner found every wire frayed and two torn free inside the connector. Pads that have worn out give keys that work only after repeated presses, and can fail again weeks after re-padding.
• If the reset switch does nothing, check its connector at J2 on the CPU board; the same connector carries +5 V for the pilot light.

Related pages

• TRS-80 Model II
• TRS-80 Model II General Maintenance
• TRS-80 Model II Capacitor Replacement Guide
• Radio Shack TRS-80 Model II Technical Reference Manual
• CRT Discharge Procedure
• Floppy Drive Repair
• Capacitor Failure Symptoms