TRS-80 Model 4 Troubleshooting Guide

The separate floppy controller and RS-232C boards of an original Model 4. On the gate array Model 4 these circuits are on the main board.

Fault finding on the TRS-80 Model 4 starts with knowing which version is on the bench. The original machine (1983) has a CPU board with separate floppy controller and RS-232C boards; the gate array Model 4 (catalogue 26-1069A, from late 1984) carries everything on one main board and has a green screen and the four arrow keys grouped together. Tandy's 1983 technical reference manual covers the original boards; Howard W. Sams' Computerfacts CS13 has a full troubleshooting guide for the gate array version. The TRS-80 Model 4D uses the gate array board, and the TRS-80 Model 4P Troubleshooting Guide covers the portable.

Common faults

These come from the Model III/4 section of the trs-80.com repair notes, matched to the Tandy parts lists where a part is named.

Model 4 faults reported by repairers

Symptom	Likely cause	What to do
Garbage on the screen at switch-on that clears when Reset is pressed	Power-on reset capacitor out of specification	Replace it. On the original board it is C20, 22 µF 16 V; on the gate array board Sams shows it as C25, 22 µF
Original Model 4 gives a scrambled screen when booting TRSDOS 6	Trimmer capacitor near the floppy cable connector, which sets horizontal sync in 80-column mode	Turn it a couple of degrees either way until the picture locks
"CASS?" appears with the drive door open, instead of "Diskette?"	Floppy controller board or the cable between it and the main board	Remove both drives, clean their edge connectors, and try to boot from each in turn
Both drives give the same errors on an original Model 4	Floppy controller board	Look closely at the board or try a known good one; the early analogue board's potentiometers need setting up
Full raster but no text	Open peaking coil L301 in the monitor	Replace L301 (39 µH)
A lit, blank raster at switch-on	Brightness set too high	Turn brightness down until the raster goes, set contrast to full, and insert a disk without closing the door: "Diskette?" should appear
Smoke from the power supply a few minutes after switch-on	Mains filter capacitor failed	Replace the mains filter capacitors with X2-rated parts: C1, C2 and C13 on Astec supplies, C32 and C33 on the Tandy supply
Keys respond slowly or only when held down	Dirty keyswitches	On four-lug switches, remove, take apart and clean them; two-lug switches can be taken apart but are fiddly

Power supply

Three supplies were used: the 38 W Astec AA11320 (8790021) in 16 KB cassette machines, and the 65 W Astec AA12090 (8790043) or Tandy 8790049 in disk machines. Sams lists the Tandy design as the supply in the gate array Model 4, with both Astec supplies as alternatives. The supplies are switching designs; the trs-80.com notes say they must run loaded, and that the Astec supplies needed full loading during troubleshooting.

Astec 65 W (8790043)

Tandy's procedure, run through an isolation transformer of at least 500 VA and a variac:

Astec AA12090 (8790043) fault finding

Symptom or stage	Check
No output	Check the fuse; if it has blown, do not power up again until the cause is found. Check the bridge DB1, power transistor Q2 and catch diode D3 for shorted junctions. With the loads disconnected, check each output for shorted rectifiers or capacitors; if the +12 V output is shorted, check the crowbar SCR1 too
B+	With 95 V AC in, the end of R14 nearest the transformer should read 260-270 V DC. If not, check R14 and DB1. An open R14 usually means one of the parts it feeds (Q2, Q1, D1, D3) has shorted
Not switching	Q2's collector should show the switching waveform. If not, check Q2 for open junctions, then look for 0.7 V on its base; if that is missing, check L3, Q1, D1 and R4
All outputs low	Check the voltage-selection jumper
+5 V and +12 V (V3) low	The supply regulates from these two outputs, so they can pull the others down. Tandy sets them by removing or adding R27 and R28
One output missing	Check that output's rectifier, then the rest of its circuit and the solder joints
Crowbar not working	Check Z1 and SCR1; if it triggers outside its limits, change Z1

Output limits and pin numbers are in the General Maintenance guide.

Astec 38 W (8790021)

The fusible resistor R25 limits inrush current and acts as the system fuse. Only the +5 V output is regulated; the ±12 V outputs follow it. If the +5 V rises too far, a zener turns on an SCR that shorts the +12 V output to ground, and a short on any output makes the supply fold back and shut down.

Tandy 65 W (8790049)

This is the TRS-80 Model 4P supply; Tandy's troubleshooting chart for it is in the Model 4P guide. Sams' procedure for the same design in the gate array Model 4:

1. Disconnect J2 so the computer cannot be damaged while the supply is being worked on.
2. If fuse F1 has blown, check bridge rectifier BR1 and amplifier transistor Q7 for shorts.
3. With F1 good, check for 120 V AC across C92; if it is missing, check line choke T2, R38, the power switch and connector J3.
4. Check for 163 V at the cathode of BR1, measured from the emitter of Q9.
5. If 163 V is present, check whether the supply is oscillating: look at the waveforms on the base of Q8, on Q1 and Q2, and at pins 5 and 9 of U1. If it is not, check around Q1-Q5, Q8, Q9 and U1, the windings of T1, and VR1 and CR1.
6. If it oscillates, check J2: +12 V on pins 2 and 5, −12 V on pin 4 and +5 V on pin 10.

Missing outputs at J2 (Sams)

Output	Check
+5 V not regulated	Regulator SCR Q6, zener VR2, windings of T1
+5 V missing	CR5, L1, C10, C11
+12 V on pin 2 missing	CR8, L4
+12 V on pin 5 missing	CR6, L2, C23, C18
−12 V missing	CR7, C19, regulator VR3

Gate array main board

Sams' checks for the gate array board:

Gate array board checks (Sams CS13)

Area	Check
Reset	Put a logic probe on pin 26 of the Z80 (U1) while switching on or pressing Reset. It should go low for about 0.1 s, then high and stay high. If it works at switch-on but not from the Reset key, check the switch, C22, C25 and R61. If neither works, check R66 and C70, and U41 and U63 by substitution
Clocks	The Z80 clock at U1 pin 6 is 4.05504 MHz. The master clock at pin 1 of the timing IC U9 is 20.2752 MHz; if it is wrong or missing, check C2, C20, R14, crystal Y1 and U9
CPU	Pins 17, 24 and 25 of U1 should be high. While Reset is held, the address and data pins should float and pins 19-23 should be high; a pin that reads high or low points at the chips connected to it. If the data lines show activity at switch-on and then stop, look at the ROMs (U4, U29) and RAM (U85-U92)
Keyboard	If it is dead, check the ribbon cable and connectors, then pulses at pin 7 of the address decoder U5. If one group of keys fails, check the input buffers U26 and U40 and the output buffers U55 and U56. For a single dead key, check its contacts with an ohmmeter
Video	Check J8 on the main board and P300 on the monitor board. The video signal leaves the video array U17 on pin 19; if it is there, check around U20 and the monitor transistors Q101-Q103. The CRT controller U11 should show activity on pins 18 and 35-38. Wrong characters point at the adder U13, character generator U19 or video RAM U16
Sync	Vertical sync leaves U11 on pin 40 for the vertical IC201 on the monitor board; horizontal sync leaves on pin 39 for Q301, Q302 and IC301
Cassette motor	Run 10 OUT 236,2: OUT 236,0: GOTO 10 and look for pulses on pins 9 and 12 of U53. If pin 9 is quiet, suspect the decoder U43; if pin 12 pulses, check Q3 and relay K1. A motor that will not stop points at sticking contacts in K1
Printer port	With no printer connected, run 10 OUT 248,255: OUT 248,0: GOTO 10. Pin 1 of the latch U107 should pulse (if not, check U43), and so should its data outputs
RS-232	Sams drives the port with a test program and a 5 V, 5 kHz square wave on J3 pin 3, then checks U82, U84 and the baud-rate generator U64 (4800 Hz at pins 3 and 17 for 300 baud)

The trs-80.com notes add that on a gate array 4D, characters typed twice with others missing is a main board fault, in one or two of the gate arrays, the CRT controller or the video RAM.

Original main board

There is no troubleshooting section for the original CPU board in Tandy's technical reference manual, only theory, schematics and parts lists. Start with the faults in the table above.

• The memory jumpers and the capacitors removed for 64 KB are listed in the General Maintenance guide; check them against the RAM fitted.
• Radio Shack's M4MEM (disk systems) and BD16K (16 KB cassette systems) test the RAM, video RAM, ROM checksums, real-time clock, sound and the printer port, which needs a loopback plug. Tandy notes that a fault in one bit can take hours to show, and suggests warming or cooling the chips to bring out heat-related faults.
• The trs-80.com notes say Z80-family parts are failing with age, that the Z80 is fussy about reset timing, and that if the address and data lines are quiet the reset timing should be checked, or the Z80 swapped if no oscilloscope is available.

Disk drives

Before a drive is used for the first time in years, the trs-80.com notes suggest these checks:

1. Open and close the door, and lubricate it if needed.
2. Make sure the head moves back and forth freely; lubricate the rails if needed.
3. Check the drive belt.
4. Inspect the head and clean it with alcohol and cotton buds if needed. Tandy specifies 91 % isopropyl alcohol for head cleaning.
5. On single-sided drives, check the felt pressure pad is present.
6. Clean the edge connector with a soft eraser.
7. Try to format a disk. If that fails, check the speed, which should be 298 rpm. If the drive still cannot read, the logic board may be faulty or the drive out of alignment.

The internal Texas Instruments drives have no select jumpers; selection is by the cable, and drive 0 has no terminator because the cable is so short. Floppy controller alignment is in the General Maintenance guide. See also Floppy Drive Repair.

Monitor

Original machines use an RCA (8492002) or TCE (8790607) monitor; Tandy gives adjustments for the RCA unit only. Replacement tubes are listed by the trs-80.com notes as 12VCMP4 or 12VCLP4 (white) and 12VCMP31 (green, Model 4 and 4D).

Related pages

• TRS-80 Model 4
• TRS-80 Model 4 General Maintenance
• TRS-80 Model 4 Capacitor Replacement Guide
• TRS-80 Model 4D Troubleshooting Guide
• TRS-80 Model 4P Troubleshooting Guide
• Radio Shack TRS-80 Model 4 Technical Reference Manual