TRS-80 Model 4P Troubleshooting Guide

TRS-80 Model 4P. The power and reset switches and the brightness and contrast controls are on the panel left of the screen.

Fault-finding on the TRS-80 Model 4P follows Tandy's TRS-80 Model 4P/4P Gate Array Service Manual (catalogue 26-1080, 1984): a general flow in section IV that isolates the fault to the power supply, display, main board or drives, then a step-by-step check for each main board and a chart for the 65 W power supply. The community notes on trs-80.com add faults seen on machines today. There are two main boards, the original and the gate array version; the checks below say which applies.

Disconnect the mains before opening the case. The supply's main filter capacitor charges to about 160 V DC and the CRT holds a charge after switch-off: see CRT Discharge Procedure and the opening steps in the TRS-80 Model 4P General Maintenance guide.

Power-up checks

Tandy's power-up flow (section IV)

Symptom	Check	Then
Power light does not come on	Mains cord and supply; power switch and lamp in the control module	Replace the switch or lamp, or repair the cord
Light on, no picture after the tube warms up and brightness and contrast are turned up	Mains reaching the power supply input	Repair the AC harness if not
Mains present at the supply, still no picture	The supply's four outputs (see below)	Use the power supply chart if they are wrong
Supply correct, no picture	Video and sync signals from the main board at J9	If present, adjust or repair the display; if absent, check the main board
Picture present, drive message on screen	The message (next section)	Drive, cable or controller

Source: section IV.

Boot messages

With a write-protected TRSDOS 6.1.1 or later disk in drive 0, pressing RESET should bring up the TRSDOS logo and a prompt for the date. Tandy's flow treats these boot ROM messages as follows:

• "The Floppy Disk Drive Is Not Ready": insert the system disk, close the door and press RESET. If the message persists, try another disk.
• "Close the Floppy Drive Door And Try Again": as above, then treat as a drive or controller fault if it repeats.
• "The Floppy Disk Drive Is Not Available", "CRC Error, Try Again Or Use Another Disk" or "Seek Error, Try Again Or Use Another Disk": a hardware fault in the floppy controller circuit or the drive.
• Any other message: see appendix B, "Startup Error Messages", of Introduction to Your Disk System, TRS-80 Model 4P.

If the 4P boots, run each test on the Model 4P diagnostics disk to find smaller faults on the main board.

Main board

Memory test

Hold the full stop key and press RESET to run the memory test in the boot ROM. If it reports a fault, check the memory circuit and replace RAM chips. The 4P uses 4164 dynamic RAMs.

No picture from the main board

If nothing appears at the video connector J9, Tandy's board checks run in this order:

Video checks by board

Check	Original board	Gate array board
Master clock	20 MHz at Y1 pin 8; replace Y1 if missing	20 MHz at U148 pin 2; replace Y1 or U148
Clock outputs	PCLK, RS232CLK and 10M from U126	PCLK and RS232CLK from gate array U148
80-column clock	12M at U148 pin 9 (NE564 loop with U147 and U149)	12M at PLL module U146 pin 8
Video timing	SHIFT*, XADR7*, CRTCLK, POT*, LOAD*, LOADS* and DCLK from U127; replace U127 or U128	DCLK in both 64 x 16 and 80 x 24 modes, then SHIFT*, XADR7*, CRTCLK, POT*, LOAD*, LOADS* from U148; replace U148 or U128
CRT controller	U85 and its inputs	U42 and its inputs
After the controller	U82 and U42, then the shift register	U102, then U142 pin 4

Will not boot

With a picture and a good memory test, Tandy checks the floppy controller's clocks (4, 2 and 1 MHz on the original board; 16 and 8 MHz on the gate array board), every input to the controller chip (U13 on the original board, U17 on the gate array board), the handshaking signals from the CPU, and the data bus and control lines, before moving to the drives.

Faults reported on trs-80.com:

• The 4P runs Model III software but not Model 4 software (which uses the 80-column screen): on an original board, the trimmer capacitor in the 80-column clock circuit has drifted. The board has to be folded down to reach it, and then the drive cable will not reach the drives, so adjust it in steps: boot a known good TRSDOS 6 disk, adjust a little, press RESET, and repeat until the screen is sharp and the disk boots. On a gate array board, check the gate array chips for a failure or a loose fit instead. The parts list calls the trimmer C148 (9-35 pF), the theory section calls the tuning capacitor C231 and the trs-80.com notes call it C210; identify it on the board.
• The screen fills with one repeated character and Model 4 disks will not boot, but the memory test passes: suspect the drive. Swap drives 0 and 1; if it boots from the other drive, the first drive is faulty. If neither works, suspect both drives or the controller.
• CRC, ID or disk errors from every disk in both drives: more often the controller on the main board than the drives. If it is the drives, they are probably out of alignment. Test with known good drives.
• Garbage on the screen at switch-on that clears when RESET is pressed: the power-on reset capacitor has aged. Tandy's theory section names it C218 on the original board and C214 on the gate array board, timed with R25; the trs-80.com notes describe it as usually a 22 µF 16 V electrolytic.
• Tandy issued bulletins 4P:9 and 4P:15 for lock-ups or reboots in 4 MHz mode; both use a 4.7 kΩ resistor (N-0247EEC).

Power supply

Measured outputs

65 W supply 8790049 (Tandy specification)

Output	Nominal	Tolerance	Ripple (max.)
V1	+5.05 V	±3 %	50 mV p-p
V2 (CRT)	+12 V	±5 %	150 mV p-p
V3 (disk)	+12 V	±5 %	150 mV p-p
V4	−12 V	+25 %, −8.3 %	150 mV p-p

Measure with the supply loaded; Tandy's tests use defined minimum and maximum loads. The crowbar on V1 fires between 5.8 and 6.8 V.

Tandy's chart

Troubleshooting chart, 65 W power supply (p. 158)

Trouble	Cause	Check or replace
Open fuse	Shorted line input filter capacitor	C30, C31, C32, C33
Open fuse	Shorted bridge	BR1
Open fuse	Shorted filter capacitor	C26, C29, R39
Open fuse	Shorted switching transistor	Q7, C37, R40, C26, T1 primary, Q3, Q1, R37
Current limit cycle	Single rectifier open in bridge	BR1
Current limit cycle	Open filter capacitor	C29
Current limit cycle	Shorted snubber capacitor or resistor	C37, R40
Current limit cycle	Open opto-coupler	U2
Current limit cycle	Shorted supply output	Clear the short in the computer on +5 V, +12 V CRT, +12 V disk or −12 V
Current limit cycle	Shorted output rectifier	CR5, CR6, CR7, CR8
Current limit cycle	Open or shorted output filter capacitor	C16, C18, C25, C23, C10, C11, C12, C19, C20
Current limit cycle	Defective crowbar	Q6
No pulses at U1 pin 8 (supply shut down)	No auxiliary DC supply	CR4, C14, T1 auxiliary winding
No pulses at U1 pin 8	No kick start	R26, Q4, Q5, VR1, CR1, C1
No pulses at U1 pin 8	No base drive	U1, Q3, R23, C8, R24
No pulses at U1 pin 8	Dead-time control divider	C2, R4, R5, U1 (reference)
No pulses at U1 pin 8	Under-voltage protect divider	R27, R25, C9, Q9
No pulses at U1 pin 8	PWM feedback	U1, U2, C3

Source: Tandy service manual, p. 158.

Start-up behaviour

• At switch-on the auxiliary capacitor C14 charges through R26 until the kick-start latch fires after two or three seconds, at about 31 V; C14 then settles at about +15 V. A supply that never gets past this point points to the kick-start parts in the chart.
• A 4P that shows nothing on the screen or drives but makes a faint ticking is in overload shutdown. The trs-80.com notes say the short is almost always in the disk drives: unplug them and switch on again; with the drives disconnected the notes expect the 4P to show its boot error message, printed in three languages.
• A pop and a puff of smoke at switch-on is a failed mains filter capacitor. On Tandy's own supply these are C32 and C33; on an Astec supply the notes name C1, C2 and C13. See the TRS-80 Model 4P Capacitor Replacement Guide.
• Bulletin 4P:4, "Power supply going into current limiting", supplied a fix kit, AXX-7098.

Display

• If the supply is correct and J9 carries video and sync, the fault is in the display or its settings. Tandy's adjustments are in the TRS-80 Model 4P General Maintenance guide.
• The display board has its own 2 A fuse.
• A picture that dims when the drives run means the display is short of voltage: reseat the connectors, then check the solder joints on them, most often the one between the video board and the power supply, and watch the +12 V line as the picture dips.

Related pages

• TRS-80 Model 4P
• TRS-80 Model 4P General Maintenance
• TRS-80 Model 4P Capacitor Replacement Guide
• Radio Shack TRS-80 Model 4P Service Manual
• TRS-80 Model 4 Troubleshooting Guide
• Capacitor Failure Symptoms