TRS-80 Model 4P General Maintenance

The Model 4P's major assemblies, from Figure 1-1 of Tandy's service manual. The power supply is on the underside of the top cover; the main logic board sits in a pan at the bottom of the chassis.

Routine care of the TRS-80 Model 4P means opening the case safely, checking the power supply and display against Tandy's figures, looking after the disk drives and checking which service bulletins have been fitted. Tandy's service manual names only one part of the 4P as needing routine maintenance: the two floppy drives, whose read/write heads should be cleaned periodically. The rest follows from age.

Opening the case

Tandy's order of disassembly:

1. Remove the power cord, printer cable, I/O bus connector and RS-232-C cable from the rear. The printer and I/O connectors are edge-card types.
2. Place the unit screen-down on a soft surface.
3. Remove the six case screws: two on each side near the front, and two under the carrying handle. Press one end of the handle down and lift it out of its recess to reach the last two.
4. Lift the case off.
5. Remove the rear mounting plate: four screws hold the handle support (two each side) and six more run round its edge.
6. The power supply is fixed to the underside of the top cover. The cover is held to the chassis by six screws and folds to the right; the supply comes off the cover with four more screws once its connectors are unplugged. Refit the mylar insulator between the supply and the cover on reassembly.
7. The front bezel has six screws. Two of them sit to the right of the partition beside the drives and need a long-shank 1/4 in nut driver, reached above and below the fan.

The trs-80.com notes describe the same route and add that the cover holding the power supply sits directly above the CRT, so it can be reached without the risk of catching the tube neck that the desktop Model III and Model 4 carry.

Main board, keyboard and drives

• The main logic board is in a metal pan at the bottom of the chassis, held by four screws on each side; nine screws fix the board to the pan, which has raised bosses to space it. The keyboard connector is at the right rear of the board, so the board has to come out of the pan to unplug the keyboard.
• The keyboard comes apart with seven screws underneath. One is under the cork strip: peel it back carefully. On reassembly, tie-wrap the cable just inside the opening as a strain relief and seat the board on its bosses.
• The drive assembly comes out as one unit before the drive screws can be reached. Do not refit a screw in the bottom rear mounting hole of a drive: Tandy warns that it can flex the drive and upset its alignment.
• The sweep board is fixed to the left side of the chassis with four screws over an insulating plate. Refit the plate.

Disk drives

The 4P's two drives are Tandon TM50-series half-height units; drive 0 is the left one. Clean the heads as part of routine maintenance; Tandy refers to the drive maintenance checks and adjustments in Tandon's manual for anything more. See Floppy Drive Repair for general drive work.

Power supply checks

The 65 W supply (Tandy 8790049) has four outputs. Measure them with the drives, display and main board connected: Tandy specifies its tests at a defined minimum load, and the trs-80.com repair notes warn that these switching supplies must run loaded.

65 W supply 8790049 outputs (Tandy specification)

Output	Nominal	Tolerance	Ripple (max.)	Wire colour
V1	+5.05 V	±3 %	50 mV p-p	Red
V2 (CRT)	+12 V	See note	150 mV p-p	Violet
V3 (disk)	+12 V	±5 %	150 mV p-p	Orange
V4	−12 V	+25 %, −8.3 %	150 mV p-p	White

V2 must not move more than 100 mV when the disk load on V3 steps, and otherwise stays within ±5 %. Ground wires are black.

• Set V1 to exactly +5.05 V with R15 on the supply board.
• The +5 V crowbar trips between 5.8 and 6.8 V, and the supply then shuts down through its current limit.
• From cold, the supply should start within two to four seconds of applying 120 V AC.
• The supply board is marked for a 3 A, 250 V fuse (F1); the parts list gives a 3 A AGC fuse and the theory section describes it as fast-acting. The power distribution drawing shows 2.5 A. Fit the value printed on your board.
• A 4P whose display dims when the drives are accessed is not getting the right voltage. The trs-80.com notes suggest reseating every connector first, then checking the solder joints on the connectors, most often the one between the video board and the supply, before measuring the +12 V line while the picture dips.

Display adjustments

Tandy's adjustments for the 9-inch display:

9-inch display adjustments

Adjustment	Control	Setting
Horizontal hold	VR301	Stop the picture scrolling sideways, then move the text to the centre of the raster
Vertical hold	VR201	Find the points either side where rolling starts and set midway between them
Vertical size	Not named in the manual	4.5 in for 24 rows on a full white screen
Horizontal size	L302	6.0 in for 80 characters on a full white screen
Focus	VR303	Full screen of "H": sharp and even in the centre and corners
Tilt	Deflection yoke	Loosen the yoke clamp and turn the yoke until a row of dashes is level
Centring	Magnets on the yoke	Equal margins left and right, top and bottom
Internal brightness	VR302	With the front brightness control central, set where the raster first appears

Capacitors

The TRS-80 Model 4P Capacitor Replacement Guide lists every electrolytic on both main boards, the power supply and the display from Tandy's parts lists. The trs-80.com notes report that the main board capacitors rarely need changing, while the power supply's mains filter capacitors are a known failure. See also Capacitor Failure Symptoms.

Service bulletins

A retyped copy of Tandy's 1991 repair check list for the Model 4 series names these mandatory bulletins for the 4P; look for evidence of them on any machine you open:

Mandatory Model 4P bulletins

Bulletin	Title	Parts named
4P:4	Power supply going into current limiting	AXX-7098 power supply fix kit
4P:10	PCB layout error on Rev. A board	-
4P:13	Correct AC harness problems	AXX-7098 AC harness
4P:15	Lockup or reboot in 4 MHz mode	N-0247EEC 4.7 kΩ resistor

The same list includes 4P:5 (glitch in certain delay lines) and 4P:9 (lock-up or reboot in 4 MHz mode) as non-mandatory bulletins. The text of the bulletins themselves is not in the hosted documents.

Parts

• The 4P uses 4164 dynamic RAMs. The trs-80.com parts page lists compatible makes and warns that some chips sold today are fakes or failures, so buy tested parts.
• A modern replacement supply that keeps the original wiring is sold for all Model 4 variants including the 4P.
• A failed speaker can be replaced with a 1 W, 8 Ω miniature speaker.

Related pages

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