TRS-80 Model I General Maintenance

TRS-80 Model I logic board in the opened keyboard unit (Binarysequence, CC BY-SA 4.0). The two large axial capacitors near the top edge are the supply reservoirs C9 and C8.

Routine care, checks and adjustments for the keyboard unit of the TRS-80 Model I, which holds the logic board, the keyboard and the regulators for all three supply rails. The procedures come from Radio Shack's TRS-80 Micro Computer Technical Reference Handbook, which covers the Model I logic board (PCB 1700069). Fault-finding is on TRS-80 Model I Troubleshooting Guide and the electrolytic capacitors are listed on TRS-80 Model I Capacitor Replacement Guide.

Before opening the unit

• The keyboard unit runs from Radio Shack's external AC adapter, a single transformer in a plastic case with a wire fuse in its primary. The primary is designed for 120 V AC and has an operating range of 105-135 V AC. Mains voltage is present only inside the adapter.
• The Video Display is a separate unit with a CRT. Read CRT Discharge Procedure before opening it.
• The RAMs are MOS parts and are easily damaged by static. Wear a grounding strap, or touch the right-hand side of capacitor C9 before handling them. Keep RAMs in conductive tube or conductive foam, never in untreated polystyrene, and do not tape them together: pulling tape off the roll generates a static charge.
• Ken Brookner, who serviced TRS-80s for Radio Shack, advises starting with a close physical inspection: treat all dust inside an old machine as conductive and blow it out, and look for socketed chips with rusty or darkened pins.

Opening the keyboard unit

Radio Shack's disassembly procedure:

1. Lay the unit keyboard-down on a soft surface.
2. Remove the six screws from the bottom of the case. They are not all the same length, so note which hole each one came from.
3. Turn the unit the right way up and separate the case halves carefully.
4. Free the power LED. On keyboards with a double-sided, plated-through PCB, the LED sits in a socket in the top half of the case on long wires: slip the ring off the socket with needle-nose pliers, let it slide down to the keyboard PCB, and push the LED down through the case with the eraser end of a pencil. Bend the leads slightly so that the ring is not lost. On a single-sided keyboard PCB the LED is soldered to the board and the top half of the case lifts straight off.
5. Lift the keyboard out of the bottom half of the case without straining the interconnect cable.
6. Note where the five rubber spacers between the two PCBs sit, so that they go back on the same plastic studs.
7. Lift both boards out and lay them side by side on the bench, again without straining the interconnect cable.

Laid out like this, the logic board is upside down relative to its position in the case.

Cleaning

Clean solder joints and tracks only with a rubber eraser, used with light pressure. Radio Shack warns never to use steel wool or a sharp tool: rubbing a solder-coated track raises fine slivers of solder that vibration then drags across neighbouring tracks, and these "solder hairs" cause shorts. If a cut or scrape has left solder slivers, reflow the area with the iron to melt them.

Loose solder balls from the factory wave-soldering machine are another known cause of shorts. They can roll around inside the case and lodge under a socket or between tracks; tapping the board can sometimes dislodge one.

Keyboard

Two Model I keyboard units. The Commons description identifies one as fitted with Alps key switches and the other with Hi-Tek switches (snuci, public domain).

Radio Shack found keyboard faults to be mostly mechanical: sticking keycaps, bouncing keys and a broken interconnect cable. A character that appears on its own straight after the ">" prompt points to a shorted key or a short on the keyboard PCB. A keyboard that is completely dead can be a loss of power, a broken interconnect cable, or the address decoder not producing the keyboard select signal, KYBD*.

A space bar that sticks usually has a weak spring. Fit a 3 oz spring; if it still sticks and there is no plastic flash to cause it, stretch the spring a little.

Radio Shack's procedure for replacing the contacts of a key with mangled contacts:

1. Take the unit apart as above.
2. Remove the keycap and keycap plunger.
3. Remove the spring.
4. Unsolder the contacts from the PCB. Make sure the contact ends that come through the board are free; this matters on double-sided boards.
5. Note the position of the two types of contact. The fingered contact is usually on the right with the keyboard the normal way round.
6. Pull both contacts out of the plastic base with strong needle-nose pliers. Fine-pointed pliers usually cannot grip the thin contacts.
7. Load a new contact set into insertion tool 773-10000 or 773-10023 with the contact fingers facing each other.
8. Push the tool into the key base until it seats against its stops. Do not rest the PCB on a hard surface while doing this, or the tool will force the contact ends through the board.
9. Withdraw the tool. Check that the new contacts are in place and that their solder ends come through the PCB.
10. Solder the contacts, refit the spring, plunger and keycap, and test the key.

Keyboard repair parts (handbook, p. 68)

Description	Manufacturer's part number
Standard plunger	171-40103
Solid contact	173-30052-2
Split contact	173-30053-2
2 oz spring	173-10012-4
3 oz spring	173-10012-2
Metal insertion tool	773-10000
Plastic insertion tool	773-10023

The handbook does not say which make of key switch these parts fit. Compare a removed contact with your own keyboard before ordering anything.

When fitting a new interconnect cable between the keyboard and logic boards, insert the contacts from the top side of the board, bend them through 90° to meet the holes in both boards, and have the contact crimps facing the boards. With the crimps facing up, refitting the boards in the case strains the cable and its contacts.

Power supply checks

Radio Shack's Figure 15 (handbook p. 61), showing the power and video sockets, C8, C9, the sense resistors R4 and R18, the meter common point and the trimmers R5, R10, R20 and R21.

Radio Shack's procedure starts every repair by testing the supply voltages. With the boards on the bench, connect the power and video DIN plugs. Put the power plug in J1, the socket nearest the power switch, and not in the cassette socket J3. Ignore the display for now and measure with a digital voltmeter, its common lead on the right-hand side of C9, the largest capacitor on the board.

Supply checks (handbook, p. 60)

Rail	Measure at	Limits	Adjust with
+12 V	Top end of R18 (the end nearest the edge of the board)	11.4-12.6 V (12.0 V ± 5 %)	R10
+5 V	Left-hand end of R4, between C8 and C9	4.75-5.25 V (5.0 V ± 5 %)	R5, only once +12 V is within limits
−5 V	Anode of CR2, to the left of the power switch	−5 V ± 5 %	None

Set +12 V first. The +5 V regulator takes its supply and reference from the regulated side of R18, so it cannot work correctly until +12 V is right. There is no adjustment on the −5 V rail; if it is out of limits, a component has failed. The design loads are about 350 mA on +12 V, about 1.2 A on +5 V and about 1 mA on −5 V. The +5 V regulator starts current limiting at about 1.82 A, and the +12 V supply's maximum design current is 480 mA. A table of the expected voltages on every pin of the two 723 regulators, Z1 and Z2, is on the troubleshooting guide.

AC adapter

The adapter's single transformer has two centre-tapped secondaries. One is rated 14 V AC at 1 A and feeds the +5 V and −5 V supplies through bridge rectifier CR8 on the logic board. The other has its own diodes inside the adapter and delivers 19.8 V DC at about 350 mA for the +12 V supply. All the outputs and centre taps come in through J1.

Radio Shack treated adapter faults as terminal: either the wire fuse in the primary has opened, or the leads have broken at the DIN plug, and in either case the adapter is replaced. If the fuse has blown, check CR8 on the logic board for shorted diodes before connecting a replacement.

Sockets, RAM and DIP shunts

• All eight system RAMs need +12 V, +5 V and −5 V. Check that all three are present on every RAM, and look for pins folded under the socket or RAMs lifted out of their sockets by shipping vibration.
• A RAM pin bent between the chip and its socket is easy to miss. Suspect one if a chip was hard to push in and then snapped home. Check that the RAMs sit level and at the same height, and look under the CPU (Z40) for folded leads. The decoupling capacitors beside the RAMs can hide the sockets, but the leads can usually be seen from above.
• DIP shunt Z3 must be programmed for the amount of RAM fitted. The ROM select signal ROM* and the memory read signal MEM* both come from DIP shunt Z71, and pulling Z71 takes the RAMs out of the system. The shunts crack if they are stressed while their bars are being broken, usually across the middle where the bars are narrowest. To test one, measure the resistance of each unbroken bar without pressing hard with the probes, which can close a cracked bar for the duration of the test. Replace a cracked shunt; solder across a broken bar only if no replacement can be had.

Video centring

After work on the sync generator, or whenever the picture is off centre, check the centring with Radio Shack's test program, which runs under Level I and Level II BASIC:

10 CLS
20 FOR X=0 TO 127
30 SET (X,0) : SET (X,47)
40 NEXT X
50 FOR Y=0 TO 47
60 SET (0,Y) : SET (127,Y)
70 NEXT Y
80 FOR X = 62 TO 65
90 SET (X,23) : SET (X,24)
100 NEXT X
110 GOTO 110

The handbook prints line 60 as SET(127,X); the right-hand edge of the rectangle needs SET(127,Y). The program draws a rectangle round the edge of the graphics area and a small block in the centre. Adjust R20 and R21 until the borders are equal on all sides, using a non-metallic screwdriver so that hand capacitance does not upset the setting. The trs-80.com repair notes suggest trying the computer on a second display first, to tell a display fault from a computer fault, and then swinging the rear controls to find their centre point before opening the keyboard unit to adjust R20 and R21.

Capacitors

The logic board has eleven aluminium electrolytic capacitors: the axial reservoirs C1, C8 and C9 and eight small radial parts. The values, ratings and Radio Shack part numbers are listed on TRS-80 Model I Capacitor Replacement Guide. See Capacitor Failure Symptoms for what a failing electrolytic looks like.

After a repair

Radio Shack's advice for a unit with an intermittent fault was to run the memory test in its SCQATS diagnostic tape for several hours, with the unit in its case so that it reaches its normal temperature. The Model I/III diagnostic ROM written by Dave Giller, Frank IZ8DWF and Adrian Black runs a March test of the video RAM and up to 48 KB of system RAM in a continuous loop, and needs no working RAM to start. On a Model I it fits a 2716 EPROM in the left-most ROM socket on the logic board, the socket the Level II ROM ribbon cable plugs into.

Related pages

• Tandy TRS 80 (Model I main page)
• TRS-80 Model I Troubleshooting Guide
• TRS-80 Model I Capacitor Replacement Guide
• Radio Shack TRS-80 Micro Computer Technical Reference Handbook
• CRT Discharge Procedure
• Recommended Tools