Tandy TRS 80

TRS-80 (Model I)

TRS-80 keyboard unit and Video Display (photo: Dave Jones, CC BY-SA 4.0)
Specifications
Manufacturer	Tandy Corporation, sold through Radio Shack
Type	Home computer
Released	3 August 1977
Discontinued	1980 (United States)
Intro price	US$599.95 with display and cassette recorder; US$399.95 for the computer alone
CPU	Zilog Z80 at about 1.77 MHz
Memory	4 KB or 16 KB; up to 48 KB with the Expansion Interface
Storage	Cassette, 250 baud (Level I) or 500 baud (Level II)
Display	64 x 16 or 32 x 16 characters, upper case; 128 x 48 block graphics
Sound	—
ROM	4 KB Level I BASIC or 12 KB Level II BASIC
Power	External AC adapter; regulated +12 V, +5 V and −5 V on the logic board
OS / Firmware	—
Successor	TRS-80 Model III
Model no.	26-1001 (computer); 26-1051, 26-1053, 26-1054, 26-1056 (systems)

The TRS-80 is the first computer Tandy Corporation sold through its Radio Shack stores. It went on sale on 3 August 1977. It was designed by Steve Leininger after the Radio Shack buyer Don French persuaded Tandy to offer an assembled personal computer. The computer, its keyboard and the logic board share one case; the display and cassette recorder are separate units. It became known as the Model I after Radio Shack introduced the Model II in 1979.

Radio Shack later used the TRS-80 name for other machines. The Model III (1980) and Model 4 (1983) run most Model I software. The Model II and the Color Computer are different designs and are not compatible with it.

History

The computer alone (26-1001) cost $399.95, the 12-inch Video Display (26-1201) $199.95 and the Realistic CTR-41 cassette recorder (14-841) $49.95. Bought together as system 26-1051 they cost $599.95. Radio Shack sold four standard configurations:

TRS-80 Model I systems

Catalogue no.	BASIC	RAM	Launch price
26-1051	Level I	4 KB	$599.95
26-1053	Level I	16 KB	$889.00
26-1054	Level II	4 KB	$698.00
26-1056	Level II	16 KB	$988.00

A Level I machine could be upgraded with the Level II BASIC ROM kit, 26-1120. Radio Shack stopped selling the Model I in the United States in late 1980, after difficulty meeting new FCC interference rules, and sold the last units in January 1981; the Model III replaced it.

Specifications

TRS-80 Model I

Item	Detail
CPU	Zilog Z80 (Z40), clocked at 10.6445 MHz from crystal Y1 divided by six, a little over 1.774 MHz
ROM	Level I: 4 KB in two 2 K x 8 ROMs, Z33 (ROM A) and Z34 (ROM B). Level II: 12 KB
RAM	Eight dynamic RAMs, Z13-Z20: 4096-bit parts for 4 KB or 16384-bit parts for 16 KB
Video RAM	1 KB in seven 2102 static RAMs (Z45-Z48, Z61-Z63)
Character generator	MCM6670 (Z29)
Display	64 x 16 characters, or 32 x 16 under Level II
Connectors	Three 5-pin DIN sockets, J1-J3, for power, video and cassette (J1 is power and J3 cassette); expansion edge connector
Logic board	PCB 1700069

Source: Radio Shack's Technical Reference Handbook, pp. 14, 28, 55 and the Level I parts list, pp. 94-97. Level I can display only the 64-character format (p. 28).

Hardware

TRS-80 Model I logic board in the opened keyboard unit, with reservoir capacitors C9 (10,000 µF 16 V) and C8 (2,200 µF 35 V) at the top (photo: Binarysequence, CC BY-SA 4.0).

Board versions

The logic board is part 1700069, and its revision letter follows the part number on the etched side: a board marked "TRS-80 1700069A" is a version A board. Radio Shack's handbook describes three major board changes, driven by which vendor was supplying the Level I ROMs at the time (pp. 100-101):

• Intel 2616 EPROMs, with a white-painted window, on version A and version D boards. An A board has many wire modifications and etch cuts; a D board has fewer. ROM A must go in Z33's socket and ROM B in Z34's.
• National MM2316 ROMs, on many D boards and a few A boards. The ceramic parts are marked "R/D" for ROM A and "S/D" for ROM B.
• Motorola 7800-series ROMs, the next type supplied.

The handbook warns against "correcting" the factory etch cuts: the board will not work without them.

Power supply

The external AC adapter holds a single transformer with a fused primary, designed for 120 V AC with an operating range of 105-135 V AC. One centre-tapped secondary supplies 14 V AC at 1 A for the +5 V and −5 V supplies; the other has its own rectifiers and gives 19.8 V DC at about 350 mA for the +12 V supply. Everything comes into the computer on DIN socket J1.

The regulators are on the logic board (pp. 52-54):

• +12 V at about 350 mA: 723 regulator Z2 with pass transistor Q6, current sensed by R18, from about 20 V filtered by C8.
• +5 V at about 1.2 A: 723 regulator Z1 with driver Q3 and pass transistor Q4, current sensed by R4, from about 7 V filtered by C9 after bridge rectifier CR8. It current-limits at about 1.82 A. A 6.2 V zener, CR1, across the +5 V bus is a crowbar against a +12 V to +5 V short in the RAM. The +5 V supply depends on the +12 V supply working.
• −5 V at about 1 mA: 5.1 V zener CR2 fed through R19 from about −11 V.

Supply checks (Technical Reference Handbook, p. 60)

Rail	Measure at	Limits	Adjust
+12 V	Top end of R18 (the end nearest the board edge)	11.4-12.6 V (± 5 %)	R10
+5 V	Left side of R4	4.75-5.25 V (± 5 %)	R5, only after +12 V is correct
−5 V	Anode of CR2	−5 V ± 5 %	None; find the faulty part

Measure with the meter's common lead on the right side of C9, the largest capacitor on the board. Connect the power DIN plug to J1, the socket nearest the power switch, and not to the cassette socket J3. Figure 19 on p. 75 lists the voltage on every pin of Z1 and Z2 in a working unit.

Electrolytic capacitors

Electrolytics on the Level I logic board (parts list, pp. 94-95)

Ref	Value	Type	Radio Shack part no.
C1	200 µF 16 V	Axial	1500059
C2, C4, C5, C10, C11, C57	10 µF 16 V	Radial	1500012
C6	100 µF 16 V	Radial	1500014
C8	2,200 µF 35 V	Axial	1500064
C9	10,000 µF 16 V	Axial	1500058
C42	22 µF 16 V	Radial	1500055

The rest of the capacitors on the list are ceramic disc and polyester film types.

Known faults

Radio Shack's handbook gives these repair notes (pp. 66-76):

• AC adapter failures are usually terminal: either the fuse link in the primary winding has opened, or the wires have broken at the DIN plug. If the fuse link has blown, check bridge rectifier CR8 on the logic board for shorted diodes, and replace the adapter.
• A dead +12 V bus can be a transistor heatsink that has worked loose and turned until it shorts against the transistor's leads, or shears them from the board. Retighten any loose heatsink.
• The +12 V and −5 V supplies are used by the system RAM, so a fault on either suggests a shorted RAM. A RAM running hotter than the others is a suspect; the handbook warns against testing for it with a finger, because a supply short can heat a RAM to soldering temperature. Pull the RAMs and refit them one at a time until the one that crashes the supply is found; there may be more than one.
• Mylar capacitors C21 and C27 in the sync generator can short after an impact, and the centring trimmers R20 and R21 can fail from heat.
• Unstable or flashing characters are usually shift register Z10 failing with heat; if circuit coolant on Z10 changes the fault, replace it. Missing graphic cells point to Z8 or Z11.
• Intermittent memory faults: run the memory test in SCQATS, a machine-language test tape Radio Shack supplied on request from its National Parts Center, for several hours with the unit in its case. The handbook suggests replacing the RAMs as a set if one bit cannot be pinned down.
• When repairing cut runs on the supply lines, use 22-gauge or heavier wire and never bridge an etch cut with solder.

After sync-generator repairs, re-centre the picture with the handbook's BASIC program, which draws a border and centre rectangle, and set R20 and R21 for equal margins with a non-metallic screwdriver (p. 76).

Documentation

• Radio Shack TRS-80 Micro Computer Technical Reference Handbook: Radio Shack's service reference for the Model I logic board, with theory of operation, adjustments, troubleshooting, parts lists and schematics. The 1978 first edition is on archive.org.

Related pages

• CRT Discharge Procedure
• Capacitor Failure Symptoms
• Recommended Tools