TRS-80 Color Computer General Maintenance
.png)
The TRS-80 Color Computer has its mains transformer inside the case, and the regulators for its four supply rails are on the main board. Radio Shack's service manual gives the disassembly order, the test points for each rail and the tolerances to check against; Tandy's technical bulletins add the faults and board changes found in service.[1][2]
Safety
[edit | edit source]Mains voltage reaches the transformer board, its 0.7 A fuse and the three 0.01 µF 1 kV capacitors C20 to C22 on the primary whenever the power cord is plugged in.[1] Unplug the computer before opening it, and keep fingers and probes away from the transformer board when testing with the power on.
Opening the case
[edit | edit source]- Disconnect every cable, including the power cord. Lay the computer face down on a padded surface and remove the seven screws from the case bottom. The screws sit deep in their holes and may not drop out until the computer is turned face up.
- Turn the computer face up and lift off the case top.
- Lift the keyboard off its plastic bosses and unplug the keyboard cable.
- Remove the top cover shield. The top cover of the RF modulator, U5, may have to come off first.
- Remove the three screws holding the transformer assembly (two on the transformer, one on the board) and unplug its jumper cables.
- Remove the ten screws holding the main board and lift it off its bosses.
- To reach the underside of the board, prise off the sixteen fasteners holding the ground plane and its insulator, using a small screwdriver.
Source: the manual's disassembly procedure.[1]
Reassembly
[edit | edit source]Refit the ground plane and insulator with all sixteen fasteners; pliers may be needed to close the tips. Pull the ends of the power cord through the square cut-out in the board where the transformer sits, and fit the board with ten #6 x 1/2 in screws. Reconnect the transformer jumpers E1 to E4 and the power cord jumpers (E6 white, E5 green, E7 black), then fit the transformer with jumper E8, two #6 x 1-1/2 in screws on the transformer and one #6 x 1/2 in screw on the board. In the case bottom, the two #6 x 7/8 in screws go at the front and the five #6 x 1-1/4 in screws at the rear: a long screw in a front position can dent the case top.[1]
When the ground plane goes back, check that no IC lead has pushed through the insulator. The manual names this as a cause of shorts, and says to trim long leads on the underside of the board and leave no loose material there.[3]
Supply checks
[edit | edit source]| Rail | Test point | Also on each RAM chip (U20 to U27) |
|---|---|---|
| +5 V | TP12 | Pin 9 |
| −5 V | TP11 | Pin 1 |
| +12 V | TP9 | Pin 8 |
| −12 V | TP10 | – |
| Ground | TP3 | – |
A rail more than 5 per cent off its nominal voltage means a power supply fault.[3] The supplies are rated at +5 V 1.35 A, +12 V 400 mA, −12 V 100 mA and −5 V 100 µA, and the +5 V regulator takes its power from the +12 V supply.[1] Figure 1 of the manual (above) marks the expected voltage at each point in the supply. Faults are covered in the TRS-80 Color Computer Troubleshooting Guide.
Board checks
[edit | edit source]- Look at C18, the 1 µF tantalum capacitor on the −5 V regulator, whenever the board is out. Tandy found revision D and E boards with it fitted backwards. Tandy's instruction for a reversed C18 is to fit a new one, because the old part may already be damaged inside.[2]
- Identify the board before ordering parts or attempting a memory upgrade. The old-style boards are revisions B to E; revision D is AXX-0338 and revision E is AX-9003. The revised 26-3002A, 26-3003A and 26-3004A use the AX-9328 board, with a different keyboard, keyboard PIA, power supply board, video mixer and cassette relay.[2]
- A revision B board will not take the 64K upgrade. Revision D and E boards need the cuts and jumpers in technical bulletin CC:15, and a 32K upgrade done to the original instructions can leave a jumper on U5 pin 16 instead of pin 17, which makes the diagnostic cartridge report U21 or U26 bad.[2]
- When replacing RAM, fit chips of the same size, 4K or 16K, as the originals.[3]
Capacitors
[edit | edit source]The main board has thirteen aluminium electrolytic and three tantalum capacitors, including the large reservoir capacitors C7 (1,500 µF), C9 (470 µF) and C10 (10,000 µF) in the power supply.[4] The full list and procedure are on the TRS-80 Color Computer Capacitor Replacement Guide.
Diagnostics
[edit | edit source]Tandy's diagnostic for the Color Computer is the Color Computer Diagnostic ROM cartridge, part AXX-2023; the upgrade bulletins call for version 2.0 or later.[2] The service manual uses it to isolate faults on a computer that still shows its sign-on message.[3]
Related pages
[edit | edit source]- TRS-80 Color Computer
- TRS-80 Color Computer Troubleshooting Guide
- TRS-80 Color Computer Capacitor Replacement Guide
- Radio Shack TRS-80 Color Computer Service Manual
- Capacitor Failure Symptoms
- Recommended Tools
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 Radio Shack, Service Manual: Color Computer, catalogue numbers 26-3001/3002, hosted as File:Radio Shack TRS-80 Color Computer Service Manual (26-3001, 26-3002).pdf, described at Radio Shack TRS-80 Color Computer Service Manual. Disassembly and reassembly (p. 14); system power supply (pp. 36 and 37); fault isolation with the supply test points and RAM supply pins (p. 47); power supply troubleshooting and Figure 1 (p. 49).
- ↑ 2.0 2.1 2.2 2.3 2.4 Tandy Corporation, Color Computer Technical Bulletins CC:1 to CC:32, hosted as File:Tandy Color Computer Technical Bulletins CC-1 to CC-32.pdf. CC:3 (C18 fitted backwards on revision D and E boards, causing −5 V failure); CC:15 (64K upgrade of revision B to E boards; the revision B board cannot be upgraded; diagnostic ROM cartridge AXX-2023, version 2.0 or later); CC:16 (U21 or U26 BAD after a 32K upgrade); CC:17 (old and new board parts).
- ↑ 3.0 3.1 3.2 3.3 Service Manual: Color Computer (26-3001/3002), Section IV, Troubleshooting, pp. 47 to 49.
- ↑ Service Manual: Color Computer (26-3001/3002), printed circuit board parts list, pp. 53 and 54.