TRS-80 Color Computer Troubleshooting Guide

Color Computer power supply with the expected voltage at each point, from the service manual (p. 49)

Radio Shack's service manual for the TRS-80 Color Computer (26-3001 and 26-3002) gives a fault-finding procedure for each part of the computer, and Tandy published further faults and fixes in its technical bulletins. The U numbers are those of the old-style boards (revisions B to E). The revised 26-3002A, 26-3003A and 26-3004A board numbers some chips differently: its VDG is U9 and its keyboard PIA U18.

Mains voltage is present on the transformer board whenever the computer is plugged in; see General Maintenance.

Where to start

The manual divides faults into input/output faults and a "dead" computer. If the computer shows its sign-on message, run the Color Computer Diagnostic ROM cartridge to find the faulty section. Garbage on the screen, or no display at all, means one of the main logic functions has failed; check the supply voltages first.

Power supply

Supply test points (Service Manual, p. 47)

Rail	Test point	RAM chip pin (U20 to U27)
+5 V	TP12	9
−5 V	TP11	1
+12 V	TP9	8
−12 V	TP10	–
Ground	TP3	–

A rail more than 5 per cent off is a power supply fault. Usually only one rail fails, but the +5 V regulator is powered from the +12 V supply, so a +12 V fault takes out +5 V as well.

1. If a rail reads zero, switch off and measure its resistance to ground. A short to ground often shows up this way.
2. The +5 V rail has an over-voltage diode, CR17, which shorts the rail to ground if the voltage rises too high. If CR17 has shorted the +5 V rail, the regulator circuit is also faulty: replace CR17, and remove R66 to disconnect the +5 V output while you work on the regulator.
3. For the +12 V, −12 V and −5 V supplies, follow the voltages along the circuit with Figure 1 of the manual. A bad voltage at one output of the bridge CR11 means the regulator or the filter capacitor for that rail; bad voltages at both outputs mean CR11 itself. Check that CR11 has the right input from the transformer. If the regulator has the right input and its output is not shorted, the regulator is faulty.
4. The +5 V supply is a 723C regulator, U13, with transistors Q3 and Q2. Work from its input to its output until the voltage goes wrong; U13, Q2 or Q3 is the usual fault.
5. A dead −5 V supply on a revision D or E board: check C18, the 1 µF tantalum capacitor at the −5 V regulator. Tandy found boards with it fitted backwards; it fails when it overheats. Fit a new one with the correct polarity, because a reversed one may already be damaged.

Garbage on the screen

If the supplies are right but the screen is full of garbage, the manual's next step is chip substitution, in this order:

Chip substitution list (Service Manual, p. 47)

Location	Part
U20 to U27	Dynamic RAM; replace with the same size, 4K or 16K
U10	MC6883 dynamic RAM controller (SAM)
U1	MC6809E CPU
U4, U8	MC6821 PIA
U3	MCM68A364 ROM

If substitution does not cure it, look for a short on the address or data bus. Remove the bottom ground shield: an IC lead may have pushed through the insulator. If the computer works with the shield off, trim the long leads on the underside of the board and leave no loose material there before refitting it.

With the shield off and the fault still present, check the CPU, U1, with an oscilloscope: the address lines (pins 8 to 23) and data lines (pins 24 to 31) should be changing fast. If they are not, check the clocks and control lines: pins 34 and 35 carry the 0.89 MHz clocks; pins 2, 3, 4, 40 and 37 should be high and pin 39 low. If the CPU is running, look for a short or open circuit on the bus, poorly soldered or bent socket pins, solder shorts and broken tracks.

Video

The manual names U7, the MC6847 video display generator, as the most common cause of video faults. U10, the SAM, can cause video faults too, and RAM faults: if repeated memory tests show random RAM failures, or replacing the RAM chip named does not help, suspect U10.

Video faults (Service Manual, pp. 47 and 48)

Symptom	Check
No picture or a distorted picture	Try another TV switch box and cable. Look for video at U7 pin 28. If there is none, check for the 3.579 MHz clock at U7 pin 33 and +5 V at pin 17; with the clock present and no video, U7 is faulty
Video at U7 pin 28 but no TV picture	Check U12: video at pin 12, 3.579 MHz clock at pin 2, +5 V at pin 11, input video at pin 9, and +8 V at the top of R16. With all inputs present, U12 is faulty. Then check the modulator input, U5 pin 1: a signal at the right level there means U5 is faulty; no signal means Q1 or its biasing resistors (Q1 collector should be at +8 V). At the wrong DC level, try adjusting R21
Fault in only one mode (graphics, semigraphics or text)	U6, U7 or U10
Dots missing from characters	U7
Wrong characters	The data reaching U7 is wrong: U6 latches it, or there is a short on the data lines
Some graphics modes fail	Check U7 pins 27, 29, 30 and 35 change from high to low as the mode changes; they come from PIA U4. If they change, the fault is U10 or U7. U10 is socketed; try it first

Joysticks, cassette, keyboard and RS-232

Interface faults

Symptom	Check
Joystick fault	Try another pair of joysticks. A fire button that fails without any keyboard fault means L2 or L3 is broken or its track is cut. Run the diagnostic joystick test: missing blocks mean a D/A bit; check U4 pins 4 to 9 and the outputs of U2, and look for an even staircase at U14 pin 8. One direction dead: look for four DC levels at U9 pin 9 and check the DC voltages from the pots at U9's inputs. With both sticks centred, U14 pin 8 should show a staircase to about 2.5 V and pin 14 should switch at each peak
Cassette fault	Run the joystick test first to prove the D/A converter. If it passes, only R41, R42 and C82 remain in the output circuit. Compare input and output of the sine-to-square converter with a long tape of steady data; the only other part is U4. Motor control: relay K1 is the usual fault; check that the collector of Q4 switches with MOTOR ON and MOTOR OFF
Keyboard fault	U8 is the only digital part; most faults are mechanical, or a short or open in the cable. One key failing is mechanical; a whole row or column failing is the cable
RS-232 fault	Run the diagnostic RS-232 test. U15 pin 2 should show a TTL waveform and pin 6 the same waveform switching between +11 and −11 V. U14 pins 4 and 6 should switch between 0 and +10 V, and U14 pins 1 and 2 should show the original TTL signal

Source: the diagnostic notes.

Faults from the technical bulletins

Tandy technical bulletins for the Color Computer

Symptom	Cause	Tandy's fix
Keys G and O work intermittently after a 32K upgrade (revision D and E boards)	The keyboard PIA U8 shares these lines with the joystick switches and RAM size select; C57 and C58 load them	Change C57 and C58 from 0.01 µF to 0.005 µF; mandatory when upgrading to 32K
SOUND does not work after CLOAD or CSAVE	A cassette signal over 5 V peak-to-peak on the CSN line damages U9	Replace U9 and fit a 5.1 V zener from the junction of R31 and R32 to ground, cathode to the junction
SOUND will not stop; TIMER does not count	Real Time Clock interrupts missing	Run 10 PRINT@0,TIMER: GOTO 10. Check for a 60 Hz negative pulse at VDG pin 37 and keyboard PIA pin 18, then at PIA pins 37 and 38 and CPU pin 3. Suspect the VDG, the keyboard PIA or the CPU
"64K MEMORY U21 BAD" (or U26) after a 32K upgrade	A jumper from U4 pin 12 wired to U5 pin 16, as the original upgrade drawing showed	Move the jumper to U5 pin 17 and check the upgrade against bulletin CC:15
Revised 26-3002A, 3003A or 3004A: after 30 minutes to two hours, random characters fill columns 2 and 9, the cursor jumps to the top left and the computer locks up	Occurs only on the revised models	Fit Tandy's satellite board kit AX-7089 as in bulletin CC:20

Source: the technical bulletins.

Related pages

• TRS-80 Color Computer
• TRS-80 Color Computer General Maintenance
• TRS-80 Color Computer Capacitor Replacement Guide
• Radio Shack TRS-80 Color Computer Service Manual
• Capacitor Failure Symptoms