ColecoVision Troubleshooting Guide

Coleco's component location drawing of the logic board, from page V-3 of the ColecoVision Technical Manual. Every U, C, L and R number on this page can be found on it.

Symptom-led fault-finding for the ColecoVision, built on the flow charts, test points and technical tips in Coleco's own service manual, with what owners and repairers have found since. Coleco's flow charts assume two things before they start: that the supply, controllers, RF switch box and game cable are good, which you prove by swapping in known-good ones, and that the copper tracks and solder joints are sound, which a careful look at the board confirms. Pin references below are written as Coleco writes them: U9:38 means U9 pin 38.

Opening the console and routine care are on ColecoVision General Maintenance; the electrolytic capacitors are on ColecoVision Capacitor Replacement Guide.

Before you start

Run through the owner's checks first. Coleco's troubleshooting chart for owners lists a switch box not set to GAME or not fully over, a game cable not plugged in properly, the TV not on the console's channel (3, or 4 if the rear switch is set to 4), a cartridge not fully seated, and a cartridge inserted while the console was on, which needs the power switch turned off and on again.

Then check the power switch. Callan Brown calls a dirty switch the most common ColecoVision repair, and a console with one usually still makes a picture, but a scrambled or black one. ColecoVision.dk ranks the supply as the most common failure of all, ahead of the video RAM and the switch, with the controller buffers U18 and U19 and the video processor U9 behind them.

Coleco's test sequence

Coleco's overview chart (sheet 1) runs in this order:

1. Connect the console to a supply, coaxial cable and RF switch box, and set the TV to channel 3. Switch on and tune the TV.
2. If the ColecoVision logo does not appear, substitute a known-good supply, cable and switch box. If those were already good, go to No picture.
3. If the logo is scrambled, go to Scrambled picture. If the colour is bad, go to Colour faults. If the picture is wavy, go to Other picture faults.
4. Insert the diagnostic ("final test") cartridge. It tests the system ROM first (a failure means U2) and the RAM (a failure means U3 or U4).
5. Compare the video test and the colour test with the colour photographs on page VI-3 of the manual.
6. Run the sound test, which plays the three tone generators and then the noise generator, and the hand controller test, in which each function on screen blanks out once it has been selected.
7. Play a game cartridge, then leave the console running for at least six hours and test it again.

Coleco's diagnostic cartridge exercises more of the circuit than a game does, so the manual tells repairers not to substitute game cartridges for it.

No picture

Measure the three supply voltages at the board with the switch on. If any is wrong, work through Power supply checks. If all are good, Coleco's sheet 2 checks the video processor's outputs:

No picture (Coleco sheet 2)

Check	What it is	If bad
U9:36	Luminance (Y) output	Check that U9 pins 17-24, the processor data bus, are active. If they are not, go to Logic section; if they are, go to Video section
U9:38	R-Y output	Video section
U9:35	B-Y output	Video section
All three good		RF output

Pin functions from the schematic redrawing.

Power supply checks

The supply plug carries +5 V on pin 1, −5 V on pin 2, +12 V on pin 3 and ground on pin 4. Inside the console the wires are white (+5 V), blue (−5 V), red (+12 V) and black (ground). The switch S2 carries +5 V and +12 V; −5 V goes straight to the board through L4 whether the console is on or off. First substitute a known-good supply; if that cures it, replace the supply. Coleco's sheet 7 then works through the rails as follows. It gives no tolerances; each step asks only whether the voltage is good.

Power supply (Coleco sheet 7)

Switch	Measure	Expect	If bad
Off	Board connection, +5 V	+5 V	Change C48
Off	Board connection, +12 V	+12 V	Change C47
On	Board connection, +12 V	+12 V	Lift one side of WJ2. If +12 V returns, check U21, Q3, U23, C57, C60, C105 and C73. If not, lift one side of L3: if +12 V then returns, check U10-U17 and C36; if not, check C50
On	J4 pin 4 (+12 V to the RF board)	+12 V	If +12 V is good at U10 pin 8, check WJ2 and L10; if it is bad there too, change L5
On	Board connection, +5 V	+5 V	Lift one side of L17. If +5 V returns, check U9, U6 and C106; if not, check every capacitor and chip on +5 V
On	U1 pin 11 (processor +5 V)	+5 V	Check +5 V at U9 pin 33: if that is bad too, check L6 and the on/off switch; if it is good, change L17
On	Board connection, −5 V	−5 V	Lift one side of L2. If −5 V returns, check U10-U17, C21, R24, C26, C27, C30 and C44; if not, check R21 and R25
On	U10 pin 1 and R21 (−5 V)	−5 V	If U10 pin 1 is bad, change L2; if U10 pin 1 is good, change L4

Lifting one end of a jumper or inductor, as Coleco does here, disconnects the part of the circuit fed through it. If the rail recovers, the fault is in that part.

Coleco's technical tips add two faults around WJ2, a wire jumper near the RF shield:

• +12 V shorted to −5 V: check whether WJ2 is touching the test points next to it.
• No voltage at the RF board: check WJ2.

The video RAM rails

The eight 4116 video RAMs take −5 V on pin 1, +12 V on pin 8 and +5 V on pin 9. The −5 V reaches them through L2, with C34 across it, and the +12 V through L3, with C36 across it. Callan Brown notes that nothing else on the board uses −5 V. When the picture is scrambled, measure −5 V at U10 pin 1 and +12 V at U10 pin 8 before replacing any RAM chip.

Scrambled picture

The title screen with no cartridge inserted when one of U10 to U13 has failed (Coleco, Technical Manual p. VIII-2).

The same for U14 to U17 (p. VIII-3).

Coleco's sheet 3 checks U9 pins 25 to 32, the data lines from the video RAM, to see whether they are all active.

• If they are not, check U9, and if it is good, change one of U10 to U17. Coleco notes that any one of the eight can cause the fault.
• If they are, check U1 and U2.

To pick out which RAM has failed, switch on with no cartridge or expansion module fitted and compare the title screen with Coleco's photographs. Each failed chip, U10 to U17, garbles the "ColecoVision" title and the "turn game off before inserting cartridge or expansion module" message in its own way, and the manual prints a photograph of each with the chip to replace. On the board the eight chips sit in two rows of four beside the video processor's heat sink; find each number on the location drawing.

Incorrect scoring in a game is a processor RAM fault: Coleco's tip is to replace U3 and/or U4.

Colour faults

Bad or missing colour (Coleco sheets 4 and 10 and the technical tips)

Symptom or check	Coleco's next step
No colour at all	Measure the frequency at J4 pin 8. It should be 3.57954 MHz ± 100 Hz; if it is not, check the clock circuit (see Clocks)
Donkey Kong's ape, Mario and the barrels purple instead of their proper colours	Replace U9
J4 pin 7 (B-Y at the RF board riser) bad	Check U9 pin 35. If that is bad, go to Video section. If it is good, check C67 on its negative side (if bad, check L8, C63 and R40), then on its positive side. If the positive side is bad, lift the positive leg of C67 and measure again: still bad means changing C67, good means changing the RF board. If both sides of C67 are good, check tracks and solder joints
J4 pin 3 (R-Y) bad	Check U9 pin 30 (as printed; the redrawing puts the R-Y output on pin 38), then the negative side of C66 (if bad, check L7, C62 and R40) and its positive side. If the positive side is bad the chart again says to lift the positive leg of C67, then change C67 if the signal is still bad or the RF board if it is now good. If both sides are good, check tracks and solder joints
J4 pins 3 and 7 good	Check J4 pin 8, the 3.58 MHz colour reference: if bad, go to Clocks. Then J4 pin 6, composite video: if J4 pin 6 is good, change the RF board. If it is bad, check the base of Q6: if Q6 base is good, check Q6, R55, R76 and C68; if not, check U9 pin 36, then L9, C64, R42 and Q6

Steps from Coleco's charts and tips. Coleco's chart writes R40 for both the C66 and the C67 branch on sheet 4; sheet 10 gives R41 for the C67 branch.

Other picture faults

• Wavy picture: R62 must be 270 Ω and C90 120 pF; replace them if they are not. If Q2 is an ITT transistor, replace it.
• Double image: replace U9.
• Vertical lines on the background where it should be solid blue: replace C106.
• The screen blacks out after a game has been playing for a short while: check that C106, the 10 µF tantalum on the video processor's supply, is fitted the right way round, and replace it if it is not.
• Sheet 5 for other picture problems: check the quality and frequency of the 10.7 MHz clock at U9 pin 40. If it is bad, check U8 pin 9; if U8 pin 9 is bad go to the 3.58 MHz clock, and if it is good go to the 10.7 MHz clock. If U9 pin 40 is good, check the sync signals at U9 pin 36: if they are good go to RF output, and if not, change U9.

Video section

Coleco's sheet 9, for faults traced to the video processor:

1. U9 pin 33 must have +5 V; if not, go to Power supply checks.
2. U9 pin 40 needs a good 10.7 MHz clock (see Clocks).
3. U9 pin 34 (RESET/SYNC) must be low. If it is not, check that U7 pin 13 is high; if U7 pin 13 is high check U7 and U1, and if it is not, check the reset switch S1, C3, R7, R9 and U7.
4. U9 pin 13 (MODE) must be good, or check U1 to U4 and U9. U9 pins 14 and 15 (the chip selects) come from U6; if they are bad, see U6.
5. U9 pins 1 (RAS), 2 (CAS) and 11 (R/W) and pins 3 to 10 (the address and data lines to the video RAM) must be good. If any is bad, check U10 to U17, and if they are good, change U9.
6. Finally check that U9 pins 25 to 32 are all active. If they are not, check U9, and if it is good change U10 to U17.

Pin functions from the schematic redrawing.

Logic section

Coleco's sheet 8 checks the Z80 and its support chips:

1. +5 V at U1 pin 11 and U6 pin 16.
2. The 3.58 MHz clock at U1 pin 6, for signal quality and frequency.
3. U1 pin 26 (RESET) must be high. If it is not, check U7 pins 11, 12 and 13; if U7 pin 13 is low, check the reset switch, C3, R7 to R10, R77 and R78.
4. U1 pins 17 (NMI), 16 (INT) and 18 (HALT), then 19 (MREQ), 20 (IORQ), 22 (WR), 23, 24 (WAIT), 25, 27 (M1) and 28 (RFSH), each with a list of the chips to check (U1 to U6, U9, U18 to U20, U22) when one is wrong.

The manual's pictures of signals show what each of these pins should look like on an oscilloscope while the game cartridge shows its blue menu screen.

Coleco's sheet 13 covers U6, one of the two 74LS138 decoders: +5 V at U6 pin 16, U6 pin 5 low (if not, check U7, U6, R35 and R17), then U1 pins 20, 37, 22, 35 and 36. If all those are good, change U6. U6 pin 5 should sit below 250 mV.

No sound

Audio (Coleco sheet 6 and the technical tips)

Symptom or check	Coleco's next step
Distorted sound	Check U20 and the RF board
No sound: U20 pin 7 (audio out) good	Check J4 pin 2 on the logic board side. If it is bad, check R18, C100 and the RF board; if it is good, change the RF board
No sound: U20 pin 7 bad	Check U20 pins 16 and 8 (supply; if bad, go to the power checks), then pins 5 and 6 (if bad, check U6), then pin 14 (if good, change U20). If pin 14 is bad, check R61 and R64 with an ohmmeter: change whichever is wrong, and if both are good, go to the 3.58 MHz clock
No explosion sound in the diagnostic cartridge's noise test	Replace U20
Game runs abnormally slowly	Replace U20

Steps from Coleco's chart and tips.

Controllers

Swap the two controllers between the sockets first. If the good controller fails in the suspect socket, the console is at fault; if it works, the first controller is.

Hand controllers (Coleco sheets 13 to 15 and the technical tips)

Symptom	Coleco's next step
One or more functions operate by themselves	Check U18 pins 3 to 9: if any is not at about 2.5 V, change U18. Then check the second buffer's pins 2 to 8 the same way, and change U19 if any is low
The blue skill-level menu is skipped	The same test with a meter: any of those pins below 2.2 V means a bad buffer
Joystick or left fire button bad on either player	Change U24
Keypad or right fire button bad on either player	Change U24
Only the joystick selects games; the keypad does not	Replace U6
Everything bad on player 1	Check U19 pins 1 and 19: if good, change U19; if not, check U6
Everything bad on player 2	Check U18 pins 1 and 19: if good, change U18; if not, check U6
Spinner (speed roller or trackball) not working	If U24 is a Texas Instruments chip, replace it. Otherwise follow sheet 15 below

For the second buffer, the technical tip prints "pins 2-8 of U18" and the flow chart prints "U9: 2-8"; both send the repairer to change U19, and ColecoVision.dk's retyped tips read U19. The tip gives 2.2 V and the chart 2.5 V. Each port's inputs pass through its own buffer.

Coleco's sheet 15 tests the spinner interface with a spinner tester running. If U7 pin 8 is bad and U7 pin 9 is good, replace U7. For player 1, check U24 pin 11: if it is good, check C61, CR1 and R24. If it is not, check pin 13 (if bad, check C17 and R19) and then pin 12 (if good, change U24; if bad, check U19 and U25). Player 2 follows the same pattern on U24 pins 8, 10 and 9, with C65, CR2 and R27, then C19 and R20, then U18 and U26. When U7 pin 8 is good, the continuation sheet goes on to the transistor stages, Q4 with R69, R72, C101 and C102 for one port and Q5 with R70, R71, C103 and C104 for the other, and U24.

Cartridges

• A cartridge inserted while the console was on: switch off, then on again.
• The console refuses some or all cartridges: look at C70, the 100 pF disc capacitor in the crystal oscillator, for a mechanical obstruction such as the capacitor bent over flat on the board, and replace U5, the decoder that selects the cartridge ROM banks.

RF output

RF (Coleco sheet 10 and the technical tips)

Symptom or check	Coleco's next step
J4 pin 4 not at 12 V	Go to the power checks
J4 pin 3 bad	Check the negative side of C66 (if bad, check L7, C62 and R40); if it is good, check C66, U23 and the RF board
J4 pin 7 bad	Check the negative side of C67 (if bad, check L8, C63 and R41); if it is good, check C67, U23 and the RF board
J4 pin 6 bad	Check U23 pin 11: if good, check Q3, C69, U23 and R55; if bad, check Q6, L9, C68, C64, R42 and U23
J4 pins 3, 6 and 7 all good	Change the RF board
Channel 3 works, but channel 4 only after switching off and on (or the reverse)	Replace the RF board

Steps from Coleco's chart and tips. The RF board is a separate part (Coleco S75748), and its components are listed in the parts list only as "Order RF board".

Clocks

3.58 MHz and 10.7 MHz clocks (Coleco sheets 11 and 12)

Check	Coleco's next step
U1 pin 6 dead	Check U22 pin 8 (if bad, check Y1, U22, C1, C70, R75, R5, R6 and U8), then U8 pin 8 (if bad, change U8), then U8 pin 9 (if bad, check Q2, U8, C4, R13 and R11). If all are good, check R60, R26 and U1
U1 pin 6 on the wrong frequency	Check Y1, U22, C1, C70, R75, R5, R6 and U8. If U22 is a Texas Instruments chip, replace it
U1 pin 6 ringing	Check U22, U8 and Q2
10.7 MHz clock	Check U8 pin 9 (if bad, go to the 3.58 MHz clock), then U22 pin 1 (if bad, check C90, R62, U22, C92 and L14), then U22 pin 3 (if bad, check C91, U22, L14 and C92), then U22 pin 4 (if bad, check U22 and U9)

The processor clock at U1 pin 6 and the colour reference at J4 pin 8 should both be 3.579545 MHz ± 100 Hz. The crystal Y1 is 7.15909 MHz, divided by two in U8.

Signal test points

Test points from Coleco's pictures of signals

Point	Signal	Expect
J4 pin 4	+12 V to the RF board	12 V DC; ripple shown at 5 mV per division, AC coupled
J4 pin 8	RF clock (colour reference)	3.579545 MHz ± 100 Hz
U1 pin 6	Processor clock	3.579545 MHz ± 100 Hz
U1 pin 16	Interrupt	Basically a steady 5 V level
U1 pin 18	HALT	Basically a steady 5 V level
U6 pin 5	Decoder gate input	A DC level below 250 mV
U1 pin 35	Address line A5	Pulses, as on every address line; no pulses means a dead line
J4 pins 2, 3, 6, 7; C66; C67; base of Q6	Audio, R-Y, composite video, B-Y, Y	Waveforms printed in the manual, taken with a game's blue menu screen showing

Values and conditions from the manual's oscilloscope pages, which recommend a scope of at least 35 MHz bandwidth.

Related pages

• ColecoVision
• ColecoVision General Maintenance
• ColecoVision Capacitor Replacement Guide
• ColecoVision Documentation
• Capacitor Failure Symptoms