Canon V-20 Troubleshooting Guide
Canon's service manual for the UK and French versions of the Canon V-20 (Canon V-20 Service Manual) includes a full set of troubleshooting flow charts. The checks below follow them in order and use Canon's designators. For opening the case, the mains warning and the adjustments, see the Canon V-20 General Maintenance guide.
Order of checks
[edit | edit source]Canon's flow chart asks, in turn, whether the machine operates, whether there is a picture on a television and (UK version) on a video monitor, whether sound is heard, and whether the cassette, joysticks and printer work; each "no" leads to one of the checks below.[1]
Power
[edit | edit source]Start with the mains lead, the power switch and the power LED, then the supply itself.[2]
| Wire | Rail | Expected |
|---|---|---|
| Brown | +5 V | 5 V ±5 % |
| Red | +12 V | +12 V |
| Orange | −12 V | −12 V |
| Black | Ground |
| Symptom | Check | Notes |
|---|---|---|
| Nothing at all, power LED off | Mains lead, power switch, fuse F2 (2.5 A) on the power board | F2 sits between the reservoir C2 and the +5 V regulator U1 (Fig. VI-2). If it has blown, look for a short in U1, C3 or the +5 V load before fitting a new one.[2][3][4] |
| Connectors | CN1 on the power board, J7 on the main board | Both must be seated.[2] |
| +5 V missing or out of ±5 % | The +5 V switching regulator | Check C1 to C3, D1 to D5, R1 to R6, TR1, U1 (STK770), VR1, L1, C10 and C11, then set +5 V with VR1 as in section II-4.[2] TR1 senses current through R4 and shuts the regulator down on overload, so a short on the main board can look like a dead regulator; disconnect J7 and measure again.[5] |
| +12 V missing | The +12 V regulator | Switch off, disconnect J7, then check C4 to C6, U2 (AN7812) and R8. The chart also names D6, which is not in the power board parts list; the +12 V and −12 V rails share bridge D5.[2][4][5] |
| −12 V missing | The −12 V regulator | Switch off, disconnect J7, then check C7 to C9 and U3 (NJM7912A).[2] |
| Picture wavers up and down or is only sometimes sharp | Reservoir capacitors C2, C4 and C7, and the output capacitor C3 | On the MSX Resource Center forum a UK V-20 with this fault was put down to a failing supply capacitor letting 50 Hz mains hum into the video.[6] Measure the rails with an oscilloscope for ripple, then see the Canon V-20 Capacitor Replacement Guide.[4] |
Main board
[edit | edit source]If the rails are correct but the machine does not start, Canon's chart works through the main board:[7]
| Check | Where | Expected or parts |
|---|---|---|
| Master oscillator | U49 pin 3 | 14.31818 MHz |
| CPU clock | U3 pin 6 | 3.579545 MHz |
| Reset | U3 pin 26 | High when running; otherwise check D5, C36, R19 to R22, TR3, U41 |
| Address bus | U4, U5, U51 | |
| Data bus | U23 | |
| Control signals | U40 | |
| Cartridge connectors | CN1, CN2 | |
| −5 V for the video RAM | Pin 1 of U31 to U38 | Needs −12 V first; then R15, D4, C18 |
| Main RAM | Dynamic RAM U8 to U15; multiplexers U6, U7; RAS, CAS and WE from U25, U41 to U44, U46, U47; SLTSL3 from U16 to U18 | |
| PPI | I/O decoders U18, U22, U28, U41, U43; M1, IORQ, RD and WR from U3 |
Canon's diagnostic cartridge, a 2764 PROM run from slot 2, checks the CPU, ROM and video RAM before its menu appears, and different cartridges were made for the UK and French versions.[8]
Picture
[edit | edit source]- Television: check the lead and the channel setting. The UK version's RF output is on UHF channel 36.[9][10]
- UK version: check the RF modulator, the PAL encoder board and JN10. French version: check the RGB encoder board and JN10 and JN12.[9]
- Both versions: check the VDP (U30), the I/O decoders (U18, U22, U28, U41, U43), the crystal frequency against the figures in section II-4, and the video RAM (U31 to U38).[9]
- The VDP clock is set with TC1, as described in the Canon V-20 General Maintenance guide.
Sound
[edit | edit source]Check the television's or amplifier's volume and input first, then the connectors, the mixing circuit (U50) and the −12 V rail, which also feeds the audio amplifier. On the board, check the PSG (U24), its clock, and the control and decoder chips U2, U21, U22, U41, U43 and U46.[11]
Keyboard
[edit | edit source]| Symptom | Check |
|---|---|
| No key works | The keyboard connector; keyboard control U18 and U19; I/O decoders U18, U22, U28, U41, U43; M1, IORQ, RD and WR from U3 |
| Some keys dead | JN1; strobe signals from U19 and U18 (PC port pins 14 to 17) and U52; return signals through JN1, U52 and U18 (pins 18 to 25); the flat cable, the keyboard board and the key switches |
| Lock key LED does not light | Connectors; U39; U18 PC port pin 11 |
[12] The keyboard's flat cable into JN1 can lose its conductive pattern if forced; reseat it with care.[13]
Cassette, joysticks and printer
[edit | edit source]| Symptom | Check |
|---|---|
| Will not load from tape | Recorder and lead; J8 pin 5; input circuit U48, C6, C7, R7 to R14 |
| Will not save to tape | Recorder and lead; J8 pin 4; output circuit R25, R26, R30, C71; PPI U18 PC port pin 12 |
| Recorder motor not controlled | Recorder and lead; J8 pins 6 and 7; R29, TR2, D6, relay RY1; PPI U18 PC port pin 13 |
| Joystick dead | Connectors J3 and J4; the general-purpose output circuit; PSG ports IOA0 to IOA5 (inputs) and IOB0 to IOB6 (outputs) |
| Printer does not work | Printer and lead; J5; control circuit U2, U29, U25; I/O decoders U22, U41, U44, U45 |
Capacitors
[edit | edit source]Every electrolytic on the power and main boards is listed in the Canon V-20 Capacitor Replacement Guide. See also Capacitor Failure Symptoms.
Related pages
[edit | edit source]- Canon V-20
- Canon V-20 General Maintenance
- Canon V-20 Capacitor Replacement Guide
- Canon V-20 Service Manual
References
[edit | edit source]- ↑ Canon, V-20 Service Manual, section II-5 "Troubleshooting" (PDF page 12): flow chart asking in turn whether the unit operates, gives a picture on a TV, gives a picture on a display monitor (UK only), gives sound, and whether the cassette, joystick and printer work, each failure leading to a numbered check. Hosted as File:Canon V-20 Service Manual.pdf.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Canon, V-20 Service Manual, section II-5, power check and power circuit check (printed page 10, PDF page 13): mains lead, power switch and power LED; fuse F2 (2.5 A); connector CN1 on the power PCB and J7 on the main PCB; J7 wires brown +5 V, red +12 V, orange −12 V, black ground; +5 V should read +5 V ±5 %; parts in the +5 V circuit C1 to C3, D1 to D5, R1 to R6, TR1, U1, VR1, L1, C10 and C11; for +12 V, switch off, disconnect J7 and check C4 to C6, U2, D6 and R8; for −12 V, switch off, disconnect J7 and check C7 to C9 and U3. See Canon V-20 Service Manual.
- ↑ Canon, V-20 Service Manual, Fig. VI-2 "Power supply section circuit diagram" (PDF page 70): bridge D1 to D4 (S2V-10) and reservoir C2 (4700 µF, 25 V), then fuse F2 (2.5 A) to pin 7 of U1 (STK770, STK772 or STK7728), L1 (150 µH) and output capacitor C3 (1000 µF, 16 V); bridge D5 (1B4B1) with C4 (3300 µF, 25 V) for U2 (AN7812) and C7 (220 µF, 25 V on the diagram) for U3 (NJM7912); C6 and C9 1 µF 50 V; C11 10 µF 50 V. See Canon V-20 Service Manual.
- ↑ 4.0 4.1 4.2 Canon, V-20 Service Manual, section V-7 "Power PCB parts" (printed page 62, PDF page 69): C1 0.1 µF 50 V ceramic; C2 4700 µF 25 V electrolytic; C3 1000 µF 16 V electrolytic; C4 3300 µF 25 V electrolytic; C5 and C8 0.01 µF 50 V Mylar; C6 and C9 1 µF 50 V electrolytic; C7 220 µF 35 V electrolytic; C10 0.1 µF 50 V ceramic; C11 10 µF 50 V electrolytic; D1 to D4 S2V-10; D5 1B4B1; F2 fuse 2.5 A; L1 toroidal coil; R4 0.22 Ω 2 W cement; R8 56 Ω 3 W cement; TR1 2SC2603; U1 STK770; U2 AN7812; U3 NJM7912A; VR1 500 Ω; ZD1 zener 4.9 to 5.3 V. See Canon V-20 Service Manual.
- ↑ 5.0 5.1 Canon, V-20 Service Manual, section III-10 "Power supply circuitry" (printed page 43, PDF page 46): about 11 V AC from the transformer is rectified by D1 to D4 to about 12 V DC and fed to the chopper regulator U1; zener ZD1 sets the reference, adjusted by VR1; TR1 detects over-current from the drop across R4 and stops U1's oscillator; about 14 V AC is rectified by bridge D5 to about +19 V and −19 V for U2 and U3; R8 across U2 bypasses part of the load. See Canon V-20 Service Manual.
- ↑ Msx canon V20, February 2016—link(accessed 2026-10-21) Opening post by repare 321, 2 February 2016: picture on a UK V-20 fluctuates up and down and is only sometimes sharp. Reply by l_oliveira, 4 February 2016: probably a dying power supply capacitor; if the large smoothing capacitors fail, 50 Hz hum from the mains leaks into the computer and causes wavy interference in the video.
- ↑ Canon, V-20 Service Manual, section II-5, main PCB check (PDF page 14): master oscillator 14.31818 MHz at U49 pin 3; CPU clock 3.579545 MHz at U3 pin 6; reset: U3 pin 26 high, check D5, C36, R19 to R22, TR3 and U41; address bus U4, U5, U51; data bus U23; control signals U40; cartridge connectors CN1 and CN2; −5 V: check −12 V, then measure pin 1 of U31 to U38, then check R15, D4 and C18; main RAM: dynamic RAM U8 to U15, address multiplexers U6 and U7, RAS, CAS and WE from U25, U41 to U44, U46 and U47, SLTSL3 from U16 to U18; PPI: I/O decoders U18, U22, U28, U41 and U43 and the control outputs M1, IORQ, RD and WR from U3. See Canon V-20 Service Manual.
- ↑ Canon, V-20 Service Manual, section III-11 "Test program" (printed page 44, PDF page 47): PROM type 2764 in a cartridge in slot 2, starting automatically at power-on; CPU, ROM and V-RAM tests before the menu; different test programs for the UK and French versions. See Canon V-20 Service Manual.
- ↑ 9.0 9.1 9.2 Canon, V-20 Service Manual, section II-5, TV connections check and computer internal check (PDF pages 15 and 16): cable and channel selector; encoder PCB check: on the UK version the RF modulator, the PAL encoder PCB and connector JN10, on the French version the RGB encoder PCB and connectors JN10 and JN12; VDP area: VDP U30, I/O decoders U18, U22, U28, U41 and U43, the crystal frequency (see II-4), video RAM U31 to U38; for a monitor (UK version), the VIDEO output and the monitor's composite input, video output pin J6, the composite video circuit and the PAL encoder PCB. See Canon V-20 Service Manual.
- ↑ Canon, V-20 Service Manual, section I-1 (PDF page 5): RF out UHF channel 36 (UK version); composite video (UK) or RGB (French). See Canon V-20 Service Manual.
- ↑ Canon, V-20 Service Manual, section II-5, sound check (PDF pages 16 and 17): TV, monitor and amplifier volume and inputs; computer connectors; mixing circuit U50; −12 V check; PSG area: PSG U24, its clock input, control signals U2, U21, U24, U46 and U43, I/O decoders U22, U41 and U43, and addresses A0 and A1 and the data bus from the CPU. Section III-10 (PDF page 46): −12 V feeds the cartridge slots and the operational amplifier for the audio output. See Canon V-20 Service Manual.
- ↑ Canon, V-20 Service Manual, section II-5 (printed page 14, PDF page 17): no input from any key, no input from some keys, and lock key LED checks, with the parts listed. See Canon V-20 Service Manual.
- ↑ Canon, V-20 Service Manual, section II-3 "Reassembly checkpoints" (PDF page 10): when inserting the keyboard's flat cable into JN1, do not let the signal pattern on the cable peel off. See Canon V-20 Service Manual.
- ↑ Canon, V-20 Service Manual, section II-5 (printed pages 14 and 15, PDF pages 17 and 18): cassette loading, saving and remote control checks, joystick check, and printer check, with the parts listed. See Canon V-20 Service Manual.