Commodore VIC-20 Power Supply and Regulator Repair
The Commodore VIC-20 was built with two power arrangements. The early board, assembly 324003 with a two-pin power socket, takes 9 V AC from its external transformer and makes its own +5 V with a bridge rectifier and a regulator under large heatsinks. The later cost-reduced board, assembly 250403 with a seven-pin DIN socket, takes +5 V DC and 9 V AC from a Commodore 64 type power supply and has no regulator of its own.[1][2] Ray Carlsen finds that power faults account for many VIC-20 failures.[1] The procedure below finds the fault on each board type.
Symptoms
[edit | edit source]- The machine is dead and the red power light stays off.[1]
- The fuse blows again as soon as a new one is fitted.[1]
- A wavy picture with hum in the sound, on an early board.[1]
- The power light is dim, or the machine works with a Commodore 64 supply that is marginal for the VIC-20.[1]
- On a later board, everything works except the cassette motor and the 9 V AC on the user port.[1]
- Intermittent operation from a loose fuse or power plug.[1]
Safety
[edit | edit source]Warning: the VIC-20's external power supply is connected to the mains. Never connect the computer's power socket to anything except its own supply.[3] Tynemouth Software notes that early VIC-20s carried a label that seemed to say the machine took 117 V AC directly, when it takes 9 V AC from the transformer, and that the two-pin socket resembles the shaver plugs of the time.[3] On an early board the regulator and bridge heatsinks run warm, so let them cool before handling the board.[3][1]
Tools and parts
[edit | edit source]- A multimeter that reads AC and DC volts.[1]
- A replacement fuse: Commodore's parts lists give a 3 A slow-blow fuse for F1 on the early board and a 1 A slow-blow fuse on the later board. Carlsen and Tynemouth fit 3.15 A on the early board.[2][1][3]
- For the early board, a replacement regulator and bridge rectifier if needed. Commodore's schematic shows VR1 as an LM323 and the bridge CR4 as a PB101; Carlsen lists an LM323K 3 A regulator and a KBPC802 5 A 50 V bridge.[4][1]
- A soldering iron, desoldering tool and small screwdrivers.[1]
- Epoxy or a nut for the bridge heatsink screw.[1]
The early board's supply
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On the 324003 board the 9 V AC from the transformer comes in at J1, through the filter choke L4, the fuse and the power switch SW1, to the bridge rectifier CR4. The reservoir capacitor C39, 4700 µF 16 V, and the regulator VR1 make the +5 V rail, and the power LED on CN3 is fed from it. VR1's pin 1 is the input, pin 2 the output and pin 3 ground.[4] Tynemouth measured an early board drawing about 1.5 A, which is why the heatsinks get warm.[3]
Commodore's Version E schematic marks the regulator SI-3554M and numbers the reservoir C1, so read the part number on your own board before ordering.[5]
Procedure
[edit | edit source]Early board (two-pin socket)
[edit | edit source]- Unplug the supply and open the case. Check that the power plug is a tight fit in the socket. Carlsen squeezes the plug's contacts gently with pliers if it is loose, taking care not to crush them.[1]
- Check F1. Clean the fuse clips and squeeze them so that the fuse is held tightly; a loose fuse heats its holder, lowers the supply to the board and eventually blows.[1]
- If a new fuse blows, look for a short in the bridge rectifier, then the regulator, then the chips on the board, in that order.[1] A partly shorted bridge can also make the external transformer run hot.[1]
- Resolder the bridge rectifier's joints. Carlsen finds them a common trouble spot and resolders them as a matter of course.[1]
- Tighten the screw that holds the bridge to its heatsink. Carlsen found it loose on every early VIC-20 he worked on, which lets the bridge run hot enough to unsolder itself or fail short. He reaches it by cutting the bottom shield between two holes at the bridge and cutting away the plastic beneath, then fixes the screw with a dab of epoxy or a nut.[1]
- Power up and measure VR1: the input at pin 1 and the output at pin 2, against pin 3. The output should be +5 V.[4]
- Replace VR1 if its output is missing or wrong with a good input. Carlsen notes that a shorted regulator can blow the fuse and damage the chips on the board, and that a failing regulator or bridge can give a wavy picture with hum in the sound.[1]
Later board (seven-pin DIN socket)
[edit | edit source]- Measure the external supply's +5 V DC output before connecting it. The board has no regulator of its own, so the supply feeds the chips directly.[2][1] See Commodore 64 Power Supply Testing and Repair for the supply itself.
- If the power light is off or dim, the +5 V from the supply is low or missing.[1]
- Use a supply with some margin. Carlsen notes that the VIC-20 draws more from this supply than a Commodore 64, so a marginal supply may run a C64 and not a VIC-20, especially with RAM expansions or cartridges fitted.[1]
- If the machine works but the cassette motor and the user port's 9 V AC do not, check the 1 A fuse F1 on the board. If it has blown, look for a short on the cassette or user port; Carlsen usually finds it was caused by connecting something with the power on.[1][2]
- If the fuse is good and the cassette motor still does not run, check the motor driver transistor, which Carlsen gives as Q3, a 2SD880, and calls a common failure.[1]
Testing
[edit | edit source]- Power up. The red power light should come on and the start-up screen should appear within about four seconds, with 3583 bytes free on an unexpanded machine.[1]
- Measure +5 V again with the machine running.[4]
- Leave the machine running and check that the bridge and regulator heatsinks get warm but not hot, and that the fuse and its clips stay cool.[1][3]
- Load from cassette to check the 9 V rail and the motor drive.[1]
Board versions
[edit | edit source]| Board | Power socket | External supply | On the board |
|---|---|---|---|
| 324003 (early) | Two-pin | 9 V AC transformer | Fuse F1 (3 A slow-blow), bridge rectifier, reservoir capacitor, regulator VR1 |
| 250403 (VIC-20CR) | Seven-pin DIN | +5 V DC and 9 V AC, Commodore 64 type | Fuse F1 (1 A slow-blow) on the 9 V AC, bridge for the cassette motor rail; no regulator |
Tynemouth notes that the two-pin connector used on United States machines differs from the one on United Kingdom machines, though both carry 9 V AC.[3] The Commodore VIC-20 General Maintenance page has the seven-pin socket's pin assignments.
Related pages
[edit | edit source]- Commodore VIC-20
- Commodore VIC-20 Troubleshooting Guide
- Commodore VIC-20 General Maintenance
- Commodore VIC-20 Capacitor Replacement Guide
- Commodore VIC-20 Technical Manual
- Commodore 64 Power Supply Testing and Repair
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 Ray Carlsen, "Commodore VIC20 computer - diagnostics and repair", updated April 2023: the two board versions and their supplies, the chip and power component failure list for boards 324003 and 250403, the fuse, plug and bridge rectifier checks, the heatsink screw, and the start-up screen.
- ↑ 2.0 2.1 2.2 2.3 2.4 Commodore Business Machines, The Friendly Computer Technical Manual, Model VIC-20, P/N 990440, hosted at Commodore VIC-20 Technical Manual: parts lists for PCB assembly 324003 (PDF pp. 7-11) and 250403 (PDF pp. 17-21), including the fuse ratings and the absence of a regulator on the 250403 board.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Tynemouth Software (Dave Curran), "VIC20 / VIC-1001 Repair", December 2024: the misleading power label, the US and UK two-pin connectors, the board's current draw and the T3.15A fuse.
- ↑ 4.0 4.1 4.2 4.3 Commodore Business Machines, The Friendly Computer Technical Manual, Model VIC-20, main logic schematic for PCB assembly 324003, sheet 2 (PDF p. 5); cropped as File:Commodore VIC-20 schematic 324003, power input (J1, SW1, CR4, C39, VR1 LM323).png.
- ↑ Commodore Business Machines, VIC-20 main logic schematic, Version E, hosted as File:Commodore VIC-20 Schematic (Version E).pdf.