Commodore VIC-20 Blank Screen Repair

An early VIC-20 board, assembly 324003, with the chip designators printed beside each position

A blank screen is the most common fault on the Commodore VIC-20 and the hardest to trace, because so many parts can cause it. Ray Carlsen finds the VIC chip and the KERNAL ROM the two chips that fail most often, and Tynemouth Software finds the 74LS138 decoders and the 74LS245 buffers behind many memory faults. The procedure below narrows a blank or white screen down to a chip, using observation, a few simple tests and a diagnostic cartridge.

Symptoms

What the screen shows, and the first suspects (Carlsen's list for board 324003)

Screen at power-on	First suspects
No change at all on the monitor	VIC at UB7
Screen darkens but shows no border or characters	KERNAL ROM at UE12
White screen, and an AM radio near the machine picks up little noise	VIC at UB7, or the 7402 master oscillator at UB9
Border but no characters	BASIC ROM at UE11
Blocks or flashing lines where the characters should be	Character ROM at UD7
Characters in the wrong colours	74LS04 at UC2, CD4066 at UD1, or the colour RAM at UE1
Garbage characters, a frozen screen after warming up, or fewer than 3583 bytes free	2114 RAM at UD2-UD6 and UE1-UE6
Garbled or frozen screen at power-on	555 reset timer at UB6
Blank screen	6502 at UE10, 74LS138 decoders at UC4 and UC5, 74LS245 buffers at UD8, UE8 and UF8, 74LS133 at UD9, or a shorted 6522 at UAB1 or UAB3

Carlsen separates a blank screen, which is dark with no border, from a white screen, which is brighter and also has no border or characters. Some cartridges working and others not points to the 74LS138 that decodes the memory blocks.

Safety

The VIC-20 has no mains voltage inside its case; its external supply does. Unplug the supply before opening the machine, and let the regulator heatsink on an early board cool. Check the supply first; see Commodore VIC-20 Power Supply and Regulator Repair.

Tools and parts

• A multimeter, and an oscilloscope or logic probe for the clock.
• An AM radio for the interference test.
• A diagnostic cartridge that runs Dead Test. Tynemouth uses the Penultimate cartridge, which starts Dead Test if its reset button is held at power-on.
• Known-good spare chips, or a working VIC-20 to test chips in.
• A vacuum desoldering iron and sockets for chips that are soldered in.
• A strip of metal and epoxy if fitting a heatsink to the VIC.

Chip positions

Commodore's parts list for board 324003, sheet 2, with each integrated circuit and its designator.

The designators above are from Commodore's parts list for the early board, assembly 324003. The later VIC-20CR board, assembly 250403, keeps most of them, but its main RAM is two 2K chips at U14 and U15 plus 2114s at UD2, UE1 and UE2. The very early board built on the VIC-1001 layout uses a different set of positions; Tynemouth found its RAM at UA2-UA6 and UB2-UB6.

Procedure

Observe

1. Check the supply and the power light first. On an early board the supply is mostly inside the computer.
2. Power up and note exactly what the screen does, using the table above. A working VIC-20 shows its start-up screen within about four seconds.
3. Tune an AM radio to the lowest point on the dial, away from any station, and hold it near the machine with the monitor or television switched off. Normal interference noises mean the processor and its support chips are running, so a blank screen points at the VIC. A white screen with little noise means nothing is running except the master oscillator, because the clock reaches the rest of the machine through the VIC.

Feel and reseat

1. Open the machine, run it for ten minutes and feel the chips. The processor and the VIC normally run very warm. A VIC that stays cold may be faulty; if a replacement also stays cold, check for +5 V on its pin 40 and ground on pin 20. Carlsen once traced a cold VIC to a socket contact on pin 20 that had been bent by an earlier chip.
2. Reseat every socketed chip by lifting each end with a small screwdriver and pressing it back down. Do not lever against the circuit board, and check for bent pins on any chip that comes out.

Remove and swap

1. The machine still starts with the two 6522 VIAs, the character ROM and UC2 removed. Take them out to see whether one of them is shorted and holding the screen blank. Without the character ROM only the border appears.
2. Swap the two 6522s; the fault moves with the faulty chip.
3. Test a suspect chip by fitting it, one at a time, in a working board.
4. Check the power-on reset: if a disk drive connected to the VIC-20 resets when the computer is switched on, the 555 at UB6 is working.

Run a diagnostic cartridge

1. Fit the diagnostic cartridge, hold its reset button and switch on to start Dead Test. It needs the KERNAL ROM and the RAM at $0100-$01FF to be at least partly working; if it does not start, check those first.
2. Read the ROM and RAM results.
3. If a run of RAM blocks fails together, check the address decoding before the RAM: Tynemouth notes that the 74LS133 and especially the 74LS138s are prone to failing and producing exactly that pattern. On one board he replaced a 74LS245 buffer and the 74LS138 that drives the RAM chip selects before finding six faulty 2114s.
4. If some cartridges fail, try games that use different memory blocks to find the block that is not working. Tynemouth suggests Adventure Land (blocks 2 and 3), Defender (blocks 3 and 5) and Donkey Kong (blocks 1 and 5).

Replace

1. Replace the faulty chip and fit a socket. The early boards have fine tracks on the top side; Tynemouth cut the legs off chips he had proved faulty and removed the legs one at a time, which risked the chips but not the board.
2. Fit a heatsink to the VIC. Carlsen epoxies a thin strip of metal to the top of the chip with its ends bent up to touch the shield cover, and has seen many VIC chips fail from overheating.
3. The KERNAL ROM is another common failure. Tynemouth suggests fitting EPROM replacements for the warm-running ROMs while the originals still work.

Testing

1. Power up. The start-up screen should appear within about four seconds with a cyan border, blue characters on a white background, the CBM BASIC banner and 3583 BYTES FREE on an unexpanded machine.
2. Run the diagnostic cartridge's self test for a long period as a soak test, then load from disk and play several games.
3. Try cartridges or programs that use every memory block.

Faults that are not chip failures

• A dark picture or weak colour on a monitor connected by A/V cable. The video level is set at the factory for the RF converter; Carlsen turns the video level control inside the VIC shield up a little, anticlockwise, until just below the point where the colour distorts, and warns not to touch the other controls.
• A white screen with diagonal lines through a television. Carlsen found the Japanese-made RF converters failing from two 10 µF 16 V capacitors inside the module leaking onto the board, and replaces both. Turning the video level too high also overloads the converter and gives a negative image with a bar through it.

Related pages

• Commodore VIC-20
• Commodore VIC-20 Troubleshooting Guide
• Commodore VIC-20 Power Supply and Regulator Repair
• Commodore VIC-20 General Maintenance
• Commodore VIC-20 Technical Manual