Commodore (VIC-20, 64, and 128) Computers

[channelmaniac]

Model: Commodore 64
Symptom: Boots normally but all text on screen flashes with random colors

Replace the PLA IC, part # 906114-01 (a.k.a. 82S100). It controls the color RAM (2114 SRAM) access.

If the screen looked normal with the exception of some wrong colored areas of the screen then replace the 2114 SRAM IC.

[channelmaniac]

Model: C64
Symptom: Dead, blank screen.

Reset line was stuck low. Found a short between the 7406 IC and the metalized cardboard RFI shield. Attached a thin piece of posterboard to the shield with double-stick tape and trimmed the pin on the IC.

[channelmaniac]

Model: C64
Symptom: Dead. Blank screen.

Found multiple machines with this symptom: Replaced bad VIC II chip on one, replaced a bad PLA chip on another, Replaced a bad 74LS629. Could also be caused by a defective crystal. PLA chip is the most common suspect.

[channelmaniac]

Model: C64
Symptom: Screen & border showing, but no text

Ran into different causes of this. One machine had a defective 4164 RAM IC, another machine had a defective PLA, and another a bad Character ROM. PLA is the most common cause for this.

[channelmaniac]

Model: C64/VIC-20
Symptom: Missing keys when typing

Commonly this is caused by dirty switches on the keyboard. If it is a single key or multiple keys that aren't in a pattern, then press them repeatedly for a few minutes to let the self wiping contacts clean the switch. If the keys are in a row or column then replace a defective 6526 CIA chip on the C64 or a 6522 VIA chip on the VIC-20.

[channelmaniac]

Model: C64
Symptom: Will not read/write to/from the serial port

Replaced a bad 6526 CIA IC and tested with diagnostics.

[channelmaniac]

Model: C64
Symptom: Missing audio channels, stuck sounds, or no audio

Replace the defective 6581 SID chip. In rare cases of no audio at all it may be the 906114-01 PLA IC. (It is a custom programmed 82S100 chip)

[channelmaniac]

Model: VIC-20
Symptom: No audio, missing sounds, or stuck audio

Replace the 6560 VIC IC. This IC does both audio and video functions.

[channelmaniac]

Model: C64
Symptom: Multiple RAM errors

If a machine has more than 1 bad RAM chip (4164) or comes back for another repair of yet another defective RAM chip then replace the external power supply. They are known for deteriorating and blowing RAM chips on the computer. They are potted and not repairable. Luckily they can be easily found.

[channelmaniac]

Model: SX-64
Symptoms: Dead. Drive light stuck on.

This one was a bugger. It had liquid spill damage.

Replaced the corroded cartridge slot, 2 40 pin sockets, 1 24-pin socket, and 2 28-pin sockets. Cleaned the pins on the VIC-II chip. Replaced a bad CPU, PLA, SID (would work, but had no white noise channel), 74LS08, 74LS06, 74LS139, 74LS258, 2x 74LS257, and 8 DRAMs (2 were bad, but went ahead and replaced them all.)

Replaced a bad 901229-05 ROM on the disk controller board to fix the drive light issue.

[channelmaniac]

Model: 64
Symptom: Dead. Garbage on screen.

The board was sent with symptoms described as "Blank blue screen."

When put on the bench it would throw garbage on the screen - a lot of @ symbols and some were flickering. The well calibrated finger found one of the DRAM chips was shorted and getting hot.

Socketed and Replaced the DRAM chip then got the reported blank blue screen. Tried a diagnostic cart to see if it would “play blind� but it wouldn’t. This ruled out a character ROM issue.

Tried a Music Composer cartridge and it tried to start it but couldn’t. The screen went garbled on graphics mode. This pointed to a PLA chip problem. Socketed and replaced the PLA then ran diagnostic loops on the board.

Informed the customer that frying DRAM chips is a pointer that the power supply may be dying.

[channelmaniac]

Model: C64
Symptom: Dead, black screen

Replaced a bad 901225-01 ROM. Board failed audio tests. Replaced bad SID and ran diagnostics in a loop to burn in unit.

[channelmaniac]

Model: C64
Symptom: Dead, Black screen

Replaced bad PLA chip and ran extended diagnostic loops to burn the unit in.

[channelmaniac]

Model: C64C
Symptom: Reports incorrect RAM count

When running diagnostics the RAM errors are random and sometimes showing Address Bus errors.

Replaced 2 4464 DRAM. No change. Replaced the 251715-01 custom IC to repair board. Ran extended diagnostic loops to burn in board.

[channelmaniac]

Model: C64
Symptom: Totally blue screen on monitor

Background and foreground were all one solid blue color. Replaced the VIC-II (6567) chip and ran diagnostics in a loop to burn in unit.

[channelmaniac]

Model: C64
Symptom: "Bad RAM 7"

Removed and socketed the RAM chips. Replaced 1 bad RAM and tested.