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

[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.