Author Topic: Commodore (VIC-20, 64, and 128) Computers (Read 19514 times)

channelmaniac · « **Reply #20 on:** July 22, 2017, 02:04:20 PM »

Model: C64
Symptom: Dead

Replaced 1 bad ROM, a bad CPU, a bad PLA, and a bad SID chip. Board would boot but the screen was scrambled and the garbage would be offset on the screen by a few characters as each row was drawn. Replaced a bad 74LS257 at IC13 and tested the board with loopbacks and a diagnostic cartridge.

channelmaniac · « **Reply #21 on:** May 22, 2018, 02:56:26 PM »

Model: VIC-20
Symptom: Dead. Garbage on screen.

Board booted enough to set the border colors and owner had already tested all the socketed chips.

Replaced bad 6116 SRAM at the bottom of memory and tested.

channelmaniac · « **Reply #22 on:** June 18, 2018, 06:28:37 AM »

Model: C64
Symptom: Dead

Board would sometimes have +5 and sometimes +.5v on the 5v rail. Replaced bad power switch and tested. The data bus was stuck low for 7 of the 8 pins. Tested the socketed ICs and replaced a bad VIC-II chip that would come up with some garbage on the screen. Replaced the BASIC ROM that was holding the data bus low and ran the diagnostic cart with loopbacks. Replaced a bad SID chip and retested, including the RF modulator. The RF output was missing. Cleaned the channel selector switch and retested successfully.

channelmaniac · « **Reply #23 on:** April 21, 2019, 12:07:39 AM »

Model: C64
Symptom: Dead

Socketed PLA tested bad. Replaced with a newly programmed PLS100. Still dead. Address lines on the RAM were not correct. Replaced bad 74LS257 IC and tested. Dead. Data bus bit D2 had a bad signal. Replaced bad RAM. Dead. Replaced bad Character ROM and board came to life but had an Interrupt Bad when running the diagnostic cart. Replaced bad 6526 at U1 and successfully tested.

channelmaniac · « **Reply #24 on:** July 17, 2019, 09:33:50 PM »

Model: C64
Symptom: Dead

Found a spot of corrosion on the board next to a DRAM IC. Patched 3 dead traces. Tested the socketed ICs by substitution into a working testbed system and found the PLA, BASIC ROM and Kernel ROM dead. Replaced those ICs and tested. Still dead. Found 2 RAMs with low output voltage and replaced them. Found missing outputs on both 74LS257 ICs and replaced them. Still dead. Found the RDY line to the CPU stuck. Replaced bad 74LS08 IC to fix that signal. Still dead. Replaced the other 6 RAM chips and found that 4 of those tested bad in a handheld DRAM tester.

Board would still not boot and would not boot a Dead Test cartridge. Replaced dead 74LS139 decoder as 2 of the outputs were stuck low. Board booted but the text placement on the screen was bad. Replaced the 74LS258 IC and the board booted normally. Left the diagnostics cartridge running looped diagnostics successfully.

channelmaniac · « **Reply #25 on:** August 01, 2019, 02:20:32 PM »

Model: C64
Symptom: Dead

Replaced a bad PAL chip and tested with loopbacks and a diagnostics cart. Replaced the very gross keyboard and tested again.

channelmaniac · « **Reply #26 on:** May 23, 2020, 11:04:36 PM »

Model: C64
Symptom: RAM Errors

Replaced bad RAM at U23. Replaced one broken keyboard stalk, spring, and keycap. Cleaned keyboard contacts, reassembled, and tested with diagnostics cart and loopbacks.

channelmaniac · « **Reply #27 on:** May 23, 2020, 11:05:57 PM »

Model: C64
Symptom: Dead

Replaced bad Kernel ROM and VIC-II chip. Replaced one broken keyboard stalk, spring, and keycap. Cleaned keyboard contacts, reassembled, and tested with diagnostics cart and loopbacks.

channelmaniac · « **Reply #28 on:** May 23, 2020, 11:07:38 PM »

Model: C64
Symptom: Loud hum in speaker and hum bar in video

Replaced bad 470uf @ 25v capacitor, SID chip, and tested. Replaced a broken keyboard stalk, spring, and keycap. Cleaned keyboard contacts, reassembled, and tested with diagnostics cart and loopbacks.

channelmaniac · « **Reply #29 on:** May 25, 2020, 10:30:41 AM »

Model: C64
Symptom: Dead

Replaced bad clock generator IC and tested with diagnostics cart and loopback connectors. Cleaned keyboard contacts, reassembled, and tested.

channelmaniac · « **Reply #30 on:** May 25, 2020, 10:30:56 AM »

Model: C64
Symptom: Dead

Replaced bad PLA chip and tested with diagnostics cart and loopback connectors.

channelmaniac · « **Reply #31 on:** May 27, 2020, 07:33:47 PM »

Model: C64
Symptom: Dead

Board had previous repairs done as the owner's Dead Test Cartridge identified one DRAM and the Color RAM as bad.

Replaced bad PLA chip and tested. Board booted and ran but had bad text on the screen and a Color RAM failure on the Diagnostics Cartridge and loopback connections. Replaced bad 4066 IC to help, but still had bad text and a Color RAM failure. Checked the owner's work and found a short between pins 10 and 11 of the 2114 Color RAM. Fixed the short and the board passed all tests.

channelmaniac · « **Reply #32 on:** May 29, 2020, 10:26:17 PM »

Model: VIC-20
Symptom: Dead

Tested RAM and ROMs with the Fluke 9010a. CPU tested good in another VIC-20. Was able to manipulate the sound registers and play audio using the Fluke. Tested the output of the video chip and found a signal. Replaced 3 capacitors on the video output to restore the output and adjusted the levels.

channelmaniac · « **Reply #33 on:** June 01, 2020, 04:24:18 PM »

Model: Commodore 64
Symptom: Dead

Replaced a bad 6510 CPU to find a video screen filled with vertical stripes. Replaced physically broken 6567 VIC-II video chip and ran diagnostics with loopbacks to test.

channelmaniac · « **Reply #34 on:** October 09, 2020, 02:05:36 PM »

Model: C64
Symptom: Boots, but with garbage on screen.

Replaced bad RAM at U9 and tested. Replaced broken key plunger, cleaned internal keyboard contacts, reassembled, and replaced missing 2 key cap and spring. Tested.

channelmaniac · « **Reply #35 on:** May 16, 2021, 05:16:57 PM »

Model: C64c
Symptom: Will not read drives

Owner requested the 64 pin custom be socketed.

Installed 64 pin socket for the custom. Board had 2 bad RAM chips. Replaced the RAM and diagnostics came up with Serial Port, User Port, and Cassette Port errors. Replaced a bad 7406 and 2 shorted protection diodes. Ran extended burn in with the diagnostics cartridge.

channelmaniac · « **Reply #36 on:** May 17, 2021, 05:23:50 PM »

Model: C64
Symptom: Dead. Nothing on screen

Replaced bad 8701 clock generator and had clock, but still a dead computer. Replaced bad 6510 CPU and tested. System had no sound. Replaced bad SID chip and ran diagnostics on an extended loop to burn in the system.

channelmaniac · « **Reply #37 on:** April 19, 2022, 12:19:13 AM »

Model: Amiga 2000
Symptom: Battery damage.

Cleaned area around the battery, replaced the damaged CPU socket, and cleaned the CPU legs. Tested.

channelmaniac · « **Reply #38 on:** December 30, 2022, 01:22:47 AM »

Model: C64C
Symptom: Dead. Black screen.

Replaced bad VIC-II chip and tested.

channelmaniac · « **Reply #39 on:** January 19, 2023, 03:02:08 AM »

Model: C64
Symptom: Dead

Dead Test Card did nothing. Reseated the VIC-II chip as it had 2 pins out of the socket. The Dead Test Cart now showed RAM and Color RAM errors. The R/W pin on the Color RAM was stuck high. Replaced the PLA. Still the same errors. Found D0 and D2 were shorted to ground reading approx 230 ohms. Desoldered D0 on each of the three ROMs to disconnect the pins. Tested and found that the Character ROM's D0 was shorted to ground. Replaced the ROM and tested.

News:

NLG Site Navigation Menu

Author Topic: Commodore (VIC-20, 64, and 128) Computers (Read 19514 times)