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NLG Users Repair Logs and Other Ramblings. Request your very own topic. Just ask any site staff. => Channelmaniac's Arcadecomponents' Old School Repair Logs => Topic started by: channelmaniac on May 04, 2014, 08:18:40 PM
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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.
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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.
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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.
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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.
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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.
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Model: C64
Symptom: Will not read/write to/from the serial port
Replaced a bad 6526 CIA IC and tested with diagnostics.
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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)
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Model: VIC-20
Symptom: No audio, missing sounds, or stuck audio
Replace the 6560 VIC IC. This IC does both audio and video functions.
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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.
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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.
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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.
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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.
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Model: C64
Symptom: Dead, Black screen
Replaced bad PLA chip and ran extended diagnostic loops to burn the unit in.
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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.
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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.
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Model: C64
Symptom: "Bad RAM 7"
Removed and socketed the RAM chips. Replaced 1 bad RAM and tested.
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Model: Commodore 64
Symptom: Dead. Black screen.
Replaced bad 6567 VIC-II video controller IC and looped diagnostics to burn in the board.
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Model: Commodore 128
Symptom: Dead
Fixed a bent pin and resocketed the IC at U68. Checked the RAMs and the /RAS was missing from one bank and the /CAS from both. Traced it backwards through the circuits and discovered that the main CPU wasn't initializing the MMU because the data buss line D3 was garbled. Checked continuity and found D3 was missing connectivity to the ROMs and the CIA chip on the left side of the board. Found corrosion next to a ROM socket that broke connectivity for D3 and had damaged the traces for D4 and D5. Patched the 3 data buss lines with 30-ga Kynar wire and tested.
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Model: Commodore 64
Symptom: Dead
Replaced a defective 7701 clock generator IC and booted a diagnostics cartridge. System came up with flashing colors on text, CIA errors, and vertical lines on screen. Reseated the CIA chips and the character ROM to fix those issues and replaced a bad PLA to fix the flashing colors. System would randomly fail the Color RAM test with an Address Error. Replaced the 2114 Color RAM IC to fix that error and left the computer looping diagnostics for burn in.
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Model: Commodore 64
Symptom: Dead
Replaced defective 906114 PLA and tested
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Model: C64
Symptom: Dead
Replaced bad clock generator IC and tested with diagnostics cart and loopback connectors. Cleaned keyboard contacts, reassembled, and tested.
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Model: C64
Symptom: Dead
Replaced bad PLA chip and tested with diagnostics cart and loopback connectors.
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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.
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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.
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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.
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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.
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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.
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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.
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Model: Amiga 2000
Symptom: Battery damage.
Cleaned area around the battery, replaced the damaged CPU socket, and cleaned the CPU legs. Tested.
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Model: C64C
Symptom: Dead. Black screen.
Replaced bad VIC-II chip and tested.
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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.
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Model: C128
Symptom: Will not work in C128 mode, only C64
Computer would run cartridges or if you held down the Commodore key on power up would boot into C64 mode. The C128 boot would result in a normal border but garbage on the screen. Swapping RAM enable lines to switch banks had no effect on symptoms. BASIC Lo and BASIC Hi ROMs checked good. CIA chips tested good. Pulled MMU and found a bent pin under the chip. Straightened pin, tested MMU in the Backbit Pro tester, and reinstalled it. Booted and ran the C128 diagnostics to test.