Commodore 64 failures - Page 1
Forum section: electronics / general

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 06, 2020, 04:35:02 pm
content: Any real C= hardware user knows that the 28 pin PLA fails often in the C64. It's the first thing I suspect after the power supply-reset-clock check I do.What are the next most failure-prone chips in a 64 or even 128? Any board revision.I simply don't repair enough of these to get a feeling for this. I've never seen a VIC-II fail for instance even though they run crazy hot. OTOH the 78XXs fail because they do run too hot. The CIAs get zapped from ESD sometimes, the DRAMs fail often. What else?

author: DrG
author_rank: Super Contributor
timestamp: April 06, 2020, 05:14:32 pm
content: Not sure I have ever seen an itemized list like that, especially considering the multiple versions. There are, of course, many "fix-it" guides on line.Here is a link to Commodore's service manualhttps://www.retro-kit.co.uk/user/custom/Commodore/C64/manuals/C64C_Service_Manual.pdfAs I recall, C64 serial numbers beginning with RP are "factory" refurbished. I had one and there were numerous wire-rapped add-ons. I looked for some kind of refurbishing guide from Commodore but never found one.Hope it helps.

author: thinkfat
author_rank: Unknown
timestamp: April 06, 2020, 05:31:01 pm
content: Check 'Adrians Digital Basement' on YouTube. He does a lot of C64 repairs.

author: Bud
author_rank: Super Contributor
timestamp: April 06, 2020, 06:02:50 pm
content: Based on repairs available on youtube it can be literally anything including the CPU, ROMs, video and audio processors or glue logic.

author: Peabody
author_rank: Super Contributor
timestamp: April 06, 2020, 08:36:34 pm
content: I had a theory that many of the C64 chip failures resulted from bus contention as the power voltage comes up or drops - two or more chips trying to drive the address or data bus at the same time. The fix I adopted for that was a power supply monitor chip (a Maxim I think), one that monitors the supply voltage and generates a hard reset (which makes all the chips let go) when the voltage drops below, say, 4.6V, and which holds reset low until after the voltage comes back up to nearly 5V. I never had a problem after installing that.

author: Bud
author_rank: Super Contributor
timestamp: April 06, 2020, 09:43:01 pm
content: Makes sense.

author: Gege34
author_rank: Contributor
timestamp: April 09, 2020, 08:47:24 am
content: Some interesting link:C64 common problems and how to fix itThe Pictorial C64 Fault GuideCommodore 64: how to repair it, step by stepPLA is one of the most common problem but not the only one.

author: Shock
author_rank: Super Contributor
timestamp: April 10, 2020, 04:40:08 pm
content: According to the guys who repair many of them, the power supply failing results in a variety of faults. The capacitors, ram, regulators seem all common EOL type failures. Intermittent joints and electrolyte leakage and corrosion damage on the board is common as well.

author: Sky Shark
author_rank: Newbie
timestamp: March 17, 2021, 12:28:52 am
content: QuoteI own several C128 computers. There is that large 80 pin PLA on the C128 which seems to fail frequently. Next in line for the C128 would probably be the MMU (8722).I am going to guess that the next most common problem would be the DRAMs failing on the C128 motherboards with the 16 individual DRAM chips (the non C128DCR motherboards) on them.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2021, 01:07:07 pm
content: I had one odd failure with my C64, that may have happed more often with the same series: The shielding used some aluminum foil coated paper and this bend with high humidity - in some cases till it touces the board to cause a short (luckily no further damage done).The supplies run rather hot and thus failing electrolytic caps are to be expected after longer use.The circuit to cause a short interrupt pulse from the "RESTORE" key was broken by design - so in the original circuit one needed to hit it hard to make is bounce and workmaybe a case if it's a feature and not a bug.AFAIK there were quite a few VIC chips were the hardware random number generator did not work. It was thus not often used.

author: DrG
author_rank: Super Contributor
timestamp: March 17, 2021, 02:46:47 pm
content: Quote from: Kleinstein on March 17, 2021, 01:07:07 pmYes! I remember that exact problem and going "ohhhhh no" I bricked it! But, as you said, no further damage done. I ended up trimming it.

author: GodIsRealUnless DefinedInt
author_rank: Contributor
timestamp: April 12, 2021, 07:39:09 pm
content: VIC, SID, PLA are always suspect. As is any board manufactured in 1983, the hell year for long lasting C64 boards. As are MOS branded RAM chips etc. where standard parts used were in short supply, nothing like having almost every RAM chip from a different factory on the same board! Plus anything on the board from the factory already mounted in a socket, usually populated with a part Commodore knew was a bit of a dodgy stop gap shoe-in and would need replacing in future.

author: GodIsRealUnless DefinedInt
author_rank: Contributor
timestamp: April 12, 2021, 07:40:21 pm
content: Maybe list the boards themselves? Anyone doing rework is almost guaranteed to lift a trace or break one. 64's were bodge city.