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Non working C64

per

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I got a NTSC C64 now, but it has certain problems.

When powered on, it sometimes sets up a frame, sometimes not, but in most cases it will fill the display with random symbols and characters before either going to black screen or lock itself in some loop where the screen flickers between two sets of characters. The nature of this is pretty random, and it is even affected if I hold my finger briefely on the CPU or the ROM closest to the CPU.

Now, I do got a PAL C64 of the same motherboard revision, but almost all chips in that one are soldered. The NTSC unit has sockets but I don't want to risk desoldering components on the PAL unit.

This far only the VIC-II has been tested and found good. May this be one of the ROMs failing?

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And here's one little bit of trivia. One of the previous owners of this machine ran it from European 220V AC mains, with the live line connected in series between the C64 and the disk drive. I'm speechless. That won't get you 110V on both devices.

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More testing. SID is missing quite a bit of voices (only one sawtooth voice heard in the title screen of "Alien"). Testing other games confirms that only one single voice works.
 
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tkc8800

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I've recently restored a couple of C64s and I found it very handy to piggy back known good chips on top of bad ones. I was able to diagnose several bad chips like that. Unfortunately for you the good machine has soldered chips. One trick you can try is to insert a cartridge and see if it comes good. Many carts run in a separate memory space and if it's a ram problem this may bring it to light.

I recall from reading that random characters on the screen points to a bad CIA. There are two CIA's you may be able to swap them to check. See here:

http://personalpages.tds.net/~rcarlsen/cbm/c64/c64-ic.txt
 

rittwage

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#1 problem is always the PLA (commonly labeled 906114, but can be an old 82S100 in original units)
#2 is RAM, usually cooked by a bad power supply (this can be diagnosed with a dead test cartridge, or feel for hot chips, or just replace it all)
#3 is failed mask ROMs
 

per

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It's not just random colors. Sometimes there's random pixels and shapes on a black background, then there's just a single color onscreen, other times the image stabilizes with border and random characters inside, sometimes with border and random characters in somewhat of a about 10x2 checkboard column pattern. The last one that might appear most often is border frame with random character, and flickering and lots of lines that jumps all over the place.

Measured the 7805, and it gives 6.2V (!!!)

*edit*

That's the 7805 on the board, not the one in the PSU (that one is known good).
 
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rittwage

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It's not just random colors. Sometimes there's random pixels and shapes on a black background, then there's just a single color onscreen, other times the image stabilizes with border and random characters inside, sometimes with border and random characters in somewhat of a about 10x2 checkboard column pattern. The last one that might appear most often is border frame with random character, and flickering and lots of lines that jumps all over the place.

Measured the 7805, and it gives 6.2V (!!!)

That's not good. May have cooked the RAM at least...
 

per

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It could be one of the LS257s. There's a video on youtube that shows the symptoms of these dead logic chips.
Not sure if that's what you're seeing though.

https://www.youtube.com/watch?v=WG-2SB716-4

Might be a possible case as well. It does got the MOS in-house-made ones, which are known to frequently fail.

I'm thinking about replacing them together with the 7805 tomorrow, in any case. I'll keep ya updated.
 

per

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Go ahead and ignore what was said and start replacing TTL parts that never go bad... Good luck with that. Misguided shotgun repairs always work, right?

Here is the authority on the subject:
http://personalpages.tds.net/~rcarlsen/cbm/c64/c64-ic.txt

Those aren't always generic TTL chips, although they function like them. Did quite a bit of reading on causes for C64 some time ago failure, and second after PLA is usually some of the MOS-made TTL equivalents. Lots of systems use generic TTL instead of the in-house MOS ones, but whenever the logic is failing it's almost always the MOS-made stuff. The semi-random nature of the failure I am seeing also leads me to believe the RAM circuits is involved. I also have to check the RAM itself.

If it isn't part of the failure, it will at least reduce the risk of those chips failing later.

That doesn't mean I'm ignoring the other suggestions. Problem is I have no other working C64 with a fully socketed chipset, so it's difficult for me to test. I can of course buy a new PLA for $40 and hope for the best, but that's quite a bit of money for something if it turns out that's not the problem.
 

rittwage

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PLA's are $10-15, and it is BY FAR the #1 cause of what you are seeing. I have repaired hundreds of C64's, and the TTL logic is at fault in maybe one or two. It's a tiny amount...

I would think if you have no way to properly troubleshoot it, just send it to someone that does, and/or buy another working C64 from a forum member. That should run you maybe $20-40. You are wasting a lot of time and money to troubleshoot it shotgun a piece at a time.
 

per

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PLA's are $10-15, and it is BY FAR the #1 cause of what you are seeing. I have repaired hundreds of C64's, and the TTL logic is at fault in maybe one or two. It's a tiny amount...

I would think if you have no way to properly troubleshoot it, just send it to someone that does, and/or buy another working C64 from a forum member. That should run you maybe $20-40. You are wasting a lot of time and money to troubleshoot it shotgun a piece at a time.

Don't forget I'm in Norway ;) Shipping's often quite a bit (about $15 for small items), and the last few PLA's I have seen on Ebay are around $25 each.
 

tkc8800

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I have three C64's that needed some sort of repairs, black screen, no basic prompt etc. In all cases the PLA's were fine. So while this may be a common problem it's not always the problem. Out of the three C64's I managed to get two working. Even the one I couldn't get working (black screen), had a good PLA, as I tested it in another machine.
 

per

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Did more testing today!

Checked the logic properties of one 74LS257, and it seems allright. Put it back on a socket. I also decided to dump the ROMs, and wouldn't you know; The first half of the Character ROM will read scrambled data, different every read! A12 problably messes with /CS in this one.

I'm using an arduino design inspired by the GBCart read design; it provides standard 16-bit address/8-bit data bus with memory R/W and one IO line. As it was originally intended for game cartridges, I have added software power control and connecors for a cart detection switch as well. It's pretty simple to use it for dumping ROMs, and it can also be used to test stateless logic with up to 16 inputs.

And a side note. I paid attention to the date codes, and it seems like the VIC-II is replaced at one point. It's a 1987 chip while the rest of the chipset is dated mid 1984. I read somewhere Commodore realized an oversight in their manufacture process sometime during 1984, but it seems like at least a couple of chips in this machine are from before that point. According to a quote from Dan Morris, to simplify manufacturing they used too much Boron in the glass sealing the various layers of the die, and the die slowly corrodes as a result.
 

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RobertB

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Shipping's often quite a bit (about $15 for small items), and the last few PLA's I have seen on Ebay are around $25 each.
Francois "Eslapion" Leveille of Montreal, Quebec, Canada produces the PLAnkton for the C64 and C128 and PLAkate for the C16, Plus/4, C116, and 1551. The PLAnkton and PLAkate are super-compatible PLA replacements which require far less power (and thus run cooler) than the original PLA.

This is what he had to say about PLAnkton pricing --

"Pricing for european customers is 12.50 Euros per unit - shipping is 7 Euros.

Also, customers ordering 10 units or more get a 25% discount - that's 10.50$ US per unit or 9.38 Euros"

Truly,
Robert Bernardo
Fresno Commodore User Group
http://www.dickestel.com/fcug.htm
 

per

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Tried to dump the PLA as a ROM, using F# as data outputs and I# as address inputs. It seems like some of the pins there has problems as well. Lots of 0xDF and 0xDE, some 0x5F and 0xD7 etc... but no 0xFF or 0xF in the upper diggit at all. Looks like there is a bit of pattern going on, but from the looks of it, it seems like IO access is pretty much stuck open among other things.
 

Bungo Pony

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The biggest problem I've had with the c-64s is the power supplies. Over time, they have all gone bad and cause random things to happen when powering up the unit.

Try replacing the power supply. There's a guy on ebay selling new replacements for a decent price.
 

per

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The biggest problem I've had with the c-64s is the power supplies. Over time, they have all gone bad and cause random things to happen when powering up the unit.

Try replacing the power supply. There's a guy on ebay selling new replacements for a decent price.

All the testing has been done on a measured good PSU, but I just tested the original PSU for good measure. It delivers nothing short of 15V DC.........

Why on earth didn't they think it a good idea to put a fuse in the 120V brick?!

Anyways, this has turned from "let's identify what's broken" into "let's identify what actually still works". From the looks of it, I need a SID, a character ROM, a PLA and problably the pair of CIAs as well. Dunno about the CPU or RAM yet, but wouldn't surprise me if I need to replace some of those.

OR

I can add the NTSC VIC-II and NTSC-speed Crystal to my working PAL C64 and add a PAL/NTSC switch.
 
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