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Old 21st Jan 2019, 10:43 pm   #1
Join Date: Jul 2010
Location: Carlisle, Cumbria, UK.
Posts: 66
Default Commodore 64 Garbled Screen

I got the old C64 out a while back, wanting to look into doing some vintage programming. It was all going well for an hour or so when the system froze completely.

I turned it off and back on, and got the blue border but the middle was filled with moving colourful symbols.

I had a quick search online and found that it could be the PLA which can become faulty if left for a long period of time unused and then switched back on.

Based on this, I decided to get a new PLA, so I bought the PLAnkton and a socket and fitted it into the board.

Powered up again, and the problem is still there however now the patterns have stopped moving.

I'm not sure where to go now. It could be RAM or Colour RAM i believe, but I'm not very experienced at computer or logic repair. I'm used to soldering etc, I do a lot of vintage audio restoration.

Any help would be appreciated I'd love to get it going again if I can. I've checked the power supply voltage just in case and the power supply is giving 5.3VDC and 9VAC. The internal voltage regulators are also working well, giving me 5VDC and 12VDC.
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Old 21st Jan 2019, 11:04 pm   #2
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Location: Maroochydore, Queensland, Australia.
Posts: 2,106
Default Re: Commodore 64 Garbled Screen

I am not familiar with this particular computer, but other vintage computers such as early IBM's and the SOL-20 etc.

Most likely this is a RAM issue as the RAM IC's (at least in my experience) are a lot less reliable than the other logic IC's (typically 74LS series) that comprise the computer.

One thing to check initially is for any stuck low or high bits on the RAM IC outputs or any anomalous or intermediate logic states with the scope that should not be there. Also check the data & address bus for stuck bits. Also if the system RAM IC's and video RAM IC's are the same type, you can swap them around if they are in sockets, if you don't have any spares on hand.

A very good place to post your question, if you don't have any luck finding the fault, is on the Vintage Computer Federation/Forum's site where there are many people with the same computer that might have worked through the same fault before, it could save you a lot of time.
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Old 22nd Jan 2019, 1:32 am   #3
Courtney Louise
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Location: Birmingham, West Midlands, UK.
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Default Re: Commodore 64 Garbled Screen

Check out Perifractics video on youtube about checking Commodore 64 ram
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Old 22nd Jan 2019, 9:32 am   #4
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Location: Falmouth, Cornwall, UK.
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Default Re: Commodore 64 Garbled Screen

if you watch youtube video Commodore 64 (C64) Black Screen Repair from time 20mins 30 seconds it shows what I had to do with mine in this situation. Its not my video though.

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Old 22nd Jan 2019, 9:37 am   #5
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Default Re: Commodore 64 Garbled Screen

RAM faults were common on these. However I would also be inclined to check any crystals (running at correct frequency) and I would scope the supply rails...not just rely on a DC voltage check. Make sure that the supply rails are smooth and not carrying any noise (faulty supply decouplers).
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Old 31st Jan 2019, 1:43 pm   #6
Join Date: Feb 2013
Location: Rainham, Kent, UK.
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Default Re: Commodore 64 Garbled Screen

Before you get to involved try a finger test on the ram chips, you might find one running hotter than the others, if so this may be the faulty one.
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Old 1st Feb 2019, 1:51 pm   #7
Join Date: Aug 2009
Location: Liverpool, Merseyside, UK.
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Default Re: Commodore 64 Garbled Screen

Also check the PSU as the Commodore ones were pretty unreliable, 7805 regulator goes faulty s/c and cooks the machine. A sign of this is it works for a little while then develops fault(s), garbage or black screen.

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