C64 With Black Screen

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C64Person
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Re: C64 With Black Screen

Post by C64Person »

I ordered the new board, and it's coming early next week, so once that's here I can get working on that.

In the meantime, I want to make sure that my power supply is stable, because the new one is coming with a psu but I definitely don't want to use a vintage psu for this. I built my own PSU using a 5VDC wall usb converter and the 9VAC transformer from the Commodore supply. Is the ground line inside the computer supposed to be connected into the ground out inside the PSU? I thought it was connected before, but maybe not.

Recently, the power led on the PSU starting blinking, and the brick had failed. I don't know if I shorted something while working inside the computer or what happened, but I want to make sure it doesn't happen again. This is just an iPad power adapter, so I doubt it just died.

Just a couple of things I figured I should check up on before I plug it into a good C64 and end up ruining it.


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Re: C64 With Black Screen

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The new board came and is running great! I swapped in all the chips I had socketed, and the VIC-II, PLA, SID and CIA's are all good. On the new board, the large chips are all socketed but none of the smaller ones are. Anything else I want to swap I will have to desolder from at least one of the boards. What's my next best step?
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Re: C64 With Black Screen

Post by rmzalbar »

It's great to hear the replacement board is good! Well, you're down to non-socketed components now. Now that you have a working board, using the oscilloscope you can compare activity between the two boards. Are you getting good 5+ volts on the new board compared to the old?

You don't need to desolder the logic chips from the working board, as those are easily available. You can probably find a good deal on ROM and CPU as well. If the CPU is not socketed on the new board, and you aren't confident that you can desolder the 40-pin CPU safely, you probably shouldn't try desoldering it from the working board. One of my 250407's had a socketed CPU and DOS ROM, the other one did not (I desoldered and installed sockets though.)

However: The CPU is a rare failure, more rare than the logic ICs and RAM. The ROM fails more often than the CPU as well. So the easiest chips are the first ones to check.
Last edited by rmzalbar on Tue Apr 21, 2020 10:52 pm, edited 1 time in total.
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Re: C64 With Black Screen

Post by C64Person »

Actually, I'm still only getting 4.41VDC on the new board. Power supply seems good and is giving 5.1V when the computer is off. It's working normally, so I imagine that it's okay?

I can definitely start comparing data lines. It would be easiest if I had both boards running at the same time. What do you recommend with the old power supply that came with the new board? Is there a better approach to making it safe than using the wall brick like I did with the other, or is there a better method for that. Just don't want to have to keep switching the psu between computers.
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Re: C64 With Black Screen

Post by rmzalbar »

I edited my last post a couple times, sorry!
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Re: C64 With Black Screen

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The ROM and CPU are socketed on the new board, but not the old. I could desolder the ROMs if they are likely, and I could probably do the CPU as well, but I too have gotten the sense that this is unlikely the problem. I would like to not desolder chips from the working board though as long as I can avoid it. Is there any way to see if the ROMs are working before removing them, or do they likely just need to be swapped before I will be able to tell?
Basically, I am comfortable soldering if need be, but clearly its safer to not if I don't need to.
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Re: C64 With Black Screen

Post by rmzalbar »

Not really any good way to check the ROM itself while still soldered to the board. However, if you have any old games on cartridges, kickman or something, sometimes those games will work as they start the C64 in Ultimax mode, bypassing most of the internal ROM.

I would still bet on logic or RAM failure first.
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Re: C64 With Black Screen

Post by C64Person »

Okay, I'll start trying to swap RAM and see where that takes me. As for logic chips, which ones would be my best starting point? Unfortunately, I don't have any cartridges except a fastload, which is definitely using internal ROM.

I'll also check RAM data lines once I have my other psu going. I'd still like some advice on that power supply if you have any, because that's really the last thing I want to mess up here!
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Re: C64 With Black Screen

Post by rmzalbar »

The power supplies tend to lose their 5V regulation and go too high. Unfortunately they are potted into a solid lump of epoxy, so there's no preventative refurbishing you can do on the PSU itself. Some people are selling "C64 Savers" which are just a dongle that sits inbetween the PSU and your C64 that monitors voltage and breaks the connection if it goes too high. This is OK if you want to continue to use original power supplies safely, but the cost of these are high enough that you may as well just get a modern power supply and chuck the old one.

- Ray Carlsen, long time professor of C64 repair, makes good PSUs as well as savers. You can get everything from cheap and ugly to expensive and pretty. https://www.carlsenelectronics.net/

- Frankencelery / KevinO makes the "NuBrick." I have one of these and they work nice and look nice. You may need to google a bit for the best way to contact him, but he can be contacted on the citrus-fruit forum, vcfed, and facebook.
http://www.vcfed.org/forum/showthread.p ... ck-64-quot

- Finally you can cobble up your own with a pair of wall-warts, one 9VAC and one 5VDC. Some people even hollow out the original case and stuff them in there. The cheapest of Ray Carlsen's options do that for you already for a reasonable price.
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Re: C64 With Black Screen

Post by C64Person »

Okay, I have plenty of wall-warts and that's easy, so I'll just do that.

I will start with checking data lines on RAM, then start swapping RAM/Logics.
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