C64 With Black Screen

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rmzalbar
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Re: C64 With Black Screen

Post by rmzalbar »

Cool!

If nothing is broken, then typically the noise is coming from the head assembly where it slides along the rails. The vibration of the motor stepping causes it to rattle against the rails, amplifying the sound. They should be cleaned, and dry is technically best because that way they stay clean, but it's also the noisiest in that condition. You can quiet things down a lot by putting a small amount of grease or a drop of a thick liquid lubricant on the rails. This will fill in the head-to-rails clearance and absorb the vibrations. Grease will quiet it the most, really thin stuff like wd-40 won't do anything. Most of my 1541s have a drop of synthetic lube on the rails, a couple of my worst offenders have a little bit of silicone or lithium grease (my Atari disk drives definitely have grease)

C64: single-wipe sockets tend to loosen up after repeated use, do any of them feel like the chip goes in too easily or wants to pop out? Dual wipe sockets generally stay good unless you've shoved adapters with round pins down into them, which tends to stretch them out.

The fuse holder near the power switch loosened up on one of my machines, due to the fuse being forced in a little off-center which over-spread one of the clips. That caused me lots of oops-i-bumped-it crashes and black screens for a while until I figured it out. I even could sometimes faintly hear arcing when the computer was on.

I've once had a cracked power jack that wasn't gripping the pins firmly as a result.

A bad power switch could be an issue, they can get grease-gummy or arc-burned inside and not make reliable contact.

Otherwise, it's got to be a dry or cracked solder joint on something. It can be a real pain in the neck, even with the board in your hands and flexing it here and there, to be sure you've isolated it to one area. Use flux if you remake any joints.


C64Person
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Re: C64 With Black Screen

Post by C64Person »

I did discover that the bumping crashes were related to the jumper I had soldered onto U9 back when I realized there was discontinuity from that chip to the other RAM. A little bit of the exposed wire would touch a nearby pin if force was exerted on the board (bumping probably would have caused this too). Now this does not happen; the computer does not crash anymore when I move it!

However, the screen has started running these lines across it that are constantly moving; this can be seen in the picture attached. Everything still operates normally in this state, but the lines are really annoying. Any idea what could cause this?

Lines running across the screen.
Lines running across the screen.
rmzalbar
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Re: C64 With Black Screen

Post by rmzalbar »

Character Rom fault? If you swap the rom from your other machine and it doesn't fix problem, then data/address lines to the rom (actual fault description said it was bad solder joints in the linked case.)

https://derbian.webs.com/c64diag/fault.htm?id=u5_2

It looks like the vertical bars are on the first column of pixels of each 8x8 character block. Each character is 8 fetches of an 8-bit word, each word is one pixel row, so the first bit in each row is "iffy," probably floating somewhere between high and low. So one data line, D0 or D7 (not sure which direction it reads) is probably coming up bad either in the ROM, or on the socket or traces connecting to those pins. Wonder what the scope would show on those data pins. I don't suspect the address lines, because that should produce wrong characters entirely, rather then one bit from otherwise normal characters.
C64Person
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Re: C64 With Black Screen

Post by C64Person »

I have finally had a free moment to work on my C64, and the character Rom was indeed causing the problem. I reseated the chip and this fixed it.

I am still slightly concerned about the socketed chips, as occasionally a bump or moving of the computer causes a system crash, but it is very much better than before. Anyway, it's workable and I can continue to investigate as necessary.

At this point my system is fully operational, and this is quite exiting. Thanks for all the help that got me here! As I continue to work on the system (and maybe even expand my vintage collection), I will definitely return with more questions.

Thanks again!
rmzalbar
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Re: C64 With Black Screen

Post by rmzalbar »

That brings a smile to my face! You're welcome.

So far as socketed chips goes, make sure the pins are clean and not oxidized. You can put contact cleaner along the pins and then partially lift and reseat the chip a couple times, this should use the socket itself to wipe/scrape the pin for a better electrical connection.

If the sockets are single-wipe (making contact on just one side of the pin) they are more prone to issues than the dual-wipe sockets. With single-wipes, its even more important for the pins of the IC and socket to be clean and not oxidized, since it only has half the contact area. If the pins of the socket have become loose, depending on the type of socket you can sometimes use a sharp pin or dental pick to get inside the wipe and spread them out again to restore tightness. I usually replace single-wipes with dual wipe or round pin sockets, but only if there's trouble.
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