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Black Screen Issue


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I have a second unit that has the LED lighting up but I have a black screen. The power supply itself, I'm assuming to be fine since the newer and nicer unit boots up fine using it.

 

I'm curious, there are several chips that are socketed already and I could easily swap them out to see if I can easily find a culprit. I'm a little hesitant to do so as I don't want to ruin my working unit. Am I likely to ruin a good, working chip using this method? And is there one chip more likely to be the issue than another. I know it's not as simple as that but if I can just swap chips and maybe find my issue that could be fast and easy.

 

And would I put the non-working chips into the working unit to test or vice versa?

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One likely culprit would be the PLA chip.

 

Writing from Seattle, Washington,

Robert Bernardo

June 10-11 Pacific Commodore Expo NW -

http://www.portcommodore.com/pacommex

July 29-30 Commodore Vegas Expo v13 -

http://www.portcommodore.com/commvex

To quote a previous poster, I am also hesitant to use my good machine to test chips from the non-working machine. Is there a real chance putting the PLA chip from the non-working unit into the working unit could cause damage to other chips in the working unit? I hear about other people using trial and error, seemingly without fear of ruining working parts.

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I agree that the PLA is the most likely culprit. I believe once it has overheated and died, it wouldn't contaminate any other chips, but generally I think it is better to insert a known good chip into an ill computer than the other way around. If it breaks, you have to order 2 PLA chips. If you do the other way around and something went further wrong, you'd have 2 entire motherboards to further troubleshoot.

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I would go one step farther, and say rather than being hesitant to swap chips, don't do it! Specially if you're just shooting in the dark.

Although the PLA might be a logical pick, there are plenty of other chips that have gone up.

You can buy a PLAnkton from a guy on Melon64 and Amibay. It's the most likely the best PLA replacement these days. There are diagnostic cartridges you can buy, but to start- why not check this out first.

 

http://www.commodore.ca/manuals/Tips/c64_tips.htm

 

Hope this helps.....

 

 

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I would go one step farther, and say rather than being hesitant to swap chips, don't do it! Specially if you're just shooting in the dark.

Although the PLA might be a logical pick, there are plenty of other chips that may have gone up.

You can buy a PLAnkton from a guy on Melon64 and Amibay. It's the most likely the best PLA replacement these days. There are diagnostic cartridges you can buy, but to start- why not check this out first.

 

http://www.commodore.ca/manuals/Tips/c64_tips.htm

 

Hope this helps.....

Edited by motrucker
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The B(lack)SOD is a real pain to diagnose. The PLA is the first thing you should check, but there's several potential sources besides that... Kernel, RAM, etc. 7800fan's link is a great place to start.

 

For what it's worth, I've ruined a good chip (A SID, nonetheless) by putting it in a non-working 64, but I've never hurt a working 64 by using it to test potentially bad chips. I may have just been lucky - YMMV.

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As for the PLA, first check what kind of motherboard you have in your non working C64. Assy 250469 boards have the PLA and some other TTL stuff integrated into a big 64 pin chip that is much more reliable than the older 28 pin PLA.

 

For testing in general, trying known good chips in non working boards is a bad idea. Unless you carefully test each and every contact for the correct voltages with a multimeter before inserting the chip you risk damage by overvoltage. There could also be multiple bad chips causing the black screen, if you only test one good chip at a time you might never find that out, so you could as well save some work and test one suspicious chip after another in a known working board.

 

The main risks of the latter method are destroying the good chips when prying them out by static discharge or ripping off traces on the board when desoldering them, but if you know what you are doing those risks are neglectible.

 

Finally, you can always pull out SID to see if that makes a difference, and the older boards should also boot to the startup screen with both CIAs pulled, so you can already test 3 chips without even touching your working C64. :)

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As for the PLA, first check what kind of motherboard you have in your non working C64. Assy 250469 boards have the PLA and some other TTL stuff integrated into a big 64 pin chip that is much more reliable than the older 28 pin PLA.

 

For testing in general, trying known good chips in non working boards is a bad idea. Unless you carefully test each and every contact for the correct voltages with a multimeter before inserting the chip you risk damage by overvoltage. There could also be multiple bad chips causing the black screen, if you only test one good chip at a time you might never find that out, so you could as well save some work and test one suspicious chip after another in a known working board.

 

The main risks of the latter method are destroying the good chips when prying them out by static discharge or ripping off traces on the board when desoldering them, but if you know what you are doing those risks are neglectible.

 

Finally, you can always pull out SID to see if that makes a difference, and the older boards should also boot to the startup screen with both CIAs pulled, so you can already test 3 chips without even touching your working C64. :)

I believe I have one of the original boards. I can remove the SID (U18) and still get blank screen. I cannot easily pull the CIAs since they are not socketed.

 

I'm a novice but I can use a multimeter but I don't know specifically how to check the voltages on each chip. Which pins should have what voltage, etc.

 

Here's a pic of my board:

20170403_1905261_zpsgmv3n9ve.jpg

Edited by Hank Rearden
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C64 will work without SID. Also it will work with one of the CIA missing (can't remember which one) but it does need all the ROM chips, VIC, and some RAM. One bad RAM chip may get you a startup screen showing less than 38911 basic bytes free. Broken or missing color RAM (next to VIC) would still work, you'd get wonky and flickering text colors.

 

Since it's blank, and you mentioned hot chips see if you can safely swap VIC and leave out SID to check later.

 

Warm chip is OK, fry-an-egg hot is not OK.

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