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Got a C64, black screen

per

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I recently got a 1985 breadbin C64, and it's giving me the black screen on boot.

From what I've read, a whole lot of things can go wrong and give a black screen. I've tried to eliminate a few things, and it seems like I might be in luck. I'll not get my hopes completely up yet though.

1. The voltages are OK (tested both AC and DC) allthough the PSU plastic case is *slightly* deformed by heat.
2. Power LED comes on, so the system is receiving power and not screwing it up.
3. When turned on, there is some brief activity on the disk drive. A sign of life, so maybe the CPU might be able to reach the ROM kernel?

I suspect a bad RAM chip, but I'd love some more input from C64-experienced people on this.

PS. This one is the big-board version, using regular TTL instead of custom chips as glue logic.
 

KC9UDX

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The disk drive is totally irrelevant unless you get to BASIC and you issue disk commands. Nothing in the KERNAL will cause any disk activity on its own. I'd sure disconnect it until you find out what is going on.

But, I agree: PLA.
 

tkc8800

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While PLA is a common failure, don't bank on it being the problem. I recently had a black screen on a breadbin and I ordered a SuperPLA for it, only to find that wasn't the problem. Turns out all the major chips including the PLA were fine. I even tested the ram using the piggy back trick and that was ok too. I was lucky enough to get a second working machine and could test all chips. This is really the only sure way to know what major chips are bad. Helps also if they're socketed as most of mine were.
 

KC9UDX

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While PLA is a common failure, don't bank on it being the problem. I recently had a black screen on a breadbin and I ordered a SuperPLA for it, only to find that wasn't the problem. Turns out all the major chips including the PLA were fine. I even tested the ram using the piggy back trick and that was ok too. I was lucky enough to get a second working machine and could test all chips. This is really the only sure way to know what major chips are bad. Helps also if they're socketed as most of mine were.

So what was the problem?
 

per

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It's a 250425 motherboard, so seems like some of the PLA replacements are available. Would be great with a RealPLA since it duplicates the original PLA's timing.

When poking around with a logic analyzer, I don't seem to get any clock at the CPU at all...

Just for the record, I got a bit of software with the machine as well (on original disks/cassettes):
  • Commodore Logo Utilities (I guess the main Logo program disk is missing)
  • Commodore 1541 test/demo disk
  • "The Ship" text-based game by Zyntax Software
  • "Imagination" sprite animation tool by George Adams
  • "Tool 64" BASIC Extension by Micro Application (the cartridge is unfortunately missing, so only got the manual and a ROM image from the internet)
  • "Matchday II" on cassette
  • "Pinball Simulator" on cassette (missing case and sleve)

Tried to record the tapes with my tape emulaor, but I don't seem to receive any data from the datasette...
 
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ClassicHasClass

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I suppose a CPU failure is possible, but they've been quite rare in my experience. It may not be getting a signal from something else.
 

tkc8800

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So what was the problem?

I still don't know what's wrong with that board. Could be caps or some other soldered component. With the benefit of having two machines I was able to use the motherboard from the second machine and swap out some parts from both to get a fully working C64.
 

per

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It may not be getting a signal from something else.
Might be that. It does not get hot at all, so I don't think it's grounding the clock input.

The CPU get its clock from the VIC2 chip, which in turn get its clock from U31. Only problem is that both of these is under the metal shield soldered to the board, and I don't have proper desoldering tools at the moment.
 

per

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More news:

Probed the clock cirquits, and seems like the 8701 at U31 is malfunctioning. On reset it first slowly toggles the color and dot clocks at around 1 KHz, then after a millisecond of doing as such, it clocks at a few MHz for some uS before quickly just pulling both clocks high. Supply voltage is good, reset input is good, and there is good connection to the crystal.

The crystal itself might be bad as well, but I've never heard of such a case.
 
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KC9UDX

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The C64 clock is downright weird. I had a bad chip cause a clock problem, which fried the PAL, VIC-II, and probably SID.
 

per

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The C64 clock is downright weird. I had a bad chip cause a clock problem, which fried the PAL, VIC-II, and probably SID.
Ordered a 8701 replacement, and I'm going to test it without the VIC-II installed when it arrives. That way interference from other potentially bad chips should be eliminated.

Otherwise it seems like the VIC-II does attempt to divide the random some-MHz-clock I described correctly according to the divisor factor (during the short few-uS period this happens).
 

CalculatorLab

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I'm getting two C64s in their boxes along with other add ons (printer, software, disk drives, etc) and the owner says there is no video output. I asked him and he said not a black screen, just no video output. Any ideas what it could be? Since it uses the phono type jack for a rf switch could I try a different switch like from a NES?
 

KC9UDX

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Sounds like he's plugging it into the wrong thing.

You either need a proper C= video cable that plugs into the DIN video port, which will plug into a "video in" on an NTSC monitor or TV

Or

An adaptor to go from the modulated RCA socket to the RF input (antenna) on an NTSC TV.

You don't need a modulator, nor a switch. The NES thing might only work with the NES.
 

CalculatorLab

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Ah, I see. I have a video cable that has the phono to rca adapter that I used with something else. The adapter I used to connect the old plug on an atari 2600 to a new tv with RF
 
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