C64 With Black Screen

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C64Person
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Re: C64 With Black Screen

Post by C64Person »

I connected the C64 to a modern display, and it seems to be outputting a mostly black screen, though occasionally some white bars are flickering across the screen. Not sure if this could mean anything.

Should I start desoldering suspect chips to see if anything changes the voltage? Kind of risky, but is that the best bet at this point? Is there a better way to test without full desoldering, as well as any often problematic chips to start with?


rmzalbar
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Re: C64 With Black Screen

Post by rmzalbar »

When you hook it to a modern display, can you tell if its detecting a valid video signal via an onscreen message? On CRT monitor, you can usually turn up the brightness and you'll see the horizontal scanlines if it's drawing a black screen as opposed to getting no signal. If the VIC-II is alive (as it should be due to its independent 5v power regulator) you'll be getting a valid video signal, just that it will be a black screen unless the VIC gets instructions from the CPU to do more.

Going back to the power issue, the absolute minimum voltage is 4.75 volts for 74- and 74LS- series logic, so try to find out why you don't have more than that available as nothing that uses the 5+ input will work until you do. (well.. the CMOS components might have more tolerance.) If you've already swapped the power supply and are satisfied it's good, and you've removed all socketed chips, then the next thing is to begin clipping the +Vcc pin on suspect soldered-in chips. Clip it right at the board so you can easily resolder it afterwards, recheck voltage after each one.

Before I did any clipping, I would also make sure I was measuring voltage at the input jack, to rule out the switch being bad. I would also make sure that I was darn certain nothing was getting warm when left on for a bit. I would clip the warmest thing first.

Capacitors and diodes can also fail short. Capacitors are almost always connected between power and ground, and diodes sometimes are as well.
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Re: C64 With Black Screen

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Well, now my PSU died. The power indicator on the PSU started flashing and the 5VDC brick I was using is dead. I swapped in another 5V brick which is now working. I am currently investigating the possible causes of its failure.

I have not started the C64 yet because I noticed the 9VAC line is not the way I remembered it. With the computer off, pin 6 of the input jack is at 10.10VAC and pin 7 is 0.035VAC. I remembered both pins being at about 4.5VAC. Is this okay or is it a problem?

Once I am sure the PSU is fine again, I will start it and see how it looks on the CRT. The modern display is definitely detecting a video signal, but it is a weak signal, as it occasionally flickers back to no signal before picking it up again.

If I start clipping pins, is a chip with a clipped pin going to be impossible to socket if I ever need to pull it for some reason? I just want to make sure that I am not going to destroy the chip by clipping it.

I could re-cap the board, if that makes sense, but I will wait on that to see if I get anywhere with the chips.
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Re: C64 With Black Screen

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With the power supply disconnected and the power switch on, check resistance between pin 5 on the power input jack (positive probe) and ground (negative probe.) You'll probably see increasing resistance for several seconds as capacitors charge up but I'd like to know what it roughly settles at. If you have a short or a near-short you should be getting a much lower ohms reading than I do through mine. I'll check mine in the same way.

Neither side of the AC is grounded, so checking pin 6 or 7 to ground would give meaningless results (there is some capacitor coupling to ground, and the negative side coming out of the rectifier is grounded.) You should check between pin 6 to 7 only. 10 to 12 VAC is normal, dipping below 9VAC input will begin the starve the 12v DC regulator.

Actually I have problems getting a good picture with the C64 on many modern displays. Weak signal, no color, or rolling picture is common. Is yours NTSC or PAL?

Clipping pins would be a last resort when you've ruled out anything else, and generally on a chip i suspect I will need to replace anyways. I'd only do the Vcc pin. Yes it would make it unsocketable later unless you soldered that pin into the socket, or left that pin out and soldered a jumper wire to where it needs to go. I'd only clip pins on the little logic chips as those are common and easily replaced.

RAM... personally I would prefer to unsolder a suspect chip and use a socket, because those are less easy to get. Whether you can do that and end up with a salvageable part afterward comes down to your skill and equipment. The clip-pin suggestion is meant to make the fault-finding process go more easily with less potentially risky rework.

If you have removed a chip and plan to put it back into a socket afterward, remember to use flux and desoldering braid to "sponge" the excess solder off each each pin of the chip so you don't wreck the socket when you try to force the chip back into a socket later.
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Re: C64 With Black Screen

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When reading resistance between pin 5 and ground, the multimeter reads 2.68 kΩ. How did it look on yours?

That explains the AC readings, looks fine now.

I have no idea whether it is PAL or NTSC. Is there an easy way to check?

I am confident that my PSU is working at this point. The problem with clipping is that I have no idea which chips to suspect. I do have some soldering experience, but not a ton so I would prefer to put it off as long as possible for the sake of avoiding damaging the C64.
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Re: C64 With Black Screen

Post by rmzalbar »

I am getting 4 kΩ. This is after I left the multimeter connected for about 20 seconds, and the reading stabilized as the capacitors have charged. This may be meaningless as neither reading is high enough to flow significant current, the true power draw is only seen when the chips are charged up and their internal logic turns on. But at least it indicates you don't have a serious short somewhere.

PAL will have a VIC-II labelled 6569. NTSC is 6567. You'd need to pry the lid off the VIC-II box to check. There's a copper tab on the lid that touches the VIC-II where some thermal grease is smeared, as a heat sink. There is also a jumper soldered inside that box that selects PAL or NTSC. I would just look at that jumper, that way you don't have to wipe the thermal grease off the top of the VIC. Make sure you put the lid back on with the thermal tab making good contact with the VIC-II, it needs that heatsink.

You're kind of getting to a point where you need another C64 to swap parts with, or at least send the motherboard to someone who does. Where are you located?
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Re: C64 With Black Screen

Post by C64Person »

My VIC-II is 6565, so NTSC. Whoever had this C64 before me broke the thermal tab off of the lid, so I have been unable to get a solid connection. I have been working on getting a new heatsink for it.

I agree that this is getting to hard to diagnose without a board to swap with. I will try to get another board, what's the recommended approach for sourcing a working motherboard?

Thanks.
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Re: C64 With Black Screen

Post by C64Person »

I found a working machine that I will order, but I just wanted to double check something. It looks the same as mine from the outside, but there is no picture of the board; will it be a problem if it is a different assembly or revision of the C64, or should any work?
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Re: C64 With Black Screen

Post by rmzalbar »

Nope, some of the brown breadbins moved the positions of the chips around a bit, but all of the brown breadbins will use the same chipset. If the new machine is PAL, you'd need to swap the crystal also if swapping the VIC-II.
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Re: C64 With Black Screen

Post by C64Person »

Great! I'll order it tonight.
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