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C64 With Black Screen

Posted: Thu Feb 13, 2020 4:02 pm
by C64Person
I have a Commodore 64 that is producing a black screen video signal. I have done a lot of troubleshooting, and am not seeing any change.

I originally had a power supply that was producing too low voltage on the DC line, so I wired a 5 volt DC power brick (for iPad) directly into the DC pin on the C64 power input, and am still using the original AC from the old psu, which seems to be working fine.

I replaced the PLA in the machine with a brand new PLAnkton, but no change.

There is a capacitor on the board that looks a little burned on the sides (C90), but after brief testing with a multimeter, appears to be functioning.

I am using a fully functional Commodore 1084 monitor, connected into the C64, which flickers normally for a CRT when the computer is off, but when connected produces a solid, black signal. It is connected using an RCA cable into the monitor, and stripped wires coming off the end of the other cable, which are sitting in the Luma and ground pin ports on the 8 pin DIN port of the back of the C64.

Voltage regulators appear to be producing the proper levels. I have attempted to boot the machine without CIAs and SID installed. The red power LED lights when the computer is switched on, as normal.

I have tried a little testing with a digital oscilloscope on some chips, but am not really sure what to look for when doing this, so it hasn't gotten me too far. This is, as of now, the extent of my troubleshooting, and I still have a black screen. Any advice as to what to check and how to check it would be really appreciated. The board is 250407 Rev. B. I am really looking forward to getting this C64 running again! I am happy to provide any further information that might help.

Thanks in advance.

Re: C64 With Black Screen

Posted: Fri Feb 14, 2020 2:13 am
by eslapion
Here: ... to-fix-it/

Go down to the section where it says ANY CHIP CAN CAUSE A BLANK SCREEN.


Re: C64 With Black Screen

Posted: Fri Feb 14, 2020 10:25 am
by rmzalbar
Unfortunately the easiest tool to diagnose a faulty C64 mainboard with is another C64 mainboard to trade parts with. But, there are a few basics you can check above what you've done so far, and if you are OK using that oscilloscope you can check a fair bit deeper.

- Pull out the SID and CIA chips and set them aside, C64 will boot without them, and if any are bad they could be pulling something down.
- Feel the chips to see if any are getting hot, particularly the RAM, ROM, and logic chips. There are 8 RAM chips, so with these you can feel for any that are markedly hot compared to the rest. Hot usually means "ouch hot!" not just pretty warm. It's normal for the PLA, SID and VIC-II for example to get much warmer than everything else.
- look for signs of rework: flux, soldering, socketed ram/logic ICs, etc would be suspect. Inspect and continuity test any you find to make sure whoever did it didn't lift a pad or create a poor solder joint.

If you don't want to get another C64, you can gamble on a "dead test" cart, available on ebay. This tries to flash the screen in code to indicate where the problem is, and can work in many (but not all) black screen situations.

You can use the oscilloscope to test the clock line, reset line, address lines, and data lines to see if there is any activity, or if any of them are being dragged down by a bad chip. Would be a good learning experience.

The page eslapion linked to is a good one.

This guy Adrian has a lot of videos of diagnosing and repairing C64s and similar computers - he uses continuity checking, part swapping, oscilloscope logic probing, and dead test carts. Not a professional by any means, but very good and gives clear explanations of what he's doing, and can make you feel comfortable tackling things you didn't know how to do before.

Have fun! I always do.

Re: C64 With Black Screen

Posted: Fri Feb 14, 2020 9:07 pm
by Gyro Gearloose
A simple logic probe is good enough to check for activity on signals.

The advice to look for very hot chips is spot-on, the DRAMs in particular are the first to fry when the 5V supply fails and the supply outputs more than 5V.

Re: C64 With Black Screen

Posted: Tue Feb 18, 2020 7:58 am
by rmzalbar
When I was a kid, I reinstalled 68000 CPU into Amiga 500 (probably after swapping out AdSpeed or AdIDE or something.) Amiga failed to start. After a few tries, I noticed 68000 CPU was HOT HOT. It was installed backward. Flipped it around, all was good then.

Tough MF that 68000.

Re: C64 With Black Screen

Posted: Tue Feb 18, 2020 2:53 pm
by C64Person
Thanks, everyone, for the quick reply.

I left the computer on for probably a good half hour, and none of the chips seemed extraordinary hot.

There are quite a few socketed chips, including a few that I did when I was testing RAM and removing CIAs.

I am mostly comfortable with using the oscilloscope, though I am a little unsure as to what to test specifically regarding the C64. For the clock, reset, address and data lines that you recommended for checking, what pins should I be checking and what exactly should they show on the oscilloscope?

Another C64 is not out of the question, but I would like to try to see if there's anything else I can do with this one before I go that route.

I will continue to check the provided resources in the meantime.

Re: C64 With Black Screen

Posted: Wed Feb 19, 2020 5:07 am
by rmzalbar
If you've already removed SID and both CIA and confirmed voltages, then I would check reset line next, using the oscilloscope.

When you turn on the C64, pin 40 of the 6510 (U7) will be low for second or so (holding the chips in reset while voltages stabilize and components charge up) then jump to 5V and stay there.

Possible fault conditions could be stays low, too short of a low time before going high (or no low time at all,) or voltages somewhere in between the low/high levels or floating. It's TTL logic so low is <0.8 volts, high is >2 volts. Reset line faults would be caused by the reset circuit itself, or by one of the chips on the reset line failed, pulling it high or low.

Aside from 6510, reset line is used by SID and CIAs, which you should already have out. It also goes to the user port, cartridge port and the IEC port (disk drive) so you shouldn't have anything plugged in there for testing.

BTW, don't get confused which socket the PLA and the SID plug into. They are both 28-pin chips and some board revisions have their location swapped. It's not entirely impossible you received a computer where the owner has done this and never figured out what the problem was, so it's worth looking to see where PLA is plugged into. PLA socket is silkscreened U17 on the board, SID is U18. Swapping them destroys them. May as well doubletake and make sure socketed chips aren't plugged in backwards.

Re: C64 With Black Screen

Posted: Wed Feb 19, 2020 7:53 am
by rmzalbar
If reset is good, then I would use oscilloscope and check clock lines. 6510 pin 1 should be receiving clock, pin 39 should be outputting a clock signal too.
At the VIC-II, check for clock at pins 21 and 22.

Then you can start checking address and data lines on RAM. Should be getting good high and low square waves, any that are much different voltage than the rest can mean a bad RAM chip or bad LS logic chip.

About those LS logic chips.. if any of those are marked as MOS manufacture, these had a high failure rate and are suspect. One is pictured here:

Re: C64 With Black Screen

Posted: Thu Feb 20, 2020 4:36 pm
by C64Person
Pins 1 and 39 of the 6510 both have clocks at 1.02MHz. Pin 21 of VIC-II has ~14MHz and 22 has ~8MHz. Does that sound normal?

The reset line stays at about 0.4V for just a fraction of a second before it jumps to about 5V. It doesn't stay low very long, but I'm not sure if the oscilloscope can refresh at the rate required to see exactly how long its staying at .4V.

The RAM address lines are definitely not showing square waves, they seem in fact really messy. I attached a couple of pictures of the oscilloscope on RAM address pins.

There are no MOS logic chips. Only the larger chips are MOS. The PLA is definitely in the U17 socket, but I'm glad that I checked and didn't accidentally destroy these by inadvertently swapping them. Good to know.


Re: C64 With Black Screen

Posted: Thu Feb 20, 2020 8:43 pm
by rmzalbar
Clocks are good.

The reset signal seems too short. There's a 555 timer, and the width of the reset pulse is defined by capacitor C24 and resistor R34. Looking it up, it seems that is should be about 1/2 second, shorter than I seem to remember.

The square waves are fine in the first two images. What are you measuring on the last image? It looks low. I'll take measurements on my own breadbin for you as a reference. I'll time my reset pulse too.