C64 With Black Screen

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C64Person
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Re: C64 With Black Screen

Post by C64Person »

C24 appears okay, measures at about 12uF. R34 too seems fine, 47KOhms. I'm not sure if this means reset line is okay or there is a problem with it.

The low signal on the RAM (3rd picture) is coming from pins 9 and 13 on all RAMs, about 3V instead of the 5V that the other pins are showing. However, the U9 RAM appears scratched up on top, and is missing half a pin on pin 13. I repaired the pin with a little solder, and checked for continuity, and it is now making contact. However, upon checking with oscilloscope, pin 13 on U9 is still reading nothing.

Thanks for the help.


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Re: C64 With Black Screen

Post by rmzalbar »

The amplitude should be the same for all address lines. Both of those address lines are serviced by the 74LS258 multiplexer at U14, pins 4 and 7. None of the other address lines are.

BTW, RESET is a 1k pullup, so you could force a reset by just by grounding pin 40 of the CPU for 1/2 second or so, but I bet your problem is with the DRAM, address lines, or multiplexer you're looking at.
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Re: C64 With Black Screen

Post by C64Person »

Yes, those pins on U14 are showing the signals seen on the RAMs. Does this mean the multiplexer is bad? Should I replace it?

I assume since pin 13 on U9 is dead, I should replace that chip as well?

Would forcing the reset cause the computer to load properly if that is the problem?

Sorry for the clarifying question, I just want to make sure I understand right.
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Re: C64 With Black Screen

Post by rmzalbar »

If the reset were the problem - not resetting long enough - then yes forcing a manual reset should allow it to boot.

One bad DRAM can cause the computer not to start, so all obvious issues with the DRAM need to be solved before moving on. A pin with no signal might mean it still doesn't have a good connection, or the dram could be bad, and any bad part on a line could be dragging it down. Can you tell why the connection went bad to begin with? Does it look as though it may have gotten hot at that pin?

Those lines also go to another multiplexer, 74LS257 at U13, but that multiplexer handles other lines too and it seems suspicious that both of U14s lines are crushed.

So I would investigate the dodgy connection at DRAM U9, DRAM U9 itself, and the multiplexer at U14 (also make sure multiplexer is getting 5vdc supply and good ground.) You can clip a pin carefully just above the solder joint to cut a suspect part out of the address line, then check signal quality and resolder if it wasn't the culprit.

If you remove any chips to replace them, install a socket first.
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Re: C64 With Black Screen

Post by rmzalbar »

Just did a little testing. I get 4V peaks on all address lines, your pin 9 and pin 13 should look no different than the rest, and are definitely showing a problem.

Reset pulse is a little less than 1/2 second. You probably need to turn off the trigger or raises it above 5V to keep it from "freezing" your display to get a real-time look at that.
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Re: C64 With Black Screen

Post by C64Person »

Since all the RAM shows this, should I assume the multiplexer is bad, or is there something else it would point to? I will replace U9 since I am getting nothing from 13.

What do you mean by turn off the trigger?

Thanks for checking.
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Re: C64 With Black Screen

Post by rmzalbar »

Each address line is connected to the same pin on all RAM chips in parallel. A problem with any chip will show up everywhere along the address line you test, so you wouldn't necessarily know which chip is bad unless you started removing them from circuit. For example, if you suspect U9, you'd clip its pin 13 at the board, so it's out of circuit and then test the line from some other point. If it was dragging it down you should see improvement. Normally when you clip a pin, you try to do it in a way that you can reverse it easily with a dab of of solder.

Since you already suspect U9, I'd clip those two address lines on that one first and then recheck the signal before you replace it completely.

When removing a bad chip, it's easiest to clip off all the pins and then you can unsolder and remove the remnants one by one.

Are you working from a schematic? Page 33 of this PDF shows the DRAM schematic for this board.
https://www.retro-kit.co.uk/user/custom ... Manual.pdf

You said you weren't sure if the scope was giving you a good view of the reset pulse. When trying to get a live view from the oscilloscope, such as to watch the reset pulse, sometimes when the scope triggers on the edge of the pulse to do a capture, it freezes the display for a moment. If it's in that mode, turn off trigger or at least adjust it up above 5V so it won't trigger when the reset pulse goes high. Then you'll get a live view.
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Re: C64 With Black Screen

Post by C64Person »

I bent up 13 on U9, and there was no change on the rest of the line, yet there is still no signal from that pin.

I figured out the trigger on the oscilloscope, and the reset seems to be staying low for about 1/2 second. The forced reset didn't do anything, most likely because the RAM is still having problems.

I will continue to do some more detailed testing of the RAM over the next few days. 4 of them are in sockets, so that makes testing a little easier. I know there are some fragile looking traces near the RAM which appear to have good continuity but I will inspect and see if any of those are at 9 or 13 on any of the RAMs. After brief inspection, it appears that there are some burned away solder pads where the sockets for the RAM are. The board around them are burned from soldering and many traces are exposed, though still look to be properly connected.

I have not been using a schematic, mostly because I’ve never used one before.
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Re: C64 With Black Screen

Post by rmzalbar »

The address signals are generated by the mutiplexers and received by the RAM, so you should not expect to see anything on the DRAM pin after lifting it from the board.

On the schematic, the MA-lines are the address lines, and each one runs to the same pin on each DRAM. The pins are numbered on the schematic, you'll see the DRAM address lines MA6 and MA7 are pins 13 and 9.

The data lines are independent (D0 through D7), one for every DRAM.

The fact that some RAM is in sockets means work was done on the board, and burned away pads are maybe a problem, check continuity to be sure. You can solder jumper wires from pin to pin to bypass a bad trace or pad. So inspect carefully, check continuity from point to point following the schematic.

Each address line from the multiplexers to each corresponding DRAM pin. There's a 33-ohm resistor pack in between the DRAM and the multiplexer, so expect that resistance to show up when checking. You can see that on the left side of the DRAM on the schematic where the address lines come out. They loop through the resistors and then pass to the multiplexer to the left, and also "under" the DRAM to the multiplexer on the right.

Each data line independently, all 8 of these go to the VIC-II can be checked to the corresponding pin there. Right side of the page I mentioned earlier.
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Re: C64 With Black Screen

Post by C64Person »

I made progress. Pin 13 on U9 is not connected to pin 13 on any other RAM, while pin 13 on all other RAMs are connected to each other. I will solder a jumper wire between the two and see if that pulls that line back up. Pin 9 is connected on all, so I will continue to inspect that.
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