c64 - ASSY 250425 Black Screen

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banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

banman wrote: Sat Jul 16, 2022 1:58 am Hi rmzalbar,


Correct!

I found this Sparkfun article on TTL logic levels


https://learn.sparkfun.com/tutorials/logic-levels/all

These C64 boards are very old. Traces and cold solder joints can be problematic in my limited experience.


I managed to take some more observations of my working 250425 C64 mainboard.




I looked at CPU pins 7, 8, 9, 10, 11, 12, 13, 14, 15 then 32 and 37. I did notice that these logic signals on dpalmeira's 250425 C64 mainboard looked unusual.

One will see that CPU pin 37 looks rough. Maybe, as rmzalbar suggests this is acceptable. These signals have been taken from a normal working 250425 C64 mainboard with the SID chip socket unpopulated.






Maybe we need to look more carefully at some of the Logic signals going into and out from the other support logic IC's on the mainboard.


Many apologies!

Something has happened here and some of my observations have disappeared.

Hopefully the signals at CPU pins 7, 8, 9, 10, 11, 12, 13, 14, 15 then 32 and 37 can be seen.


I did notice that these logic signals on dpalmeira's 250425 C64 mainboard looked unusual.




Not sure why this happened.. :o


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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Compare these observations to yours on the respective pins.
I believe pin 38 should show a repetitive signal. Where does this pin tie into?

There could be the possibility of a malfunctioning logic chip holding down (shorting) some signals. I'm not sure if the differences between our sets of observations can prove or disprove this thought.

The reasoning behind what I say is I noticed that in your observations there are some full logic cycles followed by some not so complete cycles.

It also could be what rmzalbar describes in his last post. That is multiple chips being on the same bus.
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

There are 2 things we need to be aware of. One the possibility of a hidden break or connection on the PCB. The other a failure in one of the logic IC chips.



Try this....

Pull out one by one the Basic and Kemal ROM's as well as the RAM chips. At each stage of a chip pull run the oscilloscope on the CPU pins as you have been doing and observe if there is any changes to the signals.
It's important to check for the pin 11 on the VIC II chip at these test stages too. This is the read write line from the VIC II chip.
Check too pin 14 of the VIC II chip, this is the color pin. Check pin 15 of the VIC II chip, this is the S/LUM signal pin.

At this stage. I think checking out changes in areas where we can affect easily, by removing existing socketed chips is better then desoldering logic chips.

I'll check my paperwork and see if I can find any more information about test procedures on the other chips.
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hello, many thank you for your effort and help, I really appreciate it.

Well, I spent time trying to identify if something change checking MPU pins removing one by one the chips as you suggested, but nothing has changed, so I decided to continue removing more and more, and at the end with only MPU, ROMs, VICII, U31 (8701 clock), U8, U20 (556), U6 and U26, I got a change:

A square signal at:

VICII-Pin11 = MPU Pin38
VICII-Pin11.png
VICII-Pin11.png (9.07 KiB) Viewed 1472 times
Sometimes the signal appears for a while and then goes away, and other times on power on it remains. Basically is a 140.6 KHz signal.

I don't know if it's relevant or not, but the interesting thing is that if I try to put on the board any memory, or logic chip like U13, U25, or even U15, U27 U14, U16, or U28, the signal go back again as usual, a single short pulse, and disappears.

I noticed also that when the square signal is present on MPU pin 38, then other pins like Pin 15, 16, 19, 20, 22 and 23 get a square signal. If you remember these pins are high when MPU pin 38 is bad.

To tackle the problem:

1) I will try to find a way to print on paper the circuit in order to remark with a pen all lines that I'm testing on the board, in order to verify every single line with the multimeter and discard any possible cold solder joint or cut track on the board.

2) Regarding the other front, I mean what more to test on signals, I have no idea at this point. I think we need to find a method to discard things systematically.

Thank you again for your help.
Diego
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,


Only too happy to help. I will admit that I'm not the greatest at this stuff, apologies.

That's fantastic progress and very interesting. Well done! :D


You have presented a great deal of useful observational information on what you have just done. It will take me some time to fully digest. I will get back to you on this.

The reasoning behind my request for methodically pulling socketed chips was that the CPU chip read write line is connected to many of the IC chips. I noticed that I was seeing something different to your observations.
I won't commit to any opinions as yet. My thoughts are for the time being looking into why and how the signal on pin 38 of the CPU chip is affected by the other IC chips you have been playing around with.


Have you been testing the pins on the VIC II chip as you run your observations?
I would like to know if you are getting signals on pin 14 (color signal) and 15 (S/Lum signal). Very important for video output.

Please keep observations up on the square signal at:

VICII-Pin11, which you correctly identified as being the same thing as MPU Pin38. ;)


I believe that the dead test cartridge will work with only 3 IC chips. A functional CPU Chip, a good VIC II chip and a good PLA Chip.
I think try out the dead test cartridge with the chips you are removing. Try different combinations. Making sure to leave the CPU Chip, PLA Chip and VIC II chip in place always.
You will not get much activity, however if you see the screen flashing a white screen you are well on your way to a resolution....



Yes, definitely a hard copy on paper is good. Get it on a poster print style. Much, much easier to view and make notes on.

My Epson printer has a function in the settings to print out A4 size that can be joined together for a nice large poster style print. Other printers have similar options in their print driver software. If not there are online resources to help you with this. Just Google something like "how to poster print A4 document" or similar.

It will make it nice and easy for you to navigate through the schematic layout.

Remember too, we don't need to be needlesly desoldering chips we don't have to. We need to narrow down the search area to a group of circuitry where we can fully focus our efforts on.
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Maybe start with all the socketed chips out, even both CIA chips (U1 & U2).

Only leaving in the CPU chip, VIC II chip and the PLA. Then slowly add one chip at a time until the Logic signal from Pin 38 of the CPU goes funny.

Check for the other logic signals on the other CPU pins too every time a chip is added. Look for unusual activity. Use both our observations as a reference point.

Look for the white screen flash, the CPU is trying to execute code but can't find any usable memory..
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Ok, I will try this approach this afternoon. The good thing is that I have all IC socketed at this point. I really need to fix it, I'm learning and trying it for more than 1 year so far :S , and I'm getting almost crazy!

- "Have you been testing the pins on the VIC II chip as you run your observations?
I would like to know if you are getting signals on pin 14 (color signal) and 15 (S/Lum signal). Very important for video output."

Yes, I did it. VIC II Pin 14 remains the same (high with some little ripple on top) all the time while I'm removing more and more chips.

VIC II - Pin 14
VICII_pin14_color.png
(In the good board it's totally different with more clear activity)

VIC II - Pin 15 always stays DOWN with no signal.

I'll put more images with the results of the Dead Test cartridge. (It's possible that I will get a printed version of the schematics tomorrow, I have no a printer here)

Thank you!
Diego
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi,

Just to tell you that if I remove all chips as you mentioned only with MPU/VIC/PLA + clock 8701 + 556 (for ready signal) and with the Dead Test Cartridge, a periodically signal appears on pin 38. Even putting all chips this signal remains while Dead Test Cart. is connected.

MPU pin 15 in YELLOW + pin 38 in BLUE
MPU_pin15_pin38_dead_cartridge.png
MPU_pin15_pin38_dead_cartridge.png (11.43 KiB) Viewed 1455 times

In the meantime in the VICII pin 14, nothing seems to change, here an image in yellow + MPU pin 38 in blue.
VICII-pin14_MPU-pin38_dead_cartridge.png
VICII-pin14_MPU-pin38_dead_cartridge.png (9.56 KiB) Viewed 1455 times
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

I forgot to mention that the Dead Test don't flashes as usual, in all the process of removing or putting chips :(
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,


Do you have any observations on pin 15 (S/LUM) of VIC II chip as you are running the chip swaps?

Sometimes the dead test cartridge can take some time to initialise itself. Maybe keep it in and on for a minute to be sure while conducting observations.

Keep the oscilloscope observations going too.

What is the IC chip that attaches to the serial port?
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