c64 - ASSY 250425 Black Screen

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dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi, I share here two more videos:

1) Test MPU pins using the 6510 chip from the good board (c64-2) in my bad board c64-1



I think I got similar results...

2) Second video just pins 9,10,11 and 12, removing the logic chips.



I hope it helps.

Thank you so much for your help.
Diego


banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,

Looks like the results are identical between the 2 CPUs. Good to see you pulled the logic IC chips directly attached to the CPU as well. Very interesting.
I will focus observations on my working board at logic signals from pins 7, 8, 9, 10, 11, 12, 13, 14, 32, 37. They seem to interest me the most,
The CPU pins 15, 16, 29, and 38 seem to have some noise on them. I will too check these out on my board.
Out of interest did you move the PLA from your working C64 mainboard across to your faulty board?
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

I just realized something looking at my oscilloscope just now.

Set your oscilloscope to around the 200nS range.
I did have it set like yours at 500nS and the signals looked a little off color. Changing it to 200nS cleaned up the glitches.
I think changing your oscilloscope timebase to 200nS may give you a cleaner looking signal.

See if your signals look any different.
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,



Apologies for the delay. I am not very computer literate.


Please note that the SID socket is unpopulated in all of the following observations. I did this so as to gain access to the right hand side of the 6510 CPU pins. Maybe do the same to your board so as to reflect the same basis of reference as myself.

There are no cartridges, SID or keyboard connected.


I tried to get the oscilloscope setting the same as your Oscilloscope so that we have a similar frame of reference. My machine is very old and will definitely not have the same level of detail or accuracy as your oscilloscope. However I hope we can observe similar wave shapes in attempt to find out where we need to focus efforts to repair your C64 250425 mainboard.

If you look carefully around the perimeter of my oscilloscope screen you will see the machine settings. I believe it's set to 500ns (as yours is), 2V per division, The trigger is set to 2.31 Volts (this was set to auto trigger level by the oscilloscope). I dropped the frequency to the lowest setting of 20MHz,



I have been looking at the 6510 CPU pins 15, 16, 29, and 38 presently. I looked at the Video you posted. It seemed to me that these logic levels you measured at these pins looked unusual.

I observed on my C64 main board these logic levels from the same pins.




Some of the signals looked similar to your observation. Pin 38, I think it's the Memory read write line, had activity on it.

Now on the sheet regarding the 6510 CPU I posed earlier in the discussion says Pin 38 stays High with an occasional pulse (I imagine they mean to a Low logic level).

One can see a predominately HIgh logic level with an occasional dip to Low logic level state in my observations.

Are you able to retake observations on your board and be certain that you are able to get a similar pattern?


I will continue to retest these pins to ensure I am acutely seeing the wave forms.

If you or any one else has any suggestion on oscilloscope settings please fell free to comment, thanks.
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

I have looked some more at CPU pin 38, the memory read write line.


I zoomed out on the time base to 2us to get more of on overall preview on this signal.


I noted that the frequency of the low logic pulse is around 4 MHz.
IMG_20220715_075615.jpg
I don't know why the picture looks upsidedown. It is the correct way when you click on the image.
rmzalbar
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Re: c64 - ASSY 250425 Black Screen

Post by rmzalbar »

I would venture to say that the variation in voltage levels are largely meaningless. Anything above 3.5v is "high" and less than 1.5 volts is "low." There are multiple gates sitting on the address lines, that are part of circuits with somewhat different electrical characteristics. So long as the signal doesn't dwell in the undefined zone between 1.5-3.5v, it's fine.

The very short duration spikes are also likely meaningless. They are much shorter than the clock rate.

I bet you are still looking for a broken signal path, a bad pullup/pulldown, or similar. Clearly the BA line should show activity just to display characters on the screen with a ready prompt, so the fact that it isn't changing means it's more than just bad video output for example.
Smooth operator
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

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.
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hello!

I removed the SID and take some measures as you suggested. Here are the results:

Pin 15
It seems to have some quick activity on start up, and after a second go high as usual.
Here is the pic during this period, some time is a almost 4v square signal, sometimes it looks like different.
pin15.png
pin15.png (8.56 KiB) Viewed 1408 times
pin15b.png
pin15b.png (8.57 KiB) Viewed 1408 times
Pin 16 nothing special, same as before:
pin16.png
pin16.png (7.91 KiB) Viewed 1408 times
Last edited by dpalmeira on Sun Jul 17, 2022 11:13 am, edited 1 time in total.
dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Pin 29 nothing special
pin29.png
pin29.png (7.72 KiB) Viewed 1405 times

Pin 38, interesting thing:

1) A similar pulse of yours appears on startup, but after a second no more pulses comes:
pin38.png
pin38.png (8.66 KiB) Viewed 1405 times

In 2us here is the pic on start up also...

pin38 2us.png
pin38 2us.png (8.98 KiB) Viewed 1405 times

2) Finally, if I put my Dead Test cartridge, then pin 38 have this pulse periodically every 2 sec approx.


I don't know how to interpret this.
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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