c64 - ASSY 250425 Black Screen

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dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Thank you so much for your help and advices.

I continue working hard on it...

1) I'm 90% sure PCB is ok, I spent many hours testing tracks connectivity following the schematics, etc. In fact, I've discovered that A2 issue on U25 was my fault while unsoldering that chip to install its socket, because I've found a little piece of track in the pin 6 of the removed original chip, but give me a break I just made a mistake unsoldering almost all small chips on the board. :) Honestly, I've study 8 years of digital electronics when I was young, so, I know how to soldering, but I think when I removed it, I haven't the correct tools, now, I have a good unsoldering gun, and good equipment to resolve this mystery.

Regarding logic chips U13 / U25 (74LS257), I've tested them with my logic tester, and they give me an OK too, so I decided to put the original chips to avoid any AlliExpress fake, and nothing change in the signals.

2) On the other hand, I can observe that the missing signals on CPU:
A8 pin 15
A9 pin 16
A12 pin 19
A13 pin 20
A14 pin 22
A15 pin 23

all of them, seems to have activity very similar to the good ones, but only for a second or so before go high, I guess more or less the same time the reset signal appears on CPU pin 40
I wondering what cause this behavior, and why with these address lines only...

Well, I feel we are close to discover what a hell is going on, I've spent a lot of hours with this issue :S

Thank you again,
Diego


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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,


Good to see you are making some progress. Remember it's the journey not the destination that counts.

Don't sweat it about the damaged trace on U25 (74LS257). If I had a dollar for every stuff up I did, I wouldn't need to work. Ha!

I found a good way, for me, to remove IC chips from PCB boards. I will use a solder sucker to remove as much of the solder from the IC pins and PCB holes as I can. Then I'll use the hot air gun on the back of the PCB where the IC is being worked on. This way I can gently remove the IC without getting it snagged and potentially ripping out eyelets and traces.



Okay, the board you say is in good electrical condition. That's one issue we can put to one side.
I will test the signals on CPU:
A8 pin 15
A9 pin 16
A12 pin 19
A13 pin 20
A14 pin 22
A15 pin 23
I will see if I get the same observations you get.

What about Pin CPU pin 9,10,11,12 and 15, are they still not TTL levels?

Unfortunately, the other support logic chips on the C64 mainboard can stop the computer from starting too.
Maybe probe the other support logic chips on the board. Observe the signals going into and then out of these IC chips. Look for unusual signals. Take notes and photos.

One very important point to consider. The C64 mainboard only communicates at TTL levels. CMOS level signals will cause unpredictable results.
That is to say I would recommend using only genuine 74LSxxx type IC chips. 74HCxxx chips are not alowed.
Some Far East sellers rebadge 74HCxxx chips as 74LSxxx amongst other incredibly dodgy practices.

I would only recommend getting from suppliers like Digikey or Mouser.
Having said this I do get some stuff from the Shenzhen markets with mixed results (I don't recommend this as a regular activity).

I believe the reset circuit on the C64 mainboard allows the voltages around the board to stabilize before handing over control to the 6510 CPU on powering up. A great deal of random stuff happens. I will have a look on my board.


Post your observations and I will compare them to my system.
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,

Something I want to retouch on. Do you remember this document I posted earlier in this thread?

download/file.php?id=608&mode=view


One will notice on closer inspection that there are arrows going into and out of these respective chips.

If for example, there is a weak or incorrect oscilloscope signal on these input pins then further investigation on the IC connected to this IC should be carried out.


On the other hand, if there is a weak or incorrect oscilloscope signal on the output pins of these respective 74LS257 IC chips then further investigation on these IC's should be carried out.
banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,

Something I want to retouch on. Do you remember this document I posted earlier in this thread?

download/file.php?id=608&mode=view


One will notice on closer inspection that there are arrows going into and out of these respective chips.

If for example, there is a weak or incorrect oscilloscope signal on these input pins then further investigation on the IC connected to this IC should be carried out. If you think that this IC may be still causing an issue, pull the IC out and test the input pin again. If it is still the same result maybe suspect the IC connected to this chip.


On the other hand, if there is a weak or incorrect oscilloscope signal on the output pins of these respective 74LS257 IC chips then further investigation on these IC's should be carried out.

let me know how you go.
dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hello,

I've just installed a socket on my good "c64-2" board for the U25 (74LS257) and tested my 2 ICs of "c64-1" (the bad board), and they are OK!

So, we have to discard the U13 and U25 chips, they works perfectly.

I'm afraid I'm a little lost at this point, may be your internal chips schemas could help to track the way of lines for the missing signals, but I was testing a lot of lines so far, without succeed. :(
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi,

Investigating the RDY missing signal, it should be generated in the U27 (74LS08) pin 11, but the chip seems to be OK, however the input signals on pin 12 (BA) and 13 (DMA) are different of my c64-2.

Pinout:
http://www.datasheet39.com/PDF/375337/7 ... sheet.html

If we consider that the output on U27 pin 3 is correct and it is using the same DMA as input, then the missing signal seems to be the BA which is connected to PLA pin 26, and VIC II pin 12.

BA.png

BA is always high in the BAD board (c64-1), while it's a square signal in the c64-2

Any idea about if PLA or VIC II is the responsible of generate the BA signal?

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

Post by banman »

Hi dpalmeira,


You are hard at it! That's fantastic....


I have here a printout of the mos6610 CPU Chip.
IMG_20220711_145538.jpg

If one looks closely some of the arrows are two way. This is important to note.

Of special interest to us is the address lines coming out from the CPU Chip. I noted very early in this thread that you had recorded some weak logic signals on the address lines of your 6510 CPU Chip.
I believe your 6510 CPU pins 9,10, 11, 12 and 15 look they are signalling however they don't seem to be going true low.
Please correct me if I am wrong. Test these address lines again for me please and confirm that there is something not good on these lines.


There are many varied failure modes of any of the IC chips on C64 mainboard.
Some will fail open circuit, others will short out. Still others will be a combination of the two.
I have read somewhere that the IC chips can go out of specification. They may not be able to switch at their rated speeds.
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Re: c64 - ASSY 250425 Black Screen

Post by rmzalbar »

dpalmeira wrote: Sun Jul 10, 2022 11:17 am
Any idea about if PLA or VIC II is the responsible of generate the BA signal?

Thank you,
Diego
BA is output by VIC-II. PLA is taking it as an input. High = Ready in this case. If any of DMA or BA is high, it should be high on output side of logic IC also, and all the way to the CPU pin 2. It's not showing a square wave because no instructions are executing in the system due to the mystery fault, so VIC is simply waiting for something to do. BA and/or DMA high but CPU pin 2 still low? Fault somewhere between logic IC and CPU, or fault in the logic IC itself.
Smooth operator
dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hello, I've just created a video with the current status of my MPU pins.



About the BA signal, clear @rmzalbar, thank you for the clarification.
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi rmzalbar and dpalmeira,

That is very interesting and useful information about the BA signal on the input on the PLA Chip, rmzalbar. I must investigate this further on another thread. Thank you.

That is an excellent video dpalmeira, very professional. I like it!
This has all the information I need at the moment. I will need some time to fully correlate your observations against mine.



Straight away however, I can see some weak logic signals on pin 32 (5.40 minutes). Also pins 9, 10, 11, 12 (7.50 minutes) are showing a glitch. Pin 38 R/W line is high, I think that's okay (maybe a little bit too much noise).

Pin 32 is a two way data flow. So there may be a problem at the CPU Chip end or the chip pin 32 connects to.

Regarding pins 9, 10, 11, 12 they are definitely output pins from the CPU. We could pull the logic chips out that are connected to this CPU Chip pins and re-run the observations on CPU pins 9, 10, 11, 12 and see if they are the same.
This may indicate an issue with the CPU Chip or not.

Another quick test is to swap out both the PLA Chip and the CPU Chip with the other known good set you have in your known good machine. Re-run the observations to see if you are getting nice clean logic signals on these pins.


Always remembering to keep track of which chip came out of which machine. Mixing up the known good chips with the chips under investigation will be a big pain.

I say all this because I had a machine with a partly functioning CPU Chip. That Dead test cartridge would run showing flashing on the screen as if there was a memory issue. It actually took me some time to twig on to the fact that CPU Chip was only partially working. I swapped out all the ram chips needlessly. Very frustrating it was.

While you do this I'll look into the signals on my machine.
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