c64 - ASSY 250425 Black Screen

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banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

I think it's U8. Run the oscilloscope on all of the pins observing the various signals including ground and power.

Also pull out all the 3 ROM chips, Basic, kermal and CHAR. They have a Read write line attached to the CPU chip.

Try using the PLA chip from your known good working condition C64.


We need to suspect everything here including shorts breaks and descrete components on the C64 mainboard too.

You mentioned that all of the minor IC logic chips are now socketed on the faulty C64 mainboard.
Maybe you can place one of these minor IC logic chips into your good C64 mainboard and see how it behaves. Only one at any one time to reduce the risk of getting false readings.


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Re: c64 - ASSY 250425 Black Screen

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I found some more IC chip tests.

Ignor the stuff I've written in Biro on the last image. I was checking out something on a C64 mainboard.
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banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,

I just want to touch on a subject that I discussed earlier in this thread regarding fake chips and also too chips that go out of specifications due to age, failure, etc....

I found this great YouTube video from Noel's Retro Lab on fake chips.

Very interesting!

He goes into detail on construction of a test rig test procedures to fully test out TTL Logic chip.

This could be employed by you to get an accurate characterization of your TTL chips in your suspect C64 mainboard.




He's got some other great content there too. Very methodical.

I would suggest that this is the approach to fully pass all you IC logic chips.
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Here, Adrian talks about experiences with chip testers and diagnostic cartridges



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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hello,

Here some facts so far:

1) I've tested the voltages in the board and they are OK.
2) During the ICs pull of process always the pin 15 (S/LUM) of VIC II chip remains LOW (no signal)
3) MPU, SID, CIA1, CIA2, PLA, VIC II, RAM Memories (x8), 74LS257 (U13 and U25), 3 ROMS (Kernal, Basic, Chars), U6 - 2114L3, U26 - 74LS373, were well tested on the good board. And all of them works.
4) Clock is OK in all board
5) RDY on MPU pin 40 generated by 556 (U20) + 7406 (U8) is low for a second more or less, and then goes UP, so, I think these 2 chips work well as well. For 7406 I also tested it in the IC tester and it's OK, and it's the original one.
6) I've replaced all electrolytics capacitors on the board. (big and small ones)
7) I've tested all transistors removing them from the board to be sure. Same with the 2 existing zener diodes.
8) All resistors, on array and normal ones seems to be ok.
9) IMPORTANT: I found another bad connection on MPU socket. Basically the pin 16 had no connectivity between the socket pin and the MPU pin, is like the socket has no good contact. I cleaned up all sockets with contact cleaner of course. But this was caused by an exhausted metal in the contact or similar. Now it's ok.

I continue testing lines on the board and painting them as completed in my printed schematics to keep a control over this hard work.

I'll review your videos about fake ICs, but I don't understand why I have to suspect on them, if most of them were tested in my other board, and I repeat, they work as expected. In fact, I've detected some bad memories, and I remove them from my stock.

Probably they are ok, and it's caused by a bad connection in a socket or similar. Really it's getting me crazy.

Well, let's continue reviewing, because even fixing the pin 15 of MPU things didn't change, and black screen remains, and Dead Test does not work, no flashes, nothing at all.

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

Post by banman »

Hi dpalmeira,



You are doing good work in methodically checking everything on the board. Slowly and steadily you will beat this to your will.
Remember that when you get frustrated take a break and do something else.


Am I correct in understanding that the CPU socket had a faulty wiper connection?
Is this socket an original one from the factory or it came to you this way?



When you have carefully reviewed both the videos posted above we can have a discussion regarding IC logic chips and failure modes.

For me, this hobby was/is just a time passing activity. There is no real impetus to meet deadlines. I
I initially thought or assumed that all chips purchased were the same.
I in fact was totally incorrect at every level, rmzalbar eluded to this here viewtopic.php?t=16537&start=50.
Purchasing anything from a supplier other than a reputable company or supplier like DigiKey or Mouser is more than likely going to be chancing the unknown.
That's actually fine if you like that sort of thing. I do, but I don't recommend it especially if you are trying to get things done in a timely manner.
In fact, I was testing 2 identical (from the same batch number) confirmed factory original IC logic chips, from a known good supplier, that had never been used a while back. I discovered that they had different triggering voltages. Very interesting. It appears that even chips from the same die can have variations to them.

I've read stories about Tektronix hand selecting their memory chips for their USA made oscilloscopes so that they have the best standby power consumption. That's the type of mad monk attitude that gives them a reputation for excellence.

I will say that with a great deal of certainty that a good amount of stock on sale from the Far East vendors is either someone else's ewaste or a reputable factory's substandard product that didn't meet specifications. I have had very lively and enlightening conversations with vendors. They can can reach out to their suppliers and get you any chip from any C64 or Vic 20 mainboard you desire for the right price. I imagine that they can do the same for most of the Commodore lines of hardware or other manufacturers.



Flame Wars below please!
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Was that a typo regarding CPU socket connection pin 16 or pin 15?
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi, it was the MPU pin 16
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Re: c64 - ASSY 250425 Black Screen

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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Here are some more photos of documents related to correct signals one should observe on C64 IC chips.

All 8 x 4164 Dynamic RAM IC chips should have these signals. Please ignore the writing in Biro, these were my notes on testing another machine.
IMG_20220731_152045.jpg
These are the correct signals for IC U17 (PLA). Again please ignore the writing in Biro, these were my notes on testing another machine.
IMG_20220801_122716.jpg

Here are the correct signals for IC logic chips U13 & U 25 (74ls257)
IMG_20220731_153029.jpg
Signals can be observed with an oscilloscope or alternatively on a very cheap logic probe. I imagine that it will also work on a logic analyser, though I have not tried this method as yet.
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