c64 - ASSY 250425 Black Screen

Got an issue?
dpalmeira
Member
Member
Posts: 31
Joined: Sat Apr 16, 2022 11:03 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Ic Tester - Testing the 74LS257 (x2) (U13 and U25)
ic_tester_40x_74x.png


banman
Member
Member
Posts: 419
Joined: Sat Jun 15, 2019 12:21 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,


Okay, great the signals from your VIC ii chip look normal.

I just looked at the capacitors on my board they seem a little different from what you are reporting to me. My capacitors are factory original. I looked at several other boards here and they are all the same. I think if you look carefully the values are visable. I think these large value capacitors are important to the 5Volt CAN line as well as the correct 12Volts for the VIC ii chip.

Can you confirm that you have these values? Just tap on the image to get a hi res version.
IMG_20220626_122811.jpg

I was looking through some old documents I had and found one on what the signals one should be viewing on the 74ls257 multiplexor. Note that the signals are different on each of the multiplexors. These tests were designed for a logic probe. Though I imagine using an oscilloscope will be the same.
IMG_20220626_122424.jpg
IMG_20220626_122506.jpg

I am still interested in Pins 9,10, 11, 12 and 15 of your CPU Chip. They look as if they are signalling however they don't seem to be going true low.
I will check these on my CPU Chip as well as the 74ls257 multiplexor chips and post the pictures.

I will look further into the source coming out of the 8 pin CN5 Video connector.

That looks like a nice little chip tester. Can you tell me more about this, do you have any more pictures of this?
dpalmeira
Member
Member
Posts: 31
Joined: Sat Apr 16, 2022 11:03 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hello,
again thank you for your help! I'm sure we have to discover what's happening...

- Capacitors, yes, I've replaced them with the same values, but the old ones are the same you have in your pic, and they measured well also with my capacitor tester. I've replaced them just in case, but again, I have correct voltages I think in all chips. c88 = 1000uF, c19 = 2200uF, and c90 = 470uF

- Chip tester: I bought it in Amazon some weeks ago at:


(BTW, I'll on travel next week, but I will continue with my investigation the week after next one)

Many thanks again for your help and time.
banman
Member
Member
Posts: 419
Joined: Sat Jun 15, 2019 12:21 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,



I am sure you will get it running again.

I actually appreciate the opportunity to assist you comparing like signals on my functioning C64 250425 REV board. I need to get my skills up at using an oscilloscope.


That's good regarding the Capacitors c88 = 1000uF, c19 = 2200uF, and c90 = 470uF.

I could be wrong here, I think they form a Charge Pump network that produces 12Volts from the 9 volts AC line. This voltage is then passed thru the lm 7812 (12V) and lm 7805 (5V CAN) regulators. Please correct me if I'm wrong here.

https://comp.sys.cbm.narkive.com/Gky9ZO ... ac-any-use
nzp39bY.png
I know I am going on about this for the hundredth time, please excuse. I was caught out in the worst possible way on this. Have you been able to positive confirm that the power switch is okay? You could run a few jumper wires by-passing the power switch to confirm correct current is reaching the mainboard.




Thought I would go back check your initial observations.
I have been reviewing your detailed reporting on signals coming from your failing C64 250425 board.

I can confirm that I am getting the same results as you on Pin 24 of 6510 CPU.

Here's a video....You might hear a delay between the click of the power switch and the oscilloscope reading. This is coincidentally the same delay you will see on the CPU reset line.




Are you getting the same behavior, I am not using any cartridges on a bare system only?

If so then, maybe we can agree that this is actually a good and valid signal you are observing.

I will keep looking at signals on the board I have here.

Enjoy your trip.
banman
Member
Member
Posts: 419
Joined: Sat Jun 15, 2019 12:21 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by banman »

Just checked pin 15 of the VIC ii chip.....


IMG_20220627_152954.jpg
IMG_20220627_152732.jpg
IMG_20220627_152850.jpg
In the first oscilloscope reading one will notice that here is only a little dip in the signals. I think this is the black screen.

After a second the signal becomes more complex. This is the standard C64 blue screen with a ready prompt.

I am taking measurements at the IC legs.

Do you get this on your good C64 motherboard and on your bad C64 motherboard?
banman
Member
Member
Posts: 419
Joined: Sat Jun 15, 2019 12:21 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by banman »

Something has been bothering me since I saw the oscilloscope images of the 6510 CPU output. Primarily CPU pin 9,10,11,12 and 15.

Not too sure if my eyesight is getting worse than it already is or if I am seeing things. I noticed that the oscilloscope readings show some very suspicious logic activity.

That is to say that there is definitely some logic shifting occuring, however it appears to be not valid voltage levels.
That leaves me with a question regarding whether the CPU Chip or the logic IC chips connected to the CPU Chip are damaged or faulty in some way.

I understand that you have physically tested the CPU in your good C64 motherboard and it tested good.

You also tested the logic chips in a logic chip tester that you purchased. These too apparently tested okay, or so it seemed.

I did a little digging into the matter of logic IC chip testers.

I found this very interesting and enlightening article on building a DIY Arduino IC chip tester project.

https://www.gammon.com.au/forum/?id=14898


Incidentally, this gentleman, I believe wrote some well known software for the C64 in the very early days. Thank to him for sharing some valuable information.

Reading the article I imagined that most of the IC chip testers on the market for sale go about testing IC chips in a similar fashion.

That is to say what these testers do is demonstrate that the chip's output drivers are working, and that, for the test data, they respond in an appropriate way.
They don't test for a qualitative or repetitive characteristics of an IC under investigation.

I am not for a moment saying they don't work. Though it is possible for marginal chips to be miss reported.
My reasoning for this hypothesis is I can't correlate the images from your oscilloscope to the either the 6510 CPU Chip or the logic IC chips attached to these lines.

Having said all that. Are you able to take the logic chips out of your non functioning C64 one at a time and place them in the appropriate locations on your good machine and see if any pass or fail?
Think of it as a second opinion.
dpalmeira
Member
Member
Posts: 31
Joined: Sat Apr 16, 2022 11:03 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hello!

My answers:

- CPU Pin 24 is always DOWN (you can see it in my PDF I've posted before). It behavior it not like yours, that goes up after a second or so, it remains always down. This pin 24 seems to be an output pin to control del cass motor, and it should managed by the CPU. Q1, Q2, Zener CR2, Power 9V and the associated circuit to it, seems to be OK, including the resistors R2 and R4. I'm not using any cartridges to get scopes.
In my c64-2, it works as yours!

- CPU and all big chips where tested in my good c64-2 and all of them working fine. I repeat, using my c64-2 sockets, and the IC Tester in some other cases, I've tested ALL the chips of my bad board, and all of them seems to work.

- Power Supply is OK, all voltages seems to be normal, and even my "c64-2", works perfectly. In fact, I've tested also the bad c64-1 with another original power supply and got the same result. Black screen. So I discarded the PS for now.

- VIC II pin 15 is always DOWN too, no activity in the Sync + Lum output.
In my c64-2, it works as yours!

- mmmm regarding the CPU pin 9,10,11,12 and 15, you are right they seems not to take a real low level...
In my c64-2, the activity of these pins are normal, reaching low and high levels.

- To have a second opinion about the 74LS257 chips, I will install a socket in U13 (or U25) in my c64-2, and test both chips there to be sure they are working well under normal conditions (to discard any false positive of my IC Tester). Anyway, these ICs are new, and I have about 10 more of them to test.
dpalmeira
Member
Member
Posts: 31
Joined: Sat Apr 16, 2022 11:03 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi Again,

An important discovering today.

I've detected that line A2 from U26 (74LS373) pin 12 to ROMs-pin_6/CPU-pin_9/SID-pin_11 were not connected. It only was connected to U6 pin 7.
So, if I connect it with a cable bridge, now the following address lines in my CPU appears with activity:

A2 pin 9
A3 pin 10
A4 pin 11
A5 pin 12
A2.png
But still no activity at:

A8 pin 15
A9 pin 16
A12 pin 19
A13 pin 20
A14 pin 22
A15 pin 23

(also pin 24 still down)

Please, could you check if this line is connected in your board 250425?
banman
Member
Member
Posts: 419
Joined: Sat Jun 15, 2019 12:21 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,

Good to see you are making progress. :D

Okay no problem. I'll check with mine and get back to you regarding the board wiring issues and oscilloscope signals.

We need to focus the attention on two fronts. One the possibility of a bad board . The other, issues with the support logic chips.


It is easy to pull the copper eyelets out from the PCB board when replacing chips and sockets without knowing. Looks like this could be one of those incidents. Could be worth checking out the other socketed and reworked chips on your board as well.

Have a printout of a C64 250425 schematic diagram and make notes on which traces are not connected to which chips. Make it a big poster style multi page print for easy viewing.
I know it's going to an onerous task. However in the long run you will know for certain all the board's wiring is in good condition.
Something to note... Board flexing when pressing in chips may put unwanted stresses on the traces too. Maybe try to support the underside of the board when inserting the chips. Or having the board laying on a totally flat surface will benefit.

I like to put a socket in when I am replacing a chip. It makes it that much simpler for future events. I would stay clear of the single wipe variety altogether.
With careful consideration it should be possible for you to make very good high quality repairs that look factory finished to the PCB board.


On the topic of support logic chips I think you mentioned that you have new chips. Did you purchase these from a reputable supplier such as Digikey or Mouser?

There is a possibility that if you purchased from far East supplier that you may in fact have counterfeit goods.
I have had experience with this several times before. I have gotten good at spotting fakes.
Are you able to post some hi res images front and back of these support logic chips for review?


Do a YouTube video search on 'Pace electronics' . They have some very top notch information about all things soldering. Very easy to understand.
Last edited by banman on Tue Jul 05, 2022 7:44 am, edited 1 time in total.
banman
Member
Member
Posts: 419
Joined: Sat Jun 15, 2019 12:21 am
Contact:

Re: c64 - ASSY 250425 Black Screen

Post by banman »

Post Reply Previous topicNext topic

Who is online

Users browsing this forum: No registered users and 11 guests