c64 - ASSY 250425 Black Screen

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dpalmeira
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c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hello!
I need your help to repair a c64 which is a real challenge to me. I've expending many months learning and trying to fix it without success (yet!)

I have some general digital electronics skills (when I was student :P), but I'm not an expert at all, it's just a hobbie for repair things at home.

Well, I'm trying to fix a c64, and I've bought some replacement components, chips, and tools, even another working c64 (ASSY 250407) to compare chips with a bit more agility.

Tools: Dead Test with Multi-diagnosis c64/128 cartridge (including dataset, keyboard, etc.), Digital Multimeter, Capacitor Meter, Osciloscope, Solder, and other general tools.

Glossary:
- "c64-1" = ASSY 250425 BAD one to be fixed
- "c64-2" = ASSY 250407 GOOD one to compare and help
- "OK (?)" = Means that it was measured on the board without unsoldering the component, so, I'm not completely sure that it's OK, but it seems to be OK at first look.
- "-->" = Important note regarding not expected signals, or missing signals.

Symptoms:
- Black Screen, without video output. CN5 central pin: with a low signal, very different to the c64-2 one, probably the black screen signal (not snow).
- Dead test cartridge does not flashes, or anything. (in the c64-2 it works and performs all tests as expected)
- Only a previous rework detected including a socket for the U23 memory.

What I've been done so far:

1) I cleaned up all the board with a contact cleaner. Then I've tested the c64-1 with the same power supply, cable and TV, via RF syntonized / 591000 KHz, and also via DIN to SCART video cable. Both c64-1 and c64-2 are PAL. c64-2 works perfectly with each one the two cable options, but c64-1 does not.

2) In order to accelerate the investigation, I've exchanged directly some important socketed ICs from c64-2 to c64-1
- U19 - VIC-II (6569R3): OK
- U17 - PLA (906114-01): OK
- U18 - SID (6581): OK
- U7 - MPU (6510): OK
- U4 - Kernal ROM (901227-01): OK
- U3 - Basic ROM (901226-01): OK
- U1 - CIA1 (6526): OK
- U2 - CIA2 (6526): OK

- NOTE: U5 - Char ROM (901225-01) was not moved because unfortunately it is soldered on c64-2 :(
- U31 - Clock Generator (8701) tested replacing it with a new one. It's socketed.

3) Then, I did the opposite thing, I mean, I've moved the suspected same ICs from the c64-1 to the c64-2, and all of them work properly!, that means that at least the most expensive chips are OK (good to know!)

4) Some scope measures I made following the article at: http://tech.guitarsite.de/c64_scope.html

c64-1 schematics (not exactly the same as ASSY NO 250425, but based on the same family, it is very useful to follow the circuits):
- http://www.zimmers.net/anonftp/pub/cbm/ ... 9-1of2.gif
- http://www.zimmers.net/anonftp/pub/cbm/ ... 9-2of2.gif

This is the list of checks I made on the board:

Power Supply Voltages +5V and +12V: OK (good and stable)

ICs Voltages (tested online with multimeter):
- U31 - Clock Generator (8701): OK
- U19 - VIC-II (6569R3): OK
- U17 - PLA (906114-01): OK
- U18 - SID (6581): OK
- U7 - MPU (6510): OK
- U5 - Char ROM (901225-01): OK
- U4 - Kernal ROM (901227-01): OK
- U3 - Basic ROM (901226-01): OK
- U1 - CIA1 (6526): OK
- U2 - CIA2 (6526): OK
- U20 - Dual Timer (NE556N): OK
- U8 - Logic NOT (Hex Inverter) (DM7406N): OK

- U9 - Memories (4264-15): OK
- U10 - Memories (4264-15): OK
- U11 - Memories (4264-15): OK
- U12 - Memories (4264-15): OK
- U21 - Memories (4264-15): OK
- U22 - Memories (4264-15): OK
- U23 - Memories (4264-15): OK
- U24 - Memories (4264-15): OK

- U13 - Multiplexor (74LS257): OK
- U25 - Multiplexor (74LS257): OK

- U14 - Multiplexor (74LS258P): OK
- U26 - Multiplexor (74LS373N): OK
- U15 - Demultiplexor (74LS139): OK
- U6 - SRAM (MN2114-3): OK
- U27 - Logic AND (74LS08): OK

- U16 - Quad Bilateral Switch (HEF4066BP): OK
- U28 - Quad Bilateral Switch (HEF4066BP): OK

Transistors (tested offline with multimeter):
- Q1: OK (?)
- Q2: OK (?)
- Q3: OK (?)
- Q4: OK (?)

Diodes (tested offline with multimeter):
- CR4: OK (?)
- CR5: OK (?)
- CR6: OK (?)

Voltage regulators (tested online with multimeter):
- VR1 (7812) +12V: OK
- VR2 (7805) +5V: OK

Passive components (tested offline with multimeter):
- RP1: OK
- RP2: OK
- L1: OK
- L5: OK
- C19: OK (?)
- C88: OK (?)
- C90: OK (?)
- R2: OK
- R5: OK
- R6: OK
- R7: OK
- R26: OK
- R28: OK
- R31: OK
- R34: OK
- R36: OK
- R37: OK
- R39: OK
- R42: OK
- R43: OK
- R44: OK
- R45: OK
- R48: OK
- R50: OK
- R51: OK
- R60: OK
- R100: OK

ICs Tests (tested online with osciloscope):
- U31 - Clock Generator (8701): OK
--> - U19 - VIC-II (6569R3): COLOR signal on PIN 14 not present!
- U17 - PLA (906114-01): PENDING
- U18 - SID (6581): PENDING
--> - U7 - MPU (6510): PHI0 on PIN 1, and PHI2 on PIN 39 OK. But PIN 2 not RDY signal!
- U5 - Char ROM (901225-01): OK (?)
- U4 - Kernal ROM (901227-01): PENDING
- U3 - Basic ROM (901226-01): PENDING
- U1 - CIA1 (6526): PENDING
- U2 - CIA2 (6526): PENDING
- U20 - Dual Timer (NE556N): OK (and also replaced by a new one. It's socketed)
- U8 - Logic NOT (Hex Inverter) (DM7406N): OK (?) pin 4

- U9 - Memories (4264-15): PENDING
- U10 - Memories (4264-15): PENDING
- U11 - Memories (4264-15): PENDING
- U12 - Memories (4264-15): PENDING
- U21 - Memories (4264-15): PENDING
- U22 - Memories (4264-15): PENDING
- U23 - Memories (4264-15): OK (replaced for a new one, same effect) - it's socketed (according an internal sticket in 1985!).
- U24 - Memories (4264-15): PENDING

- U13 - Multiplexor (74LS257): OK
- U25 - Multiplexor (74LS257): OK

- U14 - Multiplexor (74LS258P): OK. All signals as expected on PINs 4, 7, 9, 12
- U26 - Multiplexor (74LS373N): PENDING
- U15 - Demultiplexor (74LS139): OK - It's socketed and replaces by a new one
- U6 - SRAM (MN2114-3): PENDING
--> - U27 - Logic AND (74LS08): PIN 9 and PIN 8 looks as expected, but it have a bad RDY signal on PIN 11, not as expected.

- U16 - Quad Bilateral Switch (HEF4066BP): PENDING
- U28 - Quad Bilateral Switch (HEF4066BP): PENDING

Others:
- TV Syntonized: OK
- Antena cable RF: OK
- A/V to Scart (Euroconector) cable: OK


Well, in order to discover what is happening, could you guide me about what to measure next?

Thank you for your time!
Diego


banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,


Sounds like you have a good range of test equipment. That will definitely help you in testing.

I once found when trouble shooting a C64 board the actual rocker power switch was not making good contact. I would always test good on all the various voltages around the board as one would expect. However the board wouldn't start up.
It took me ages to realise this. At first I thought it was the PLA or both the voltage regulators.
I desoldered everything off the board. All IC's including the RF modulator (which was a real pain).
It was only by chance after I had removed everything off the board and I was testing current flow around the RF modulator area on the bare board that the issue became apparent.

After determining this. I used a bit of lubricant and elbow grease and worked the power switch till it came good. Maybe about a good 5 minutes worth of switching.
So maybe a good hard look at the power switch is in order.

I also use this particular Commodore factory service manual troubleshooting routine.
I found it to be better that some of the others.

Have a look at this thread for details.

viewtopic.php?p=18856&hilit=Troubleshooting#p18856




It always a satisfying feeling getting back to rights a C64.

Let us know how you go with your repairs. :D
dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi, thank you for your help and service manual, I added it to my collection of docs about c64.

I want to share with you my progress on it, but still in the same situation.

1) I've socketed all the board, and installed new IC's for all chips, except the big ones I already tested in my other working board. So I'm almost sure all chips are good.

2) I have following this excellent video: where you can find at 13:10 (min), an excellent sequence of scope checkpoints.

3) After test it with my oscilloscope, I could see good signals, but I've detected some missing signals:

G RW = PLA pin 13
IO = PLA pin 12
P5 = CPU pin 24 (2.5-3v)
READY = CPU pin 2, VIC pin 12, PLA pin 26
R/W = CPU pin 38, VIC pin 11, PLA pin 24
IRQ = CPU pin 3, VIC pin 8
COLOR = VIC pin 14
LUMA + SYNC = VIC pin 15

If I look for this signals in my working board, they are there as expected, but they are not present in the bad board.

I've trying to follow the missing signals connections in terms of circuit/components involved on each end of the missing signal, but no way.

At this point, I only suspect about PCB board, but at this moment all checks I made about continuity are ok.

Any idea?? I'm getting crazy...
dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi, thank you for your help and service manual, I added it to my collection of docs about c64.

I want to share with you my progress on it, but still in the same situation.

1) I've socketed all the board, and installed new IC's for all chips, except the big ones I already tested in my other working board. So I'm almost sure all chips are good.

2) I have following this excellent video where you can find at 13:10 (min) an excellent sequence of scope checkpoints:



3) After test it with my oscilloscope, I could see good signals, but I've detected some missing signals:

G RW = PLA pin 13
IO = PLA pin 12
P5 = CPU pin 24 (2.5-3v)
READY = CPU pin 2, VIC pin 12, PLA pin 26
R/W = CPU pin 38, VIC pin 11, PLA pin 24
IRQ = CPU pin 3, VIC pin 8
COLOR = VIC pin 14
LUMA + SYNC = VIC pin 15

If I look for this signals in my working board, they are there as expected, but they are not present in the bad board.

I've trying to follow the missing signals connections in terms of circuit/components involved on each end of the missing signal, but no way.

At this point, I only suspect about PCB board, but at this moment all checks I made about continuity are ok.

Any idea?? I'm getting crazy...
rmzalbar
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Re: c64 - ASSY 250425 Black Screen

Post by rmzalbar »

Have you checked reset line? You can check at pin 40 of 6510 CPU. It should be high. (when switching power on, it will go low for a moment, then high.) If it is low, then some chips are being held in RESET, and the CPU will also not execute, so the system will not show activity on many pins.

Also, have you verified clock signal at the CPU? Pins 1 and 39 on 6510. It is about 1 MHz. Pin 1 is generated by the VIC-II. Pin 39 is generated by the 6510.
Smooth operator
dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi, yes I did it. Reset and Clock seems to be there. That's part of the signals indicated in the previous video. I mean, from that video I get all signals OK, except the indicated ones:

G RW = PLA pin 13
IO = PLA pin 12
P5 = CPU pin 24 (2.5-3v)
READY = CPU pin 2, VIC pin 12, PLA pin 26
R/W = CPU pin 38, VIC pin 11, PLA pin 24
IRQ = CPU pin 3, VIC pin 8
COLOR = VIC pin 14
LUMA + SYNC = VIC pin 15

BTW: Here another good signals help I've found:
http://tech.guitarsite.de/c64_scope.html#clk_407

Then, I don't know what more to check. :(
banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,

It looks like you have been very thorough in your testing. That's awesome!

One thing I noted you said you hadn't changed out the bigger sockets. Have you checked the Vic II socket and CPU socket for decent continuity?

I find the Commodore factory VIC II and CPU sockets terrible things. Also did you check the electrolytic capacitors for leaks and correct values?

I am thinking that maybe you could try to carefully trace back with your probes from a faulty point until you detect a good signal pickup. Then try to determine where the break or maybe the faulty connection is.

Pull out the SID chip, it's not necessary . Have you got a decent replacement PLA chip? Eslapion sells a good one here called the PLAnkton. It's cheap and bulletproof. Worth just having one in you toolbox.
I once had a CPU that appeared to work but was faulty. They do partly fail. Put known good one in while trouble shooting.
dpalmeira
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Hi banman, thank you so much for your reply.

- All chips have sockets in the bad board "c64-1". The one hasn't socket is U5 - Char ROM (901225-01) on the "c64-2" which is the GOOD board that I'm using to compare things.

- Yes, I've tested the continuity of all chips mounted in the sockets from each leg to the soldered pin on the other side of the board. I spend some time with my son testing all of them with the multimeter, doing this work on each of us on a both side of the board by putting the board in vertical position :) (I know that, it's a brute force approach)

- "I am thinking that maybe you could try to carefully trace back with your probes from a faulty point until you detect a good signal pickup. Then try to determine where the break or maybe the faulty connection is." --> that's exactly what I'm trying to test now. Probably I have to focus in only one missing signal first. Any suggestion? for instance the RDY = CPU pin 2, VIC pin 12, PLA pin 26. It seems to came from U27 pin 11, which have as input pin 12 BA and pin 13 DMA, but this 2 signals appear, but output does not appear in U27 pin 11 (it appears well in the "c64-2" GOOD board). I've replaced the U27 with a new chip, but nothing happens.

- SID, PLA, CPU, VIC II and all big chips (except the U5 Char ROM I cannot exchange) works well, because I put them as I mentioned in the "c64-2" GOOD board and they works perfectly.

- About electrolytic capacitors, the 3 big ones on the power and regulation area seems to work, because I obtain the correct 9 and 12 volts in chips and regulators with little ripple, that means they are filtering well I think.
I did not remove yet the 2 electrolytic capacitors (smalls ones), C13 (10uF x 25V) and C24 (22uF x 25V). I will try it, because I just got a capacitor tester instrument, so, I can verify them.

Well, I will continue my checks and put a summary so far with more info.

Thank you for your support, I really getting crazy with this. :S
PS. is there some way to paste images or screenshots in this forum without upload them in another place and reference them by url?
banman
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Re: c64 - ASSY 250425 Black Screen

Post by banman »

Hi dpalmeira,


Don't get too discouraged. You've already done so much already. The added bonus is that you have spent quality time with your son, that's priceless.
Doing this slow methodical approach will give you an intimate understanding of the C64. After all that's why we're all here, right?

I'm glad you have all known good working major chips. These can be very expensive and hard to source. So I think you're ahead of the game already.
Stay calm always. If you're getting tired or frustrated take a break and come back to it tomorrow. Thankfully doing this is only a hobby and not our day jobs.


I actually think I have a mainboard ASSY 250407. Have a look on your board above the assy no., Is it a 'HiPric' marked board?

Maybe I could pull it out and what we can do is compare signals on mine to yours. Forum members like rmzalbar and eslapion may be kind enough to guide us on the troubleshooting routines too.

Just something on a completely different tangent. I had a C64 mainboard power switch that gave me a whole lot of trouble. It would pass enough voltage to make the RF modulator produce a black screen on the monitor however after much painful trouble shooting found that it was only allowing about 50 mA to energize the C64 mainboard.
How much current is your C64 mainboard drawing. Off the top of my head it's about 900mA. We can properly check this further down the road.

I did have a board that had some very fine breaks on the PCB board traces on the backside of where the PLA chip is located.
I'm not sure how they were broken. I bought the machine this way. The only way I discovered this was carefully tracing from the PLA chip pins to where the line joined to the next physical thru hole component on the C64 mainboard. The traces on the mainboard opposite the PLA chip are very thin and fragile.
Can you confirm that all the traces coming out of the PLA chip to the next IC chip connection is a good connection (all I would use is the continuity selection on your multimeter).


By the way I upload things here.....
Screenshot_2022-06-06-17-13-00-095_com.android.chrome.png
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Re: c64 - ASSY 250425 Black Screen

Post by dpalmeira »

Thank you banman, your words really motivated me :)
Just a litte update:

I've replaced all electrolytic capacitors: C88, C19, C90 C91, C13 and C24
CR4 Bridge also OK, I removed it and test it with the multimeter.

- All PLA lines OK
- All U15 and U16 lines OK
- All U7 - MPU (6510) lines OK

I can continue testing lines using the schematics, but it's a a hard work for sure. Any suggestion what chip should I continue testing in this way?

I will try to take some screenshots on my oscilloscope to upload signals here. The point is why I have no the missing signals:

RDY = CPU pin 2, VIC pin 12, PLA pin 26
G RW = PLA pin 13
IO = PLA pin 12
P5 = CPU pin 24 (2.5-3v)
R/W = CPU pin 38, VIC pin 11, PLA pin 24
IRQ = CPU pin 3, VIC pin 8
COLOR = VIC pin 14
LUMA + SYNC = VIC pin 15

Circuit for missing RDY signal:
RDY_circuit.png
Thank you again for your support.
Diego
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