c64 - ASSY 250425 Black Screen

[banman]

Hi dpalmeira,


I'm impressed with your skill at removing the CR4 Bridge rectifier. That is very hard to heat up the pins effectively to remove. Was there any damage to the board after doing this?

I must admit that I think if you could post some nice clear images of your C64 mainboard on this thread. I will see if I have the same board as you.
We can then both run over the boards with some oscilloscope probes and compare signals.

Also, is there a way you can confirm that your power switch is working correctly? I think it's always worth checking out the simplest things first.

I'll wait for your C64 mainboard pictures.

[banman]

Hi dpalmeira,


This is an interesting article about the black screen on a C64 from Ray Carlsen...

https://cbm8bit.com/articles/ray-charlsen/article36


Looks like this one is a rehash of Ray Carlsen's work


https://www.classic-computing.org/c64-d ... nk-screen/



I almost forgot about this site.
This is a good site for pictorial representations of C64 mainboard failure modes.

https://derbian.webs.com/c64diag/

[dpalmeira]

23 June 2022 - UPDATE

Hello, and thank you for your links, some of them I already knew, but anyway I will review them carefully just in case I missing something. BTW, I was watching also tons of youtube videos, and I've learned a lot of thing regarding c64 aspects.

Here some update, but still same black screen issue, even with the Dead Test cartridge (no flashes at all with CR5 or RF)

BAD or missing Signals

P5 = CPU pin 24 (2.5-3v)
RDY = 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
G RW = PLA pin 13
IO = PLA pin 12
COLOR = VIC pin 14
LUMA + SYNC = VIC pin 15

Glossary:
- "c64-1" = ASSY 250425 BAD one to be fixed
- "c64-2" = ASSY 250407 GOOD one to compare and help

1) "c64-1" board has correct voltages (9AC, 5DC, 5v REG, 12v DC REG, etc.) also tested on user port not only in board points.
2) All ICs have correct voltages too.
3) All ICs were tested and they are working in the "c64-2" and/or tested with a 74x/40x series IC tester I've bought to discard small chips failures. To achieve it, I also installed some sockets in the "c62-2" working board, e.g for one memory, and I was able to test all chips. Interesting thing here is I was able to detect some bad new ICs I bought on AlliExpress. grrrrr. But at the end I got a good set of all these chips well tested.

So ALL CHIPS are OK!

4) Cleaned up all sockets (old and new ones I've installed for small chips). All sockets were tested with multimeter.
5) The 2 zener diodes were removed and tested OK
6) All main board transistors were removed and tested OK
7) Replaced all electrolytic capacitors - OK
8) Tested lines on the board so far:

- All U7 - MPU (6510) lines OK
- All U19 - VIC-II (6569R3) lines OK
- All PLA lines (total of 28) OK
- All U14 lines OK
- All U15 lines OK
- All U16 lines OK
- All U3 - Basic ROM (901226-01) lines OK
- All U4 - Kernal ROM (901227-01) lines OK
- All U5 - Char ROM (901225-01) lines OK
- All U8 - Logic NOT (Hex Inverter) (DM7406N) lines OK
- All U27 lines OK

Next steps:
- Put here some scope signals to analyze.
- Start thinking if modulator has some problem maybe?? it controls signals to RF and Video CN5... but strange thing is why VIC II don't produce the correct signal... may be if the problem is the modulator should the VIC II generate the signal well anyway?

"c64-1" = ASSY 250425 BAD one to be fixed

BTW, I continue getting crazy with this...

[rmzalbar]

In your photo, the third DRAM from the bottom-left, in the blue socket, U23. The socket itself looks like it's displaced downward by one pin position! In fact I can see the top edge of the through-hole pad for pin 1.

[dpalmeira]

mmm @rmzalbar very nice observation...
I've tested pins, and they are well connected to back part of the board. I put some pics here.

I think it's caused by it is a different kind of socket. In fact, this was the only socket I've detected as re-work on first time I've saw this board with a paper inside saying.... "Repaired in 18-09-1985", but this should say "good luck my friend, this machine will guide you to the hell !"

The fight to achieve it continues... I really want to fix it.

[banman]

Hi dpalmeira and rmzalbar,

Yes, it does look like the socket is down one row of pins.

They look like single wipe sockets. It would be a good idea to replace these with double wipe sockets at the minimum.


By the way, the VIC ii chip you have is a PAL B version 12 volt job.

https://en.m.wikipedia.org/wiki/MOS_Technology_VIC-II

I noticed that there has been something modified on the short PAL G jumper pad.
Are you using the RF modulator to output your video signal exclusively?
Maybe try using the 8 pin a/v video connection instead. This way you can bypass the "non factory" modification on the RF modulator.

https://www.c64-wiki.com/wiki/A/V_Jack

Try plugging in a game cartridge or even trying to load something from a disk drive and see if you have any sound or disk light activity. You won't need the use of the screen to observe these things.

[dpalmeira]

Ohhh thank you I've removed the short of PAL G jumper. Originally it was connected. I will re-soldered it again.

Regarding the RF, I'm testing both, RF (591 KHz) and 5 pin DIN to SCART cable that I've made. It works well on the other "c64-2" computer (in both cases).
In fact, in the bad one ("c64-1"), I get black screen in RF when I turn it on (no snow), just back screen after some quick pixel colors on the bottom of the screen (purple or similar color).

But I got the similar color signal from VIC II (pin 14) as the guy in the following video, but him has the problem of permanent reset, not my case, my reset in CPU is low, and then high after a second or so:
min: 10:30

[banman]

Hi dpalmeira,

Ok that's good.

I think I have a working board the same as your bad board.

We can do a run over the board with an oscilloscope to find out more about the failing signals.

I don't have any experience with the coloured sockets in your board.
However, I know from past experience with the Commodore factory single wipe sockets they are rubbish and should be replaced with more suitable versions even if they look ok.

Keep going over your board with your eyes for anything that looks out of place. Nine times out of 10 it is usually some very small detail that is the cause of all the issues.

Did you check the 5v CAN line. I think it comes off the 12v line somewhere. Pull up you 425 schematic diagram and look for the CAN line.


Give me some time to set my ...425 up with the oscilloscope and I'll get back to you.

[dpalmeira]

Thank you!
Yes, all voltages are in place, 12, 5, 5 can, etc. I will soldering the things I removed and take some scope signals in order. Clock, Reset, Address, Data bus, etc. in the CPU and VICII

[dpalmeira]

Scope 1:

Some images for test:
(BTW, I have to convert them from bmp to png to upload them here)

Clock - CPU - PIN 1

Clock - CPU - PIN 39