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Revival attempt of C64C mobo rev 250469

V

VintageVic

Experienced Member
Joined
Dec 9, 2019
Messages
340
Location
Finland
I got this black screen C64 motherboard recently and yesterday begun my attempt
to bring it back to life.

I first connected the pcb to power cable and to scart. Switched the power on.
There was nothing, absolutely nothing. I would expect some sort of flash or blink
from TV black screen change into a C64 black screen but there was no any kind of reaction on the television
to turning power on.

I then measured with a multimeter that there were at least some voltages coming to the pcb. Noticed that
small green resistor near the tv modulator was getting awfully hot. There was also strange / bad odor coming
from the tv modulator area. And below the pcb I noticed that someone had at some point removed (or tried to remove)
that tv modulator before me. I had another same revision pcb that I have used as spares before and it was helpful again
to donate tv modulator to this one. After replacing the tv-modulator, I have now clear flash/blink that I would expect to see
when power is switched on to the pcb - yet the screen remains black. the resistor is no longer getting hot after power on.
I'm hoping that this part of the issue is solved anyway.

I let the pcb be on for a few minutes and ram chip at U10 was getting very hot. So I pulled that and it did not work on
other pcb either - that was one issue, but still black screen. I noticed physical damage on one of the caps (pic 3) and cut one leg from it
just to make sure it is not shorted and causing issue, but it was not. VIC chip and 8701 were on sockets and are tested good on
working pcb.

I think my next steps to check are the second ram at U11, both 6526 chips and the cpu. Hopefully I dont have to pull
everything out before I reach the fried chip (assuming it is a chip issue in the first place). I need to find jupiter lander also,
as it bypasses kernal and might reveal something.


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Oh man, this is one unforgiving pcb !

Tested so far by unsoldering and checking with working pcb:
- Tv modulator BAD
- U10 ram BAD
- U11 ram BAD
- U22 7406 logic BAD
- U2 6526 GOOD !
- U1 6526 - boots to blue screen but cursor missing - BAD
- CPU - BAD

This is one of those motherboards that make you wonder what on earth has happened to
it in the past. Of course there is the possibility, that I have damaged some (or more) of the
pulled chips while desoldering, but I usually get good chips out of the board without damaging them.
At least I was successful in pulling the chips to avoid damaging traces on the motherboard.

Still, I socketed all the slots that I had removed chips and tried the motherboard with tested and working chips but the
screen remains black. There is still something wrong with it. And I'm soon running out of good candidates to check.

Let's see... I guess not every unit is such that I can fix.




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Yes, probably overvoltage from power supply, with or without lightning accident.

I continued troubleshooting last night. The board was powered on for a while on my worktable
and I noticed U4 kernal/basic rom getting a bit hot. Heating chips are always suspects. They normally
get warm/hot too, but there is the fine line of detecting what may be caused by shorted chip.

I often check back on Ray Carlsen's work and comments on chips, quote from Ray's site for U4 here:
"U4 251913-01 BASIC and KERNAL ROM (same IC as U32 in C128 )
Blank screen, no border. Cartridges don't work except the few that bypass the Kernal ROM such as CBM Kickman and Jupiter Lander. Those can be used as a diagnostic for a bad Kernal."

Now, I have been trying the 'bypass' trick here, with jupiter lander and I also have tried
the pcb with deat test cart and it remained black. I tried to be smart here and checked
the schematics. U3 (7408 ) logic chip seems to control CS of U4 chip - so maybe that logic
chip is the cause then for example keeping the chip selected at all times. Makes sense yes ?

Well, I pulled 7408 out and replaced with a new, but pcb remained black. Have not tested that
logic chip if it was faulty or not, but I will.

Followed by that I decided that I'm anyway going to pull kernal rom U4 from the pcb. And I managed
to finally break a trace there :/. Well, it is bound to happen some times even though you try to be careful.

It was late and I had no time or energy to fix the trace yet, but I was still going to chect the board
without U4 connected. As Jupiter lander or dead test cart might still bring it live. And you know what - it did !
Yeah, finally light at the end of the tunnel =) .

Now, I have some fixing and tidying up to do with this. I also want to test it comprehensively, before I can
assume it fixed.

Just a comment on Ray's advices, I appreciate and value his work and willingness to share information very, very much.
He also gives a disclamer, that this is not an exact science to repair old commodores. Eventhough at this case the symptom
or failure did not match Ray's suggestion, does not mean that in most cases his comment may be correct. Ray has probably
helped many to bring old commodores back to life and I give all my thumbs up for him.

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