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[C64] Black screen and power LED fading till almost out.

M

MBaNL

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Aug 20, 2011
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I just bought a Commodore 64 which was in working order as far as I can trust the seller. When I try to power it up the TV screen turns black and the power LED goes on. Within a second the same power LED is almost off (almost, since when I turn off the C64 you can see there was some light left).

When I measure the PSU I have a nice 5V= and the 9VAC is 11V... but that's normal since it's an unbalanced 9VAC.
Both the fuses (from the PSU and C64) are O.K.

I've read multiple items about black svreens on the web but none of them describes my LED sequence...
 
The remains of a C64C I found lying on the curb displayed a similar symptom and it turned out to be the power switch - its contacts had become resistive so the computer wasn't getting full voltage (the switch acted as a series dropping resistor). Check continuity across the switch contacts or bypass them altogether using jumper leads as a quick test.
 
It was indeed the 5V= from the PSU that is dropping voltage. When I bypass it with another 5V= the led stays on but I won't get anything on screen... The screen flickers a sec. when I turn the C64 on and also flickers a sec. when I turn it off but that's the only thing happening.

About the 5V=... must it be perfectly 5V=? since the biggest adapter I have is dropping back to 4.5 to 4.7 volts...
 
You may also check the fuse of the c64..in mine the LED was on but I had no picture and a burnt fuse was the problem.You can also try to get 5v from an old pc power supply.
 
@angel_grig, as stated in the first post... both the fuses are O.K.

I also found an old XT style power supply but I get the same result as earlier (flickering screen).

EDIT: some progression....
When I insert a cartridge (Final Cartridge III) I got something on screen (blue border with weird colored characters). When I read THIS text file it says that a/some ROM chips are dead.
The annoying thing is though when I turn it one sometimes is stays black (with flickering) and sometimes it show the image as described above.

EDIT 2:
Looks like someone already did something to the internals... any of you knows what this could be?

IMAG0437.jpg
The lead on the right goed to the Caps Lock key...
 
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@dabone, well I could if I have one... problem is that it's my first and only C64 actually...

When I pull the PCB it actually replaces U4 which is the Kernel ROM if I read it correctly...

Does someone know the correct ROM chip to put in the C64 (PCB ASY NO. 2500407)
I suspect I need the 901227-01 since The 901225-01 and 901226-01 are there... problem is that revision 01 could not be found on ebay... Is there no problem with sticking in a revision 2 or 3 of this chip?

EDIT: actually 901227-03 is on the extra PCB... problem is though that it's soldered in a socket... why would someone do that? soldering a socket to the PCB and soldering the IC to the socket...

I think that I'm better off buying another C64 and using this as a spare system... Since the 901127 chip is actually in the C64 and the whole unit is not working I don't think it will ever work within the costs of another system...
 
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@angel_grig, thank you, that's some useful information.

As it seems now I can make an arrangement with the seller of my current C64 and I will get another one and keeping this one for spares...
 
What happens when you have the shift lock down (caps lock) when you power on? It looks like that circuit allows switching between custom kernals, so try using it.
Do the wires go to the underside of the keyboard where the 2 wires are soldered to a switch?

Later,
dabone
 
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