• Please review our updated Terms and Rules here

C64 - black screen

G

gary2000

Experienced Member
Joined
Feb 14, 2015
Messages
84
Location
Florida, USA
I recently bought a nice one-owner C64. At power-up, the original monitor flickers and gives a black screen.

Voltages look good (12v,5v,9v)/ 1541 resets. Tried loading disk, but nothing.

The PLA seems to be abnormally hot (like the SID) and no pulses from pins 10-13.

Voltages are not 5v on VIC, pins 15=1.48v,10=3.32 & 11=2.68

On cn5, pin 4=.62v,1=0,6=.75.

I believe the video signal gets formed in the modulator, which I hope is not bad.

All chips are soldered except VIC, so any suggestions before I start replacing PLA IMG_1332.jpgon a best guess basis?
 
It has been a while so maybe someone else can pipe up, but as you are probably aware, the PLA is a common (perhaps most common) failure point on these, and since you indicate the PLA is hot that lends some additional skepticism to that part.

Any wiggling of signals on the input side of PLA? How are you measuring the signals?
 
geoffm3 said:
It has been a while so maybe someone else can pipe up, but as you are probably aware, the PLA is a common (perhaps most common) failure point on these, and since you indicate the PLA is hot that lends some additional skepticism to that part.
Click to expand...
I second that. In 9 out of 10 cases of C64's with a generated black screen I have to repair the PLA is to blame. With "generated" I mean you can see that the picture is generated by the VIC. If the VIC doesn't generate a picture, my TV is so kind to say there is no signal :)
 
Ok, I've removed the unsocketed PLA - it does not work in a friend's 64. We'll see if that's all that is wrong.

Also, I meant the composite signal gets formed in the modulator.
 
The composite signal gets formed in the VIC-II. :)

So it, and the clock generator are working well.

Do clean all the old heat sink compound off the VIC and put new stuff on.
 
Ok, PLA swapped and we have a blue border w/ garbage characters!

I suspect character ROM or RAM...
 
Last edited:
gary2000 said:
Ok, PLA swapped and we have a blue border w/ garbage characters!

I suspect character ROM or RAM...
Click to expand...

It's more likely to be a dead CIA chip at U2.
Can you blindly load up a game or cartridge? Does it "play" except fir the garbage on screen?
If so, then it's the CIA
 
No, not the CIA! (they're not socketed)

Yes, cartridges seem to work except for the garbage on screen.

What about trying to replace the two LS257? Just want to eliminate other possibilities.

IMG_1349.jpg
 
Last edited:
gary2000 said:
What about trying to replace the two LS257? Just want to eliminate other possibilities.
Click to expand...

Well, you can eliminate possibilities by socketing and replacing chips as you go. :)

That garbage screen looks a little different to a CIA failure however. You actually get discernable characters on screen.
Have a look at this guide: http://derbian.webs.com/c64diag/

I'd say it's either RAM or the 257s
 
Last edited:
Would be interested to see what the problem ends up being, purely nostalgia based. This was the reason we sold ours all those years ago at a trunk sale. The guy took it for peanuts with all the games because we started getting gobble on the screen. He reckoned he'd worked on them before and could probably sort the issue out but who knows whether he was genuinely clued-up and whether he managed to get it sorted in the end. I missed my C64 for years, it was the computer that provokes the most interesting and fun memories. Probably because we used to grab the games from the local shops and it was pot luck whether they ever worked. That was all part of the fun though. So many solid games I could never complete as a child. Would love to revisit them and see whether it was just me being too long or whether they were flawed and you couldn't actually complete them. Guessing it was just me being too young and the fact that pretty much every game used to crash at some point.
 
Will be replacing borrowed PLA with PLAnkton when it arrives (before replacing any more).

Previously replaced one ls257 and RAM was ok (according to dead test cart). We'll see...
 
You must log in or register to reply here.
Back
Top