Sunday, September 1, 2019

Repair : C64 No signal, black screen fix - part 3 - the final chapter, success!

Well . . . I was totally wrong about the ROM's I replaced both to no effect.

Back to the drawing board.

Looking at diagnostic pictures and my remaining socketed chips I was getting down to a last man standing situation.

I only had 5 chips left, CPU, CIA2, PLA, Color Ram and two identical logic chips.

PLA - ease don't


I guessed the PLA ( I had two spare ) and got to work de-soldering it very, very carefully
WARNING : apparently it is very easy to kill it by removing it.



I eventually got it out very cleanly.  I put in a socket and then inserted a new PLA and got this.



Ugh Oh,  that's not right. I put the original PLA back in the socked and got the same picture. I pulled out the multi-meter and began checking all of the connections, nope everything was fine.  I then checked for shorts.  I already knew that two of the pins short when a cart is attached so I removed it.  I quickly found the short between two pins connected to the Colour RAM.  The solder had joined under the socket.  With a piece of copper solder wick I jammed it against the side of the socket and it cleared the short and I went back to the black screen.

The Final Culprit - CIA2


Time to pull CIA2.  So I got to work and removed it and powered it up without CIA2.


Now we are getting somewhere.  I quickly socketed it and jammed in my two spare good CIA's.


Now it's all working . . . so what was the damage?

I replaced 3 x logic chips ( status unknown, one was probably dead ).

1 x ram chip ( dead ), 2 x CIA chips ( dead ), 1 x SID ( dead ), 1 x 8701 ( dead )

Was it worth the repair.  Cost wise I'd have to say no.  If you wanted to have C64 that works just buy a working one.

Education wise, yes it was a great exercise. On the plus side I now have a test board with almost all of the IC's socketed.  I buy this dead stuff to learn skills and my through hole soldering is so much better now.

I'm also pretty pleased I got the PLA out in one piece.  I've read it's quiet hard to do without killing it.

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