On 12/19/2018 3:31 PM, Ethan Dicks via vcf-midatlantic wrote:
On Wed, Dec 19, 2018 at 11:57 AM Michael Pohl via vcf-midatlantic <vcf-midatlantic@lists.vcfed.org> wrote:
I signed up to bring our non-working Commodore 64 to the workshop on 12 JAN. The power light comes on, but there is no video from either the RF modulator or the 8 pin video jack. We ran some basic daignotics - power supply produces the correct voltages, voltage is present at specific places on the motherboard, fuse is good - but now we are stuck. "Black screen" Commodore 64 is commonly the PLA chip. It's a programmed 82S100 PLA that implements the memory map. Replacements are available. You can borrow one from another machine and see if that's the problem.
There is a smaller chance it's a bad DRAM chip. Those occasionally fail.
-ethan
To get more technical, the actual Signetics 82s100 PLAs rarely fail, nor do the toshiba ones that very late breadbox c64s had, but the NMOS 'in house hacked from signetics masks to nmos' ones have an obscene failure rate. IIRC the fusemap from the real 82s100 PLA (two versions of it!) is known/dumped, so if you manage to find a blank NOS 82s100 PLA you can burn a replacement using that. -- Jonathan Gevaryahu jgevaryahu@gmail.com jgevaryahu@hotmail.com