[vcf-midatlantic] Museum Report 2018-11-16

[Jonathan Gevaryahu]

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 at 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 at gmail.com
jgevaryahu at hotmail.com