Commodore 64 question

On Thursday 24 May 2007 23:02, Bill Machacek wrote: Maybe, maybe not. This was *the* most common symptom back when I was making a living fixing those units... First thing I'd check is the power supplies. The regulators internal to tbe unit provide +5 and +12 for the video section and SID only, the rest of the unit runs off the +5 coming out of the "brick" power supply. Next thing I'd check is to see if any of the RAM (usually 8 of them) are running warm. Touching them with your finger works but be warned that I've had some of them run so hot I ended up making a small thermistor probe to use instead. :-) Following that, look for the PLA chip. This will be marked either 82S100 or 906114-01 (I think) and is one of the only two 28-pin parts in there, the other one being the 6581 or 8580 SID chip. Starting at the end opposite the notch, pin 14 is ground, and 12, 13, 15, 16, and 17 are various select lines for different parts of the machine. These should be either at a high level or pulsing, never low, which shows a bad chip (and does anyone know of a source for these?). The other thing that can be checked with a scope or logic probe is the data bus, easiest by hitting pin 2 of each RAM chip, sometimes one will get stuck low and that'll do it too. Lots and lots of other stuff in there will cause this symptom, or a problem with data or address lines. I got to the point where I'd pull chips out and put sockets in where they were, allowing me to try the chip in a test board and put it back if it worked okay or replace it if not. Got lots of chips here too, if anybody needs some. -- Member of the toughest, meanest, deadliest, most unrelenting -- and ablest -- form of life in this section of space, ?a critter that can be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters" - Information is more dangerous than cannon to a society ruled by lies. --James M Dakin