Posted By
TLC
on 2012-05-15
05:13:40
|  Re: Faulty Plus 4
Hi!
I'd try swapping the other socketed chips first (simplest test to start with if you already have some spares). Judging from the symptoms that you're describing, I couldn't name a safe "bet". If it's not the socketed chips, with probably the PLA (251641-02) being the first one, you can probably start suspecting the ram chips, the ram multiplexers (2*74ls257), maybe some other ttl gates. It's all very strange, digital circuits rarely fail like that (they're usually either alive or dead). Ram chips don't usually fail, nor do standard ttl gates (unless there's some shock, say, overvoltage or similar). I'd probably check the supply voltage around the chips using the scope (would be a pity if these symptoms were just caused by badly filtered supply voltage, because of, say, some aging capacitors). In any case, I wish you good luck for the quest!... 
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