tezza
Veteran Member
When I first started playing around with vintage tech I used to think socketed chips were great. You could swap suspected faulty chips in and out and enable a repair easily.
Now I've come to the conclusion that it kinda depends on the socket. In a few of my vintage models (Commodore Pet, Apple II+) it has been the sockets that have caused the problem in the first place!
I've just diagnosed a fault on my spare Apple II+ board. The board appeared dead. One clue was a TTL chip servicing both the keyboard and RAM was getting very hot. Investigating this I discovered it was not a short in the chip itself. Rather the OUPUT of a gate was indicating -3.5 volts when the inputs were positive (err...at least one was, the other was -0.75...but only with the chip in place. The true input (if the chip was removed) was positive).
Anyway, this gate output was connected to pin 2 of a RAM chip. What was happening was there was a short across pins 1 and 2 in the RAM chip socket. Not underneath the socket, or in the RAM chip, but somewhere IN THE SOCKET. This short occurred ONLY WHEN a RAM chip was inserted and pressed down securely. Pin 1 carried -5 volts. Being shorted to pin 2, high negative voltage then appeared on the output of the gate, thereby cooking the logic chip.
It took me a while to figure it out. I still don't get video even though the socket, RAM chip and cooked chip have been replaced. Something else has probably been fried from high -ve voltage being where it shouldn't be.
Anyway, the moral of the story is faulty socket issues are common in some systems and can be hard to track down.
Tez
Now I've come to the conclusion that it kinda depends on the socket. In a few of my vintage models (Commodore Pet, Apple II+) it has been the sockets that have caused the problem in the first place!
I've just diagnosed a fault on my spare Apple II+ board. The board appeared dead. One clue was a TTL chip servicing both the keyboard and RAM was getting very hot. Investigating this I discovered it was not a short in the chip itself. Rather the OUPUT of a gate was indicating -3.5 volts when the inputs were positive (err...at least one was, the other was -0.75...but only with the chip in place. The true input (if the chip was removed) was positive).
Anyway, this gate output was connected to pin 2 of a RAM chip. What was happening was there was a short across pins 1 and 2 in the RAM chip socket. Not underneath the socket, or in the RAM chip, but somewhere IN THE SOCKET. This short occurred ONLY WHEN a RAM chip was inserted and pressed down securely. Pin 1 carried -5 volts. Being shorted to pin 2, high negative voltage then appeared on the output of the gate, thereby cooking the logic chip.
It took me a while to figure it out. I still don't get video even though the socket, RAM chip and cooked chip have been replaced. Something else has probably been fried from high -ve voltage being where it shouldn't be.
Anyway, the moral of the story is faulty socket issues are common in some systems and can be hard to track down.
Tez
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