15 Jul
2013
15 Jul
'13
11:42 a.m.
Just ran across an interesting oddity --
I ordered a few 7475s to replace the ones with corroded legs on my
Imlac's memory control boards; upon replacement I note a dead short
between 5V and GND (as evidenced by the 5V supply being drawn down to
1.5V or so on powerup). After pulling the chip out, it's pretty clear
that pin 5 (VCC) is shorted to pin 12 (GND), as are pins 4 and 13.
Are you saying all 4 pins are shorted together?
There are ICs with the middle pins all commoned like that. Some
darlington driver type thigns have the common emitter (0V) connection
wired to the centre 4 pins, the ideal is that they can go to 'heat sink
traces' on the PCB.
Verrry odd. I grab the other replacements I ordered
and they test the
same way.
My curiosity getting the better of me, I break the top off the chip (not
entirely successfully). You can see the results here:
http://yahozna.dyndns.org/scratch/imlac/fake7475.jpg
Do you have a picture of the markings on an unbroken IC? Is it posible
that the '7475' is the date code or soemthing? Who claims to have made
the IC?
-tony
