On 10/28/16 8:31 AM, Peter Reilley wrote:
Looking around with my scope it seems that the output driver chip is bad
as I expected. It is a TI 14 pin surface mount DIP. It says S30 on it
which if it is a 74S30 it is an 8 input positive NAND gate. The board
layout confirms this as the 10 MHz signal is connected to pin 2 and all
other inputs are tied high. Pin 8 is connected to the output.
The chip is run off 12 volts so it must be CMOS. But I cannot find any
chip like that that will run off 12 volts. Any suggestions for a
replacement?
Also, using an 8 input NAND chip for a driver seems an odd choice.
could it be some sort of 74C series? if it's intended to run off 12V, a
lot of the CMOS parts can run at almost any Vdd (a virtue of 4000 series
CMOS - 15V power, no problem)
Maybe they got a good price on the 8 input NAND parts?
If unused pins are tied high, it's unlikely that they are outputs - most
parts are either a totem pole or open collector output, neither one of
which would like being tied high.
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