[time-nuts] Opening an Isotemp OCXO

jimlux jimlux at earthlink.net
Fri Oct 28 12:46:41 EDT 2016

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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