[time-nuts] Opening an Isotemp OCXO

[Adrian Godwin]

The 1k resistor doesn't seem to feed the 'S30'. It looks as though pin 14
(Vcc) goes via that thick track to the +12 input.

On Fri, Oct 28, 2016 at 5:46 PM, jimlux <jimlux at earthlink.net> wrote:

> 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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