Replacement for a DEC 7474 Chip

[Paul Koning]

> On May 14, 2022, at 1:41 PM, John Robertson via cctalk <cctalk at classiccmp.org> wrote:
> 
> On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote:
>> Hello,
>> 
>> I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
>> 74<something>74 D flip flop. The problem is I don't know which modern series
>> would be the best one to replace it with. I am sure I have seen a list
>> somewhere of modern equivalents for some DEC chip numbers, but I can't
>> remember where.
>> 
>> If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the
>> schematic (p157 of the PDF).
>> 
>> Picture of the failed chip here:
>> https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg
>> 
>> Can anyone tell me what the best modern equivalent is likely to be?
>> 
>> Thanks
>> 
>> Rob
>> 
> You are stuck with using an original 7474 family assuming this is driving other early TTL. 74LS74, and others simply don't have the drive capability to work.

I know LS has less fanout, but is it not able to drive plain 74xx at all?  That doesn't sound right.  If the circuit in question runs near the fanout spec of plain 74 the yes, 74LS won't work.  

Spec sheets and the actual schematic will give a definitive answer.

	paul