Replacement for a DEC 7474 Chip

[Chris Quayle]

On 05/15/22 18:00, cctech-request at classiccmp.org wrote:
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> Today's Topics:
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>     1. Replacement for a DEC 7474 Chip (Rob Jarratt)
>     2. Re: Replacement for a DEC 7474 Chip (John Robertson)
>     3. Re: Replacement for a DEC 7474 Chip (Paul Koning)
>     4. Re: Replacement for a DEC 7474 Chip (Nigel Johnson Ham)
>     5. Re: Replacement for a DEC 7474 Chip (ben)
>     6. Re: Replacement for a DEC 7474 Chip (Chuck Guzis)
>     7. Re: Replacement for a DEC 7474 Chip (ben)
>     8. Re: Replacement for a DEC 7474 Chip (dwight)
>     9. Re: Replacement for a DEC 7474 Chip (Chris Zach)
>    10. RE: Replacement for a DEC 7474 Chip (Rob Jarratt)
>    11. Re: Replacement for a DEC 7474 Chip (Eric Smith)
>    12. Re: Replacement for a DEC 7474 Chip (Eric Smith)
>    13. Re: Replacement for a DEC 7474 Chip (Chuck Guzis)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sat, 14 May 2022 18:11:32 +0100
> From: "Rob Jarratt"<robert.jarratt at ntlworld.com>
> To: "General Discussion: On-Topic and Off-Topic Posts"
> 	<cctalk at classiccmp.org>
> Subject: Replacement for a DEC 7474 Chip
> Message-ID:<007b01d867b5$a8c90f00$fa5b2d00$@ntlworld.com>
> Content-Type: text/plain;	charset="us-ascii"
>
> 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
>
>
>
> ------------------------------
>
> Message: 2
> Date: Sat, 14 May 2022 10:41:34 -0700
> From: John Robertson<jrr at flippers.com>
> To: Rob Jarratt via cctalk<cctalk at classiccmp.org>
> Subject: Re: Replacement for a DEC 7474 Chip
> Message-ID:<aeed1725-343e-30e4-743e-2480a11ba286 at flippers.com>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> 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. You can use 74S74 or 74F74 as they have the same
> output current, the "S" is Schottky, and the other is "F"aster.
>
> I find that https:///unicornelectronics.com is a reliable source of TTL.
>
> John :-#)#
>

Original 74 series TTL can still be found, Ebay often, new old stock
and other surplus vendors.

Would need to take care replacing with later ttl series, as the gate
delay, switching times and fanout probably won't match. Always use the
original series if you can...

Chris