Replacement for a DEC 7474 Chip

[Rob Jarratt]

Oh dear, while I was ordering an original 7474 I ordered some other parts
that were connected to the same bad chip in case other chips are damaged,
and I ordered a Fairchild 74LS08! I will ask them to change it for a
Motorola part they also have.

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of dwight via
cctalk
> Sent: 14 May 2022 23:36
> To: Paul Koning via cctalk <cctalk at classiccmp.org>
> Subject: Re: Replacement for a DEC 7474 Chip
> 
> What ever you do, don't use a Fairchild part. When I worked for Intel in
the
> 80's, we finally band using Fairchild for any latching device. They failed
on
> pullup current, even when the parts were sent back and they claimed they
> were good. We just gave up on them, we couldn't hold production while
> they figured it out.
> We had a similar problem with PowerOne, a manufacture of power supplies.
> Since it was a custom supply, we had to send someone to their plant to fix
> their final test.
> Dwight
> 
> 
> ________________________________
> From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Nigel Johnson
> Ham via cctalk <cctalk at classiccmp.org>
> Sent: Saturday, May 14, 2022 10:50 AM
> To: Paul Koning via cctalk <cctalk at classiccmp.org>
> Subject: Re: Replacement for a DEC 7474 Chip
> 
> AFAIR LS can only drive one unit TTL load.
> 
> I may have some 7474, even of that vintage, if you cannot find any
anywhere
> else.
> 
> cheers,
> 
> Nigel
> 
> 
> Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU Amateur Radio,
> the origin of the open-source concept!
> Skype:  TILBURY2591
> 
> 
> On 2022-05-14 13:48, Paul Koning via cctalk wrote:
> >
> >> 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.j
> >>> pg
> >>>
> >>> 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
> >
> >